diff --git a/app/api/dimensions/__tests__/create-dimension.test.ts b/app/api/dimensions/__tests__/create-dimension.test.ts
new file mode 100644
index 00000000..76ffc3a1
--- /dev/null
+++ b/app/api/dimensions/__tests__/create-dimension.test.ts
@@ -0,0 +1,166 @@
+/**
+ * Tests for POST /api/dimensions (create a custom dimension — PR10).
+ *
+ * Covers: auto-picking the next free SIE number >= 20 (proved both via the
+ * insert result and via the self-parent guard, which names the picked
+ * number), 409 on an explicitly taken number, 400 on an invalid parent, and
+ * the 201 { data: { dimension } } contract for an explicit number.
+ *
+ * Queue order per request: ensure_company_dimensions RPC → existing-numbers
+ * select → insert returning the row.
+ */
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { NextResponse } from 'next/server'
+import { createQueuedMockSupabase, createMockRequest, parseJsonResponse } from '@/tests/helpers'
+
+const { supabase, enqueue, reset } = createQueuedMockSupabase()
+
+const requireAuthMock = vi.fn()
+vi.mock('@/lib/auth/require-auth', () => ({
+  requireAuth: (...args: unknown[]) => requireAuthMock(...args),
+}))
+
+vi.mock('@/lib/company/context', () => ({
+  getActiveCompanyId: vi.fn().mockResolvedValue('company-1'),
+  requireCompanyId: vi.fn().mockResolvedValue('company-1'),
+}))
+
+vi.mock('@/lib/auth/require-write', () => ({
+  requireWritePermission: vi.fn().mockResolvedValue({ ok: true }),
+}))
+
+vi.mock('@/lib/init', () => ({ ensureInitialized: vi.fn() }))
+
+import { POST } from '../route'
+
+const noParams = { params: Promise.resolve({}) }
+const postRequest = (body: Record<string, unknown>) =>
+  createMockRequest('/api/dimensions', { method: 'POST', body })
+
+interface DimensionRow {
+  id: string
+  sie_dim_no: number
+  name: string
+  parent_sie_dim_no: number | null
+  resets_annually: boolean
+  is_system: boolean
+  is_active: boolean
+  sort_order: number
+}
+
+type DimensionBody = { data: { dimension: DimensionRow } }
+type ErrorBody = { error: { code: string; message: string } }
+
+function makeDimensionRow(overrides: Partial<DimensionRow> = {}): DimensionRow {
+  return {
+    id: 'dim-new',
+    sie_dim_no: 21,
+    name: 'Avdelning',
+    parent_sie_dim_no: null,
+    resets_annually: true,
+    is_system: false,
+    is_active: true,
+    sort_order: 100,
+    ...overrides,
+  }
+}
+
+/** Enqueue the ensure RPC + the existing-numbers select. */
+function enqueuePreamble(existingNumbers: number[]) {
+  enqueue({ data: null }) // ensure_company_dimensions
+  enqueue({ data: existingNumbers.map((n) => ({ sie_dim_no: n })) })
+}
+
+beforeEach(() => {
+  vi.clearAllMocks()
+  reset()
+  requireAuthMock.mockResolvedValue({ user: { id: 'user-1' }, supabase })
+})
+
+describe('POST /api/dimensions', () => {
+  it('returns 401 when not authenticated', async () => {
+    requireAuthMock.mockResolvedValue({
+      error: NextResponse.json({ error: 'Unauthorized' }, { status: 401 }),
+    })
+
+    const response = await POST(postRequest({ name: 'Avdelning' }), noParams)
+
+    expect(response.status).toBe(401)
+  })
+
+  it('picks the next free number >= 20 when sie_dim_no is omitted ([1,6,20] → 21)', async () => {
+    enqueuePreamble([1, 6, 20])
+    enqueue({ data: makeDimensionRow({ sie_dim_no: 21 }) })
+
+    const response = await POST(postRequest({ name: 'Avdelning' }), noParams)
+    const { status, body } = await parseJsonResponse<DimensionBody>(response)
+
+    expect(status).toBe(201)
+    expect(body.data.dimension.sie_dim_no).toBe(21)
+    expect(body.data.dimension.is_system).toBe(false)
+  })
+
+  it('auto-picks exactly 21 — pinned via the self-parent guard message', async () => {
+    // The queued mock cannot capture insert payloads, so pin the computed
+    // number through an observable branch: parent 21 collides with the pick
+    // ONLY if the route picked 21 (any other pick yields the "finns inte"
+    // message instead, since 21 is not a registered number).
+    enqueuePreamble([1, 6, 20])
+
+    const response = await POST(
+      postRequest({ name: 'Avdelning', parent_sie_dim_no: 21 }),
+      noParams,
+    )
+    const { status, body } = await parseJsonResponse<ErrorBody>(response)
+
+    expect(status).toBe(400)
+    expect(body.error.code).toBe('DIMENSION_PARENT_INVALID')
+    expect(body.error.message).toBe(
+      'En dimension kan inte vara sin egen överordnade dimension.',
+    )
+  })
+
+  it('returns 409 DIMENSION_NUMBER_TAKEN for an explicitly taken number', async () => {
+    enqueuePreamble([1, 6])
+
+    const response = await POST(postRequest({ name: 'Projekt igen', sie_dim_no: 6 }), noParams)
+    const { status, body } = await parseJsonResponse<ErrorBody>(response)
+
+    expect(status).toBe(409)
+    expect(body.error.code).toBe('DIMENSION_NUMBER_TAKEN')
+  })
+
+  it('returns 400 DIMENSION_PARENT_INVALID for an unknown parent', async () => {
+    enqueuePreamble([1, 6])
+
+    const response = await POST(
+      postRequest({ name: 'Avdelning', sie_dim_no: 30, parent_sie_dim_no: 99 }),
+      noParams,
+    )
+    const { status, body } = await parseJsonResponse<ErrorBody>(response)
+
+    expect(status).toBe(400)
+    expect(body.error.code).toBe('DIMENSION_PARENT_INVALID')
+    expect(body.error.message).toContain('99')
+  })
+
+  it('creates an explicit-number dimension with the 201 { data: { dimension } } shape', async () => {
+    enqueuePreamble([1, 6])
+    const row = makeDimensionRow({
+      sie_dim_no: 30,
+      name: 'Maskin',
+      parent_sie_dim_no: 6,
+      resets_annually: false,
+    })
+    enqueue({ data: row })
+
+    const response = await POST(
+      postRequest({ name: 'Maskin', sie_dim_no: 30, parent_sie_dim_no: 6, resets_annually: false }),
+      noParams,
+    )
+    const { status, body } = await parseJsonResponse<DimensionBody>(response)
+
+    expect(status).toBe(201)
+    expect(body.data.dimension).toEqual(row)
+  })
+})
diff --git a/app/api/dimensions/__tests__/rules.test.ts b/app/api/dimensions/__tests__/rules.test.ts
new file mode 100644
index 00000000..2897c1ad
--- /dev/null
+++ b/app/api/dimensions/__tests__/rules.test.ts
@@ -0,0 +1,292 @@
+/**
+ * Tests for /api/dimensions/rules (list + create) and
+ * /api/dimensions/rules/[id] (update + delete) — dimensions PR10.
+ *
+ * Exercises the routes through the real withRouteContext wrapper, mocking
+ * only its auth/company dependencies and injecting a queued Supabase mock via
+ * requireAuth. Covers: 401, the DTO mapping contract, the account filter
+ * validation, the schema's value-presence superRefine, referential 404s, the
+ * 23505 → 409 duplicate mapping, PATCH's effective-type validation, and
+ * DELETE's count-based 404.
+ */
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { NextResponse } from 'next/server'
+import {
+  createQueuedMockSupabase,
+  createMockRequest,
+  createMockRouteParams,
+  parseJsonResponse,
+} from '@/tests/helpers'
+
+const { supabase, enqueue, reset } = createQueuedMockSupabase()
+
+const requireAuthMock = vi.fn()
+vi.mock('@/lib/auth/require-auth', () => ({
+  requireAuth: (...args: unknown[]) => requireAuthMock(...args),
+}))
+
+vi.mock('@/lib/company/context', () => ({
+  getActiveCompanyId: vi.fn().mockResolvedValue('company-1'),
+  requireCompanyId: vi.fn().mockResolvedValue('company-1'),
+}))
+
+vi.mock('@/lib/auth/require-write', () => ({
+  requireWritePermission: vi.fn().mockResolvedValue({ ok: true }),
+}))
+
+vi.mock('@/lib/init', () => ({ ensureInitialized: vi.fn() }))
+
+import { GET, POST } from '../rules/route'
+import { PATCH, DELETE } from '../rules/[id]/route'
+
+const DIM_ID = '11111111-1111-4111-8111-111111111111'
+const VALUE_ID = '22222222-2222-4222-8222-222222222222'
+const RULE_ID = '33333333-3333-4333-8333-333333333333'
+
+const noParams = { params: Promise.resolve({}) }
+const idParams = createMockRouteParams({ id: RULE_ID })
+
+interface RuleDto {
+  account_dimension_rule_id: string
+  account_number: string
+  dimension_id: string
+  sie_dim_no: number
+  dimension_name: string
+  rule_type: string
+  value_id: string | null
+  value_code: string | null
+  value_name: string | null
+  is_active: boolean
+}
+
+type RuleBody = { data: { rule: RuleDto } }
+type RulesBody = { data: { rules: RuleDto[] } }
+type ErrorBody = { error: { code: string; message: string } }
+
+/** Raw row exactly as RULE_SELECT returns it (joined registry aliases). */
+function makeRawRule(overrides: Record<string, unknown> = {}) {
+  return {
+    id: RULE_ID,
+    account_number: '4010',
+    rule_type: 'default',
+    value_id: VALUE_ID,
+    is_active: true,
+    dimension: { id: DIM_ID, sie_dim_no: 6, name: 'Projekt' },
+    value: { code: 'P001', name: 'Projekt Alpha' },
+    ...overrides,
+  }
+}
+
+beforeEach(() => {
+  vi.clearAllMocks()
+  reset()
+  requireAuthMock.mockResolvedValue({ user: { id: 'user-1' }, supabase })
+})
+
+describe('GET /api/dimensions/rules', () => {
+  it('returns 401 when not authenticated', async () => {
+    requireAuthMock.mockResolvedValue({
+      error: NextResponse.json({ error: 'Unauthorized' }, { status: 401 }),
+    })
+
+    const response = await GET(createMockRequest('/api/dimensions/rules'), noParams)
+
+    expect(response.status).toBe(401)
+  })
+
+  it('maps rows through the DTO (account_dimension_rule_id + value fields)', async () => {
+    enqueue({
+      data: [
+        makeRawRule(),
+        makeRawRule({
+          id: '44444444-4444-4444-8444-444444444444',
+          account_number: '5010',
+          rule_type: 'required',
+          value_id: null,
+          value: null,
+        }),
+      ],
+    })
+
+    const response = await GET(createMockRequest('/api/dimensions/rules'), noParams)
+    const { status, body } = await parseJsonResponse<RulesBody>(response)
+
+    expect(status).toBe(200)
+    expect(body.data.rules).toHaveLength(2)
+    expect(body.data.rules[0]).toEqual({
+      account_dimension_rule_id: RULE_ID,
+      account_number: '4010',
+      dimension_id: DIM_ID,
+      sie_dim_no: 6,
+      dimension_name: 'Projekt',
+      rule_type: 'default',
+      value_id: VALUE_ID,
+      value_code: 'P001',
+      value_name: 'Projekt Alpha',
+      is_active: true,
+    })
+    // A required rule has no value — the DTO carries explicit nulls.
+    expect(body.data.rules[1]).toMatchObject({
+      rule_type: 'required',
+      value_id: null,
+      value_code: null,
+      value_name: null,
+    })
+  })
+
+  it('rejects a malformed account_number filter with 400', async () => {
+    const response = await GET(
+      createMockRequest('/api/dimensions/rules', { searchParams: { account_number: '40' } }),
+      noParams,
+    )
+    const { status, body } = await parseJsonResponse<{ type: string }>(response)
+
+    expect(status).toBe(400)
+    // validateQuery's canonical envelope (review fix: inline regex → schema).
+    expect(body.type).toBe('validation_error')
+  })
+})
+
+describe('POST /api/dimensions/rules', () => {
+  const postRequest = (body: Record<string, unknown>) =>
+    createMockRequest('/api/dimensions/rules', { method: 'POST', body })
+
+  const validDefaultBody = {
+    account_number: '4010',
+    dimension_id: DIM_ID,
+    rule_type: 'default',
+    value_id: VALUE_ID,
+  }
+
+  it('creates a default rule (dimension → value → account → insert) with 201', async () => {
+    enqueue({ data: { id: DIM_ID, is_active: true } }) // dimension lookup
+    enqueue({ data: { id: VALUE_ID, is_active: true } }) // value lookup
+    enqueue({ data: { account_number: '4010' } }) // chart lookup
+    enqueue({ data: makeRawRule() }) // insert returning RULE_SELECT
+
+    const response = await POST(postRequest(validDefaultBody), noParams)
+    const { status, body } = await parseJsonResponse<RuleBody>(response)
+
+    expect(status).toBe(201)
+    expect(body.data.rule.account_dimension_rule_id).toBe(RULE_ID)
+    expect(body.data.rule.rule_type).toBe('default')
+    expect(body.data.rule.value_code).toBe('P001')
+  })
+
+  it('rejects a required rule that carries a value (schema superRefine)', async () => {
+    const response = await POST(
+      postRequest({
+        account_number: '4010',
+        dimension_id: DIM_ID,
+        rule_type: 'required',
+        value_id: VALUE_ID,
+      }),
+      noParams,
+    )
+
+    expect(response.status).toBe(400)
+  })
+
+  it('rejects a default rule without a value (schema superRefine)', async () => {
+    const response = await POST(
+      postRequest({ account_number: '4010', dimension_id: DIM_ID, rule_type: 'default' }),
+      noParams,
+    )
+
+    expect(response.status).toBe(400)
+  })
+
+  it('returns 404 for a dimension the company does not have', async () => {
+    enqueue({ data: null }) // dimension lookup misses
+
+    const response = await POST(postRequest(validDefaultBody), noParams)
+    const { status, body } = await parseJsonResponse<ErrorBody>(response)
+
+    expect(status).toBe(404)
+    expect(body.error.code).toBe('DIMENSION_NOT_FOUND')
+  })
+
+  it('maps the UNIQUE violation (23505) to 409 DIMENSION_RULE_EXISTS', async () => {
+    enqueue({ data: { id: DIM_ID, is_active: true } })
+    enqueue({ data: { id: VALUE_ID, is_active: true } })
+    enqueue({ data: { account_number: '4010' } })
+    enqueue({
+      error: { code: '23505', message: 'duplicate key value violates unique constraint' },
+    })
+
+    const response = await POST(postRequest(validDefaultBody), noParams)
+    const { status, body } = await parseJsonResponse<ErrorBody>(response)
+
+    expect(status).toBe(409)
+    expect(body.error.code).toBe('DIMENSION_RULE_EXISTS')
+  })
+})
+
+describe('PATCH /api/dimensions/rules/[id]', () => {
+  const patchRequest = (body: Record<string, unknown>) =>
+    createMockRequest(`/api/dimensions/rules/${RULE_ID}`, { method: 'PATCH', body })
+
+  it('pauses a rule via is_active without touching the value', async () => {
+    enqueue({
+      data: { id: RULE_ID, rule_type: 'default', value_id: VALUE_ID, dimension_id: DIM_ID },
+    }) // existing lookup
+    enqueue({ data: makeRawRule({ is_active: false }) }) // update returning RULE_SELECT
+
+    const response = await PATCH(patchRequest({ is_active: false }), idParams)
+    const { status, body } = await parseJsonResponse<RuleBody>(response)
+
+    expect(status).toBe(200)
+    expect(body.data.rule.is_active).toBe(false)
+    expect(body.data.rule.account_dimension_rule_id).toBe(RULE_ID)
+  })
+
+  it('rejects switching to required while the stored value remains (effective type)', async () => {
+    enqueue({
+      data: { id: RULE_ID, rule_type: 'default', value_id: VALUE_ID, dimension_id: DIM_ID },
+    })
+
+    const response = await PATCH(patchRequest({ rule_type: 'required' }), idParams)
+    const { status, body } = await parseJsonResponse<ErrorBody>(response)
+
+    expect(status).toBe(400)
+    expect(body.error.code).toBe('VALIDATION_FAILED')
+  })
+
+  it('returns 404 for a rule outside the company', async () => {
+    enqueue({ data: null })
+
+    const response = await PATCH(patchRequest({ is_active: false }), idParams)
+    const { status, body } = await parseJsonResponse<ErrorBody>(response)
+
+    expect(status).toBe(404)
+    expect(body.error.code).toBe('DIMENSION_RULE_NOT_FOUND')
+  })
+})
+
+describe('DELETE /api/dimensions/rules/[id]', () => {
+  it('deletes the rule and confirms', async () => {
+    enqueue({ count: 1 })
+
+    const response = await DELETE(
+      createMockRequest(`/api/dimensions/rules/${RULE_ID}`, { method: 'DELETE' }),
+      idParams,
+    )
+    const { status, body } = await parseJsonResponse<{ data: { deleted: boolean } }>(response)
+
+    expect(status).toBe(200)
+    expect(body.data.deleted).toBe(true)
+  })
+
+  it('returns 404 when nothing was deleted (count 0)', async () => {
+    enqueue({ count: 0 })
+
+    const response = await DELETE(
+      createMockRequest(`/api/dimensions/rules/${RULE_ID}`, { method: 'DELETE' }),
+      idParams,
+    )
+    const { status, body } = await parseJsonResponse<ErrorBody>(response)
+
+    expect(status).toBe(404)
+    expect(body.error.code).toBe('DIMENSION_RULE_NOT_FOUND')
+  })
+})
diff --git a/app/api/dimensions/route.ts b/app/api/dimensions/route.ts
index 5897b089..16dac498 100644
--- a/app/api/dimensions/route.ts
+++ b/app/api/dimensions/route.ts
@@ -15,6 +15,8 @@
 import { NextResponse } from 'next/server'
 import { ensureInitialized } from '@/lib/init'
 import { withRouteContext } from '@/lib/api/with-route-context'
+import { validateBody } from '@/lib/api/validate'
+import { CreateDimensionSchema } from '@/lib/api/schemas'
 import { errorResponse } from '@/lib/errors/get-structured-error'
 
 ensureInitialized()
@@ -33,6 +35,7 @@ interface DimensionRow {
   id: string
   sie_dim_no: number
   name: string
+  parent_sie_dim_no: number | null
   resets_annually: boolean
   is_system: boolean
   is_active: boolean
@@ -58,7 +61,7 @@ export const GET = withRouteContext(
 
     const { data: dims, error: dimsError } = await supabase
       .from('dimensions')
-      .select('id, sie_dim_no, name, resets_annually, is_system, is_active, sort_order')
+      .select('id, sie_dim_no, name, parent_sie_dim_no, resets_annually, is_system, is_active, sort_order')
       .eq('company_id', companyId)
       .order('sort_order', { ascending: true })
       .order('sie_dim_no', { ascending: true })
@@ -101,3 +104,141 @@ export const GET = withRouteContext(
     return NextResponse.json({ dimensions })
   },
 )
+
+/**
+ * POST /api/dimensions — create a custom dimension (dimensions PR10).
+ *
+ * SIE reserves numbers 1-19 for standardized meanings (1 kostnadsställe,
+ * 6 projekt, 7 anställd, …) and leaves 20+ free — when sie_dim_no is
+ * omitted the server picks the next free number >= 20. Explicit numbers are
+ * allowed across the whole 1-9999 range (SIE import already creates
+ * reserved-number dims like 7 Anställd; manual creation of one you know is
+ * the same operation), uniqueness enforced per company.
+ *
+ * parent_sie_dim_no (optional) declares an #UNDERDIM hierarchy — it must
+ * reference an existing dimension in the company registry; SIE export emits
+ * the declaration parent-before-child (lib/reports/sie-export.ts).
+ */
+export const POST = withRouteContext(
+  'dimension.create',
+  async (request, ctx) => {
+    const { supabase, companyId, log, requestId } = ctx
+
+    const validation = await validateBody(request, CreateDimensionSchema)
+    if (!validation.success) return validation.response
+    const body = validation.data
+
+    const { error: ensureError } = await supabase.rpc('ensure_company_dimensions', {
+      p_company_id: companyId,
+    })
+    if (ensureError) {
+      log.error('ensure_company_dimensions failed', ensureError)
+      return errorResponse(ensureError, log, { requestId })
+    }
+
+    const { data: existing, error: existingError } = await supabase
+      .from('dimensions')
+      .select('sie_dim_no')
+      .eq('company_id', companyId)
+
+    if (existingError) {
+      log.error('dimension number lookup failed', existingError)
+      return errorResponse(existingError, log, { requestId })
+    }
+    const taken = new Set(
+      ((existing ?? []) as { sie_dim_no: number }[]).map((d) => d.sie_dim_no),
+    )
+
+    let sieDimNo = body.sie_dim_no
+    if (sieDimNo === undefined) {
+      // Next free custom number — SIE leaves 20+ unreserved.
+      sieDimNo = 20
+      while (taken.has(sieDimNo)) sieDimNo++
+    } else if (taken.has(sieDimNo)) {
+      return NextResponse.json(
+        {
+          error: {
+            code: 'DIMENSION_NUMBER_TAKEN',
+            message: `Dimension ${sieDimNo} finns redan i registret.`,
+          },
+        },
+        { status: 409 },
+      )
+    }
+
+    if (body.parent_sie_dim_no != null) {
+      if (body.parent_sie_dim_no === sieDimNo) {
+        return NextResponse.json(
+          {
+            error: {
+              code: 'DIMENSION_PARENT_INVALID',
+              message: 'En dimension kan inte vara sin egen överordnade dimension.',
+            },
+          },
+          { status: 400 },
+        )
+      }
+      if (!taken.has(body.parent_sie_dim_no)) {
+        return NextResponse.json(
+          {
+            error: {
+              code: 'DIMENSION_PARENT_INVALID',
+              message: `Överordnad dimension ${body.parent_sie_dim_no} finns inte i registret.`,
+            },
+          },
+          { status: 400 },
+        )
+      }
+    }
+
+    const autoPicked = body.sie_dim_no === undefined
+    const insertDimension = (dimNo: number) =>
+      supabase
+        .from('dimensions')
+        .insert({
+          company_id: companyId,
+          sie_dim_no: dimNo,
+          name: body.name,
+          parent_sie_dim_no: body.parent_sie_dim_no ?? null,
+          resets_annually: body.resets_annually ?? true,
+          is_system: false,
+          is_active: true,
+          // System dims 1/6 sit at sort_order 10/20 (substrate seeding); custom
+          // dims trail them by default.
+          sort_order: 100,
+        })
+        .select('id, sie_dim_no, name, parent_sie_dim_no, resets_annually, is_system, is_active, sort_order')
+        .single()
+
+    let { data: dimension, error: insertError } = await insertDimension(sieDimNo)
+
+    // Auto-picked numbers can race a concurrent create/SIE import between the
+    // read and the insert — the UNIQUE is the arbiter; retry once past the
+    // loser instead of surfacing a spurious "finns redan" for a number the
+    // user never chose. Explicitly chosen numbers still 409.
+    if (insertError?.code === '23505' && autoPicked) {
+      sieDimNo++
+      while (taken.has(sieDimNo)) sieDimNo++
+      ;({ data: dimension, error: insertError } = await insertDimension(sieDimNo))
+    }
+
+    if (insertError) {
+      if (insertError.code === '23505') {
+        return NextResponse.json(
+          {
+            error: {
+              code: 'DIMENSION_NUMBER_TAKEN',
+              message: `Dimension ${sieDimNo} finns redan i registret.`,
+            },
+          },
+          { status: 409 },
+        )
+      }
+      log.error('dimension create failed', insertError)
+      return errorResponse(insertError, log, { requestId })
+    }
+
+    return NextResponse.json({ data: { dimension } }, { status: 201 })
+  },
+  { requireWrite: true },
+)
diff --git a/app/api/dimensions/rules/[id]/route.ts b/app/api/dimensions/rules/[id]/route.ts
new file mode 100644
index 00000000..e6d490fd
--- /dev/null
+++ b/app/api/dimensions/rules/[id]/route.ts
@@ -0,0 +1,138 @@
+/**
+ * /api/dimensions/rules/[id] — mutate one account dimension rule (PR10).
+ *
+ * PATCH  { rule_type?, value_id?, is_active? } — value presence is
+ *        re-validated against the EFFECTIVE rule_type (required ⇔ no value).
+ * DELETE — removes the rule; enforcement stops immediately. Pausing without
+ *          losing the configuration is is_active: false.
+ */
+import { NextResponse } from 'next/server'
+import { ensureInitialized } from '@/lib/init'
+import { withRouteContext } from '@/lib/api/with-route-context'
+import { validateBody } from '@/lib/api/validate'
+import { UpdateAccountDimensionRuleSchema } from '@/lib/api/schemas'
+import { errorResponse } from '@/lib/errors/get-structured-error'
+import { RULE_SELECT, toRuleDto, type RawRule } from '../dto'
+
+ensureInitialized()
+
+export const PATCH = withRouteContext<{ params: Promise<{ id: string }> }>(
+  'dimension.rules.update',
+  async (request, ctx, { params }) => {
+    const { id } = await params
+    const { supabase, companyId, log, requestId } = ctx
+
+    const validation = await validateBody(request, UpdateAccountDimensionRuleSchema)
+    if (!validation.success) return validation.response
+    const body = validation.data
+
+    const { data: existing, error: existingError } = await supabase
+      .from('account_dimension_rules')
+      .select('id, rule_type, value_id, dimension_id')
+      .eq('id', id)
+      .eq('company_id', companyId)
+      .maybeSingle()
+    if (existingError) return errorResponse(existingError, log, { requestId })
+    if (!existing) {
+      return NextResponse.json(
+        { error: { code: 'DIMENSION_RULE_NOT_FOUND', message: 'Regeln finns inte.' } },
+        { status: 404 },
+      )
+    }
+
+    const effectiveType = body.rule_type ?? (existing.rule_type as string)
+    const effectiveValueId =
+      body.value_id !== undefined ? body.value_id : (existing.value_id as string | null)
+
+    if (effectiveType === 'required' && effectiveValueId) {
+      return NextResponse.json(
+        { error: { code: 'VALIDATION_FAILED', message: 'En obligatorisk regel har inget värde — ta bort värdet eller byt regeltyp.' } },
+        { status: 400 },
+      )
+    }
+    if (effectiveType !== 'required' && !effectiveValueId) {
+      return NextResponse.json(
+        { error: { code: 'VALIDATION_FAILED', message: 'Välj vilket värde regeln ska använda.' } },
+        { status: 400 },
+      )
+    }
+
+    if (body.value_id) {
+      const { data: value, error: valueError } = await supabase
+        .from('dimension_values')
+        .select('id, is_active')
+        .eq('id', body.value_id)
+        .eq('company_id', companyId)
+        .eq('dimension_id', existing.dimension_id)
+        .maybeSingle()
+      if (valueError) return errorResponse(valueError, log, { requestId })
+      if (!value) {
+        return NextResponse.json(
+          { error: { code: 'DIMENSION_VALUE_NOT_FOUND', message: 'Värdet finns inte under regelns dimension.' } },
+          { status: 404 },
+        )
+      }
+      if (!value.is_active) {
+        return NextResponse.json(
+          { error: { code: 'DIMENSION_VALUE_ARCHIVED', message: 'Värdet är arkiverat — återaktivera det innan det används i en regel.' } },
+          { status: 400 },
+        )
+      }
+    }
+
+    const updates: Record<string, unknown> = {}
+    if (body.rule_type !== undefined) updates.rule_type = body.rule_type
+    if (body.value_id !== undefined) updates.value_id = body.value_id
+    if (body.is_active !== undefined) updates.is_active = body.is_active
+    if (Object.keys(updates).length === 0) {
+      return NextResponse.json(
+        { error: { code: 'VALIDATION_FAILED', message: 'Ingen ändring angiven.' } },
+        { status: 400 },
+      )
+    }
+
+    const { data: rule, error: updateError } = await supabase
+      .from('account_dimension_rules')
+      .update(updates)
+      .eq('id', id)
+      .eq('company_id', companyId)
+      .select(RULE_SELECT)
+      .single()
+
+    if (updateError) {
+      log.error('dimension rule update failed', updateError)
+      return errorResponse(updateError, log, { requestId })
+    }
+
+    return NextResponse.json({ data: { rule: toRuleDto(rule as unknown as RawRule) } })
+  },
+  { requireWrite: true },
+)
+
+export const DELETE = withRouteContext<{ params: Promise<{ id: string }> }>(
+  'dimension.rules.delete',
+  async (_request, ctx, { params }) => {
+    const { id } = await params
+    const { supabase, companyId, log, requestId } = ctx
+
+    const { error, count } = await supabase
+      .from('account_dimension_rules')
+      .delete({ count: 'exact' })
+      .eq('id', id)
+      .eq('company_id', companyId)
+
+    if (error) {
+      log.error('dimension rule delete failed', error)
+      return errorResponse(error, log, { requestId })
+    }
+    if (!count) {
+      return NextResponse.json(
+        { error: { code: 'DIMENSION_RULE_NOT_FOUND', message: 'Regeln finns inte.' } },
+        { status: 404 },
+      )
+    }
+
+    return NextResponse.json({ data: { deleted: true } })
+  },
+  { requireWrite: true },
+)
diff --git a/app/api/dimensions/rules/dto.ts b/app/api/dimensions/rules/dto.ts
new file mode 100644
index 00000000..58c420f8
--- /dev/null
+++ b/app/api/dimensions/rules/dto.ts
@@ -0,0 +1,29 @@
+/** Shared select + DTO mapper for the account_dimension_rules routes (PR10). */
+
+export const RULE_SELECT =
+  'id, account_number, rule_type, value_id, is_active, dimension:dimensions!account_dimension_rules_dimension_id_company_id_fkey(id, sie_dim_no, name), value:dimension_values!account_dimension_rules_value_id_fkey(code, name)'
+
+export interface RawRule {
+  id: string
+  account_number: string
+  rule_type: 'required' | 'default' | 'fixed'
+  value_id: string | null
+  is_active: boolean
+  dimension: { id: string; sie_dim_no: number; name: string }
+  value: { code: string; name: string } | null
+}
+
+export function toRuleDto(row: RawRule) {
+  return {
+    account_dimension_rule_id: row.id,
+    account_number: row.account_number,
+    dimension_id: row.dimension.id,
+    sie_dim_no: row.dimension.sie_dim_no,
+    dimension_name: row.dimension.name,
+    rule_type: row.rule_type,
+    value_id: row.value_id,
+    value_code: row.value?.code ?? null,
+    value_name: row.value?.name ?? null,
+    is_active: row.is_active,
+  }
+}
diff --git a/app/api/dimensions/rules/route.ts b/app/api/dimensions/rules/route.ts
new file mode 100644
index 00000000..d310cd19
--- /dev/null
+++ b/app/api/dimensions/rules/route.ts
@@ -0,0 +1,157 @@
+/**
+ * /api/dimensions/rules — per-account dimension policy (dimensions PR10).
+ *
+ * GET  ?account_number=4010 (optional) → every rule (or the account's).
+ * POST → create a rule. 'required' blocks posting on the account without a
+ * value for the dimension (enforced at commitEntry + the bulk-book route);
+ * 'default' pre-fills at draft creation; 'fixed' always applies.
+ *
+ * Opt-in by construction: zero rules = the engine behaves exactly as before.
+ * There is deliberately NO settings toggle for enforcement — a rule that
+ * exists but is ignored would be worse than either extreme; pausing a single
+ * rule is what is_active is for.
+ */
+import { NextResponse } from 'next/server'
+import { ensureInitialized } from '@/lib/init'
+import { withRouteContext } from '@/lib/api/with-route-context'
+import { validateBody, validateQuery } from '@/lib/api/validate'
+import { CreateAccountDimensionRuleSchema, ListDimensionRulesQuerySchema } from '@/lib/api/schemas'
+import { errorResponse } from '@/lib/errors/get-structured-error'
+import { RULE_SELECT, toRuleDto, type RawRule } from './dto'
+
+ensureInitialized()
+
+
+export const GET = withRouteContext(
+  'dimension.rules.list',
+  async (request, ctx) => {
+    const { supabase, companyId, log, requestId } = ctx
+
+    const queryValidation = validateQuery(request, ListDimensionRulesQuerySchema, {
+      log,
+      operation: 'dimension.rules.list',
+    })
+    if (!queryValidation.success) return queryValidation.response
+    const { account_number: accountNumber } = queryValidation.data
+
+    let query = supabase
+      .from('account_dimension_rules')
+      .select(RULE_SELECT)
+      .eq('company_id', companyId)
+      .order('account_number', { ascending: true })
+
+    if (accountNumber) {
+      query = query.eq('account_number', accountNumber)
+    }
+
+    const { data, error } = await query
+    if (error) {
+      log.error('dimension rule list failed', error)
+      return errorResponse(error, log, { requestId })
+    }
+
+    return NextResponse.json({
+      data: { rules: ((data ?? []) as unknown as RawRule[]).map(toRuleDto) },
+    })
+  },
+)
+
+export const POST = withRouteContext(
+  'dimension.rules.create',
+  async (request, ctx) => {
+    const { supabase, companyId, log, requestId } = ctx
+
+    const validation = await validateBody(request, CreateAccountDimensionRuleSchema)
+    if (!validation.success) return validation.response
+    const body = validation.data
+
+    // The dimension must belong to this company (RLS backstops; this gives a
+    // clean Swedish 400 instead of an FK error).
+    const { data: dimension, error: dimensionError } = await supabase
+      .from('dimensions')
+      .select('id, is_active')
+      .eq('id', body.dimension_id)
+      .eq('company_id', companyId)
+      .maybeSingle()
+    if (dimensionError) return errorResponse(dimensionError, log, { requestId })
+    if (!dimension) {
+      return NextResponse.json(
+        { error: { code: 'DIMENSION_NOT_FOUND', message: 'Dimensionen finns inte i registret.' } },
+        { status: 404 },
+      )
+    }
+
+    // default/fixed: the value must belong to the SAME dimension + company
+    // and be active — a rule pointing at a foreign or archived value would
+    // make every booking on the account fail registry validation.
+    if (body.value_id) {
+      const { data: value, error: valueError } = await supabase
+        .from('dimension_values')
+        .select('id, is_active')
+        .eq('id', body.value_id)
+        .eq('company_id', companyId)
+        .eq('dimension_id', body.dimension_id)
+        .maybeSingle()
+      if (valueError) return errorResponse(valueError, log, { requestId })
+      if (!value) {
+        return NextResponse.json(
+          { error: { code: 'DIMENSION_VALUE_NOT_FOUND', message: 'Värdet finns inte under den valda dimensionen.' } },
+          { status: 404 },
+        )
+      }
+      if (!value.is_active) {
+        return NextResponse.json(
+          { error: { code: 'DIMENSION_VALUE_ARCHIVED', message: 'Värdet är arkiverat — återaktivera det innan det används i en regel.' } },
+          { status: 400 },
+        )
+      }
+    }
+
+    // The account must exist and be active in the company chart — a rule on
+    // a nonexistent account can never fire and only confuses.
+    const { data: account, error: accountError } = await supabase
+      .from('chart_of_accounts')
+      .select('account_number')
+      .eq('company_id', companyId)
+      .eq('account_number', body.account_number)
+      .eq('is_active', true)
+      .maybeSingle()
+    if (accountError) return errorResponse(accountError, log, { requestId })
+    if (!account) {
+      return NextResponse.json(
+        { error: { code: 'ACCOUNT_NOT_FOUND', message: `Konto ${body.account_number} finns inte som aktivt konto i kontoplanen.` } },
+        { status: 404 },
+      )
+    }
+
+    const { data: rule, error: insertError } = await supabase
+      .from('account_dimension_rules')
+      .insert({
+        company_id: companyId,
+        account_number: body.account_number,
+        dimension_id: body.dimension_id,
+        rule_type: body.rule_type,
+        value_id: body.value_id ?? null,
+        is_active: body.is_active ?? true,
+      })
+      .select(RULE_SELECT)
+      .single()
+
+    if (insertError) {
+      if (insertError.code === '23505') {
+        return NextResponse.json(
+          { error: { code: 'DIMENSION_RULE_EXISTS', message: `Konto ${body.account_number} har redan en regel för den dimensionen.` } },
+          { status: 409 },
+        )
+      }
+      log.error('dimension rule create failed', insertError)
+      return errorResponse(insertError, log, { requestId })
+    }
+
+    return NextResponse.json(
+      { data: { rule: toRuleDto(rule as unknown as RawRule) } },
+      { status: 201 },
+    )
+  },
+  { requireWrite: true },
+)
diff --git a/app/api/transactions/bulk-book/__tests__/route.test.ts b/app/api/transactions/bulk-book/__tests__/route.test.ts
index 1f1a1e3f..afa4815d 100644
--- a/app/api/transactions/bulk-book/__tests__/route.test.ts
+++ b/app/api/transactions/bulk-book/__tests__/route.test.ts
@@ -139,6 +139,8 @@ describe('POST /api/transactions/bulk-book', () => {
       { account_number: '2611', debit_amount: '', credit_amount: String(total * 0.2), line_description: 'Utg moms 25%' },
     ])
 
+    // Account dimension rules pre-check (PR10) — none configured.
+    enqueue({ data: [], error: null })
     // RPC returns happy path.
     enqueue({
       data: {
diff --git a/app/api/transactions/bulk-book/route.ts b/app/api/transactions/bulk-book/route.ts
index 8296f854..626f8aa6 100644
--- a/app/api/transactions/bulk-book/route.ts
+++ b/app/api/transactions/bulk-book/route.ts
@@ -5,6 +5,12 @@ import { BulkBookSchema } from '@/lib/api/schemas'
 import { errorResponseFromCode } from '@/lib/errors/get-structured-error'
 import { applyTemplate } from '@/lib/bookkeeping/template-library'
 import { mergeDimensionBags } from '@/lib/bookkeeping/dimension-resolver'
+import {
+  applyDimensionRules,
+  assertMandatoryDimensions,
+  fetchActiveDimensionRules,
+} from '@/lib/bookkeeping/dimension-rules'
+import { bookkeepingErrorResponse } from '@/lib/bookkeeping/errors'
 import { eventBus } from '@/lib/events/bus'
 import { ensureInitialized } from '@/lib/init'
 import type { BookingTemplateLibraryLine, Transaction } from '@/types'
@@ -242,6 +248,28 @@ export const POST = withRouteContext(
       }
     }
 
+    // Account dimension rules (dimensions PR10): the bulk-book RPC bypasses
+    // the TS engine, so the policy layer runs here — defaults/fixed applied
+    // to the computed lines, then 'required' asserted. Zero rules (the
+    // default) or a failed fetch changes nothing (fail-open, same posture as
+    // the engine).
+    if (newEntryPayload) {
+      const rules = await fetchActiveDimensionRules(supabase, companyId!)
+      if (rules === null) {
+        opLog.warn('dimension rule fetch failed — policy skipped (fail-open)')
+      }
+      if (rules && rules.length > 0) {
+        newEntryPayload.lines = applyDimensionRules(newEntryPayload.lines, rules)
+        try {
+          assertMandatoryDimensions(newEntryPayload.lines, rules)
+        } catch (err) {
+          const mapped = bookkeepingErrorResponse(err)
+          if (mapped) return mapped
+          throw err
+        }
+      }
+    }
+
     // p_user_id removed in PR #608 (round-3 hardening pattern applied
     // consistently). RPC resolves the caller via auth.uid().
     const { data, error } = await supabase.rpc('bulk_book_transactions', {
diff --git a/components/bookkeeping/EditAccountDialog.tsx b/components/bookkeeping/EditAccountDialog.tsx
index b4aac9ff..f21a7043 100644
--- a/components/bookkeeping/EditAccountDialog.tsx
+++ b/components/bookkeeping/EditAccountDialog.tsx
@@ -1,6 +1,6 @@
 'use client'
 
-import { useState } from 'react'
+import { useEffect, useState } from 'react'
 import {
   Dialog,
   DialogContent,
@@ -8,12 +8,29 @@ import {
   DialogTitle,
   DialogFooter,
 } from '@/components/ui/dialog'
+import {
+  Select,
+  SelectContent,
+  SelectItem,
+  SelectTrigger,
+  SelectValue,
+} from '@/components/ui/select'
 import { Button } from '@/components/ui/button'
 import { Input } from '@/components/ui/input'
 import { Label } from '@/components/ui/label'
+import { Skeleton } from '@/components/ui/skeleton'
 import { Textarea } from '@/components/ui/textarea'
 import { Switch } from '@/components/ui/switch'
-import { Loader2 } from 'lucide-react'
+import { useToast } from '@/components/ui/use-toast'
+import { getErrorMessage } from '@/lib/errors/get-error-message'
+import { Loader2, Plus, X } from 'lucide-react'
+import DimensionCombobox from '@/components/dimensions/DimensionCombobox'
+import {
+  fetchDimensions,
+  type AccountDimensionRuleDto,
+  type DimensionDto,
+  type DimensionRuleType,
+} from '@/components/dimensions/types'
 import type { BASAccount } from '@/types'
 
 interface EditAccountDialogProps {
@@ -23,7 +40,23 @@ interface EditAccountDialogProps {
   onSaved: () => void
 }
 
+// Hardcoded Swedish per the file's convention (chart-of-accounts editing is a
+// bookkeeping surface). Labels mirror the rule semantics enforced by the
+// engine at commit time.
+const RULE_TYPE_LABELS: Record<DimensionRuleType, string> = {
+  required: 'Krävs',
+  default: 'Förval',
+  fixed: 'Låst',
+}
+
+const RULE_TYPE_HELP: Record<DimensionRuleType, string> = {
+  required: 'Krävs — verifikat på kontot kan inte bokföras utan värde',
+  default: 'Förval — värdet föreslås men kan ändras',
+  fixed: 'Låst — värdet sätts alltid automatiskt',
+}
+
 export function EditAccountDialog({ open, onOpenChange, account, onSaved }: EditAccountDialogProps) {
+  const { toast } = useToast()
   const [accountName, setAccountName] = useState(account.account_name)
   const [description, setDescription] = useState(account.description || '')
   const [defaultVatCode, setDefaultVatCode] = useState(account.default_vat_code || '')
@@ -31,6 +64,183 @@ export function EditAccountDialog({ open, onOpenChange, account, onSaved }: Edit
   const [isActive, setIsActive] = useState(account.is_active)
   const [isSaving, setIsSaving] = useState(false)
 
+  // Dimension rules ("Dimensionsregler") — visible only when the company has
+  // dimensions enabled (same /api/settings gate as JournalEntryForm). Rule
+  // mutations apply immediately via their own fetches + toasts; they are
+  // deliberately independent of the account PUT below.
+  const [dimensionsEnabled, setDimensionsEnabled] = useState(false)
+  const [dims, setDims] = useState<DimensionDto[]>([])
+  const [rules, setRules] = useState<AccountDimensionRuleDto[]>([])
+  const [rulesLoading, setRulesLoading] = useState(false)
+  const [addRuleOpen, setAddRuleOpen] = useState(false)
+  const [newRuleDimensionId, setNewRuleDimensionId] = useState('')
+  const [newRuleType, setNewRuleType] = useState<DimensionRuleType>('required')
+  const [newRuleValueCode, setNewRuleValueCode] = useState<string | null>(null)
+  const [isAddingRule, setIsAddingRule] = useState(false)
+
+  useEffect(() => {
+    let cancelled = false
+    fetch('/api/settings')
+      .then((r) => r.json())
+      .then(({ data }) => {
+        if (!cancelled && data?.dimensions_enabled === true) setDimensionsEnabled(true)
+      })
+      .catch(() => {
+        /* keep the section hidden */
+      })
+    return () => {
+      cancelled = true
+    }
+  }, [])
+
+  useEffect(() => {
+    if (!dimensionsEnabled) return
+    let cancelled = false
+    setRulesLoading(true)
+    Promise.all([
+      fetchDimensions().catch(() => [] as DimensionDto[]),
+      fetch(`/api/dimensions/rules?account_number=${account.account_number}`)
+        .then(async (r) => ({ ok: r.ok, json: await r.json().catch(() => null) }))
+        .catch(() => ({ ok: false, json: null })),
+    ]).then(([fetchedDims, rulesRes]) => {
+      if (cancelled) return
+      setDims(fetchedDims)
+      if (rulesRes.ok) {
+        setRules((rulesRes.json?.data?.rules ?? []) as AccountDimensionRuleDto[])
+      }
+      setRulesLoading(false)
+    })
+    return () => {
+      cancelled = true
+    }
+  }, [dimensionsEnabled, account.account_number])
+
+  const activeDims = dims.filter((d) => d.is_active)
+  const newRuleDim = activeDims.find((d) => d.id === newRuleDimensionId) ?? null
+  const newRuleNeedsValue = newRuleType === 'default' || newRuleType === 'fixed'
+
+  function resetAddRuleForm() {
+    setAddRuleOpen(false)
+    setNewRuleDimensionId('')
+    setNewRuleType('required')
+    setNewRuleValueCode(null)
+  }
+
+  async function handleAddRule() {
+    if (!newRuleDim) return
+    setIsAddingRule(true)
+    try {
+      // Resolve the picked code to a value id. The combobox can create values
+      // inline, so a code missing from the mount-time registry snapshot means
+      // we refetch once before giving up.
+      let valueId: string | null = null
+      if (newRuleNeedsValue) {
+        const code = newRuleValueCode
+        if (!code) return
+        const findValueId = (list: DimensionDto[]) =>
+          list
+            .find((d) => d.id === newRuleDim.id)
+            ?.values.find((v) => v.code === code)?.id ?? null
+        valueId = findValueId(dims)
+        if (!valueId) {
+          const refreshed = await fetchDimensions().catch(() => null)
+          if (refreshed) {
+            setDims(refreshed)
+            valueId = findValueId(refreshed)
+          }
+        }
+        if (!valueId) {
+          toast({
+            title: 'Kunde inte lägga till regeln',
+            description: `Värdet ${code} hittades inte i registret.`,
+            variant: 'destructive',
+          })
+          return
+        }
+      }
+
+      const body: Record<string, unknown> = {
+        account_number: account.account_number,
+        dimension_id: newRuleDim.id,
+        rule_type: newRuleType,
+      }
+      if (valueId) body.value_id = valueId
+      const res = await fetch('/api/dimensions/rules', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify(body),
+      })
+      const json = await res.json().catch(() => null)
+      if (!res.ok) {
+        toast({
+          title: 'Kunde inte lägga till regeln',
+          description: getErrorMessage(json, { locale: 'sv' }),
+          variant: 'destructive',
+        })
+        return
+      }
+      const created = json?.data?.rule as AccountDimensionRuleDto | undefined
+      if (created) setRules((prev) => [...prev, created])
+      toast({ title: 'Regel tillagd' })
+      resetAddRuleForm()
+    } finally {
+      setIsAddingRule(false)
+    }
+  }
+
+  async function handleToggleRule(rule: AccountDimensionRuleDto, checked: boolean) {
+    const ruleId = rule.account_dimension_rule_id
+    // Optimistic — the switch flips immediately and reverts on failure.
+    setRules((prev) =>
+      prev.map((r) =>
+        r.account_dimension_rule_id === ruleId ? { ...r, is_active: checked } : r,
+      ),
+    )
+    const res = await fetch(`/api/dimensions/rules/${ruleId}`, {
+      method: 'PATCH',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ is_active: checked }),
+    }).catch(() => null)
+    const json = await res?.json().catch(() => null)
+    if (!res?.ok) {
+      setRules((prev) =>
+        prev.map((r) =>
+          r.account_dimension_rule_id === ruleId ? { ...r, is_active: rule.is_active } : r,
+        ),
+      )
+      toast({
+        title: 'Kunde inte uppdatera regeln',
+        description: getErrorMessage(json, { locale: 'sv' }),
+        variant: 'destructive',
+      })
+      return
+    }
+    const updated = json?.data?.rule as AccountDimensionRuleDto | undefined
+    if (updated) {
+      setRules((prev) =>
+        prev.map((r) => (r.account_dimension_rule_id === ruleId ? updated : r)),
+      )
+    }
+  }
+
+  async function handleDeleteRule(rule: AccountDimensionRuleDto) {
+    const ruleId = rule.account_dimension_rule_id
+    const res = await fetch(`/api/dimensions/rules/${ruleId}`, {
+      method: 'DELETE',
+    }).catch(() => null)
+    if (!res?.ok) {
+      const json = await res?.json().catch(() => null)
+      toast({
+        title: 'Kunde inte ta bort regeln',
+        description: getErrorMessage(json, { locale: 'sv' }),
+        variant: 'destructive',
+      })
+      return
+    }
+    setRules((prev) => prev.filter((r) => r.account_dimension_rule_id !== ruleId))
+    toast({ title: 'Regel borttagen' })
+  }
+
   async function handleSave() {
     setIsSaving(true)
     try {
@@ -62,7 +272,7 @@ export function EditAccountDialog({ open, onOpenChange, account, onSaved }: Edit
 
   return (
     <Dialog open={open} onOpenChange={onOpenChange}>
-      <DialogContent>
+      <DialogContent className="max-h-[95dvh] sm:max-h-[90vh] overflow-y-auto">
         <DialogHeader>
           <DialogTitle>
             Redigera konto {account.account_number}
@@ -107,6 +317,173 @@ export function EditAccountDialog({ open, onOpenChange, account, onSaved }: Edit
             </div>
           </div>
 
+          {dimensionsEnabled && (
+            <div className="space-y-3">
+              <p className="text-sm font-medium">Dimensionsregler</p>
+
+              {rulesLoading ? (
+                <Skeleton className="h-10 w-full" />
+              ) : (
+                <>
+                  {rules.length === 0 && !addRuleOpen && (
+                    <div className="flex items-center justify-between gap-3">
+                      <p className="text-xs text-muted-foreground">
+                        Inga dimensionsregler för det här kontot.
+                      </p>
+                      <Button
+                        variant="ghost"
+                        size="sm"
+                        onClick={() => setAddRuleOpen(true)}
+                      >
+                        <Plus className="mr-1 h-3.5 w-3.5" />
+                        Lägg till regel
+                      </Button>
+                    </div>
+                  )}
+
+                  {rules.map((rule) => (
+                    <div
+                      key={rule.account_dimension_rule_id}
+                      className="flex items-center gap-3 rounded-lg border p-3"
+                    >
+                      <div className="min-w-0 flex-1">
+                        <p className="text-sm font-medium truncate">
+                          {rule.dimension_name}
+                        </p>
+                        <p className="text-xs text-muted-foreground truncate">
+                          {RULE_TYPE_LABELS[rule.rule_type]}
+                          {rule.value_code && (
+                            <>
+                              {' · '}
+                              <span className="font-mono">{rule.value_code}</span>
+                              {rule.value_name && rule.value_name !== rule.value_code
+                                ? ` ${rule.value_name}`
+                                : ''}
+                            </>
+                          )}
+                        </p>
+                      </div>
+                      <Switch
+                        checked={rule.is_active}
+                        onCheckedChange={(checked) => handleToggleRule(rule, checked)}
+                        aria-label={`Regel för ${rule.dimension_name} aktiv`}
+                      />
+                      <Button
+                        variant="ghost"
+                        size="icon"
+                        aria-label={`Ta bort regel för ${rule.dimension_name}`}
+                        onClick={() => handleDeleteRule(rule)}
+                      >
+                        <X className="h-4 w-4" />
+                      </Button>
+                    </div>
+                  ))}
+
+                  {addRuleOpen ? (
+                    <div className="space-y-3 rounded-lg border border-dashed p-3">
+                      <div className="grid grid-cols-1 sm:grid-cols-2 gap-3">
+                        <div className="space-y-1">
+                          <Label className="text-xs text-muted-foreground">
+                            Dimension
+                          </Label>
+                          <Select
+                            value={newRuleDimensionId || undefined}
+                            onValueChange={(id) => {
+                              setNewRuleDimensionId(id)
+                              setNewRuleValueCode(null)
+                            }}
+                          >
+                            <SelectTrigger>
+                              <SelectValue placeholder="Välj dimension" />
+                            </SelectTrigger>
+                            <SelectContent>
+                              {activeDims.map((dim) => (
+                                <SelectItem key={dim.id} value={dim.id}>
+                                  {dim.name}
+                                </SelectItem>
+                              ))}
+                            </SelectContent>
+                          </Select>
+                        </div>
+                        <div className="space-y-1">
+                          <Label className="text-xs text-muted-foreground">Typ</Label>
+                          <Select
+                            value={newRuleType}
+                            onValueChange={(v) => {
+                              setNewRuleType(v as DimensionRuleType)
+                              if (v === 'required') setNewRuleValueCode(null)
+                            }}
+                          >
+                            <SelectTrigger>
+                              <SelectValue />
+                            </SelectTrigger>
+                            <SelectContent>
+                              {(Object.keys(RULE_TYPE_LABELS) as DimensionRuleType[]).map(
+                                (type) => (
+                                  <SelectItem key={type} value={type}>
+                                    {RULE_TYPE_LABELS[type]}
+                                  </SelectItem>
+                                ),
+                              )}
+                            </SelectContent>
+                          </Select>
+                        </div>
+                      </div>
+                      <p className="text-xs text-muted-foreground">
+                        {RULE_TYPE_HELP[newRuleType]}
+                      </p>
+                      {newRuleNeedsValue && newRuleDim && (
+                        <div className="space-y-1">
+                          <Label className="text-xs text-muted-foreground">Värde</Label>
+                          <DimensionCombobox
+                            sieDimNo={String(newRuleDim.sie_dim_no)}
+                            value={newRuleValueCode}
+                            onChange={setNewRuleValueCode}
+                          />
+                        </div>
+                      )}
+                      <div className="flex justify-end gap-2">
+                        <Button
+                          variant="ghost"
+                          size="sm"
+                          disabled={isAddingRule}
+                          onClick={resetAddRuleForm}
+                        >
+                          Avbryt
+                        </Button>
+                        <Button
+                          size="sm"
+                          disabled={
+                            isAddingRule ||
+                            !newRuleDim ||
+                            (newRuleNeedsValue && !newRuleValueCode)
+                          }
+                          onClick={handleAddRule}
+                        >
+                          {isAddingRule && (
+                            <Loader2 className="mr-2 h-4 w-4 animate-spin" />
+                          )}
+                          Lägg till
+                        </Button>
+                      </div>
+                    </div>
+                  ) : (
+                    rules.length > 0 && (
+                      <Button
+                        variant="ghost"
+                        size="sm"
+                        onClick={() => setAddRuleOpen(true)}
+                      >
+                        <Plus className="mr-1 h-3.5 w-3.5" />
+                        Lägg till regel
+                      </Button>
+                    )
+                  )}
+                </>
+              )}
+            </div>
+          )}
+
           <div className="flex items-center justify-between rounded-lg border p-3">
             <div>
               <p className="text-sm font-medium">Aktivt konto</p>
diff --git a/components/dimensions/DimensionsManager.tsx b/components/dimensions/DimensionsManager.tsx
index 638604a3..e98c1d1d 100644
--- a/components/dimensions/DimensionsManager.tsx
+++ b/components/dimensions/DimensionsManager.tsx
@@ -6,7 +6,9 @@ import { Button } from '@/components/ui/button'
 import { Card, CardContent, CardHeader } from '@/components/ui/card'
 import { Badge } from '@/components/ui/badge'
 import { Input } from '@/components/ui/input'
+import { Label } from '@/components/ui/label'
 import { Skeleton } from '@/components/ui/skeleton'
+import { Switch } from '@/components/ui/switch'
 import { Tabs, TabsList, TabsTrigger } from '@/components/ui/tabs'
 import {
   Dialog,
@@ -14,6 +16,13 @@ import {
   DialogHeader,
   DialogTitle,
 } from '@/components/ui/dialog'
+import {
+  Select,
+  SelectContent,
+  SelectItem,
+  SelectTrigger,
+  SelectValue,
+} from '@/components/ui/select'
 import {
   Table,
   TableBody,
@@ -34,7 +43,9 @@ import {
   Tags,
   ChevronUp,
   ChevronDown,
+  ChevronRight,
   ChevronsUpDown,
+  Loader2,
 } from 'lucide-react'
 import DimensionValueForm, {
   type DimensionValueFormInput,
@@ -81,6 +92,8 @@ export default function DimensionsManager() {
   const [sortDir, setSortDir] = useState<SortDir>('asc')
   const [dialog, setDialog] = useState<DialogState>(null)
   const [isSaving, setIsSaving] = useState(false)
+  const [newDimDialogOpen, setNewDimDialogOpen] = useState(false)
+  const [isCreatingDimension, setIsCreatingDimension] = useState(false)
 
   const loadDimensions = useCallback(
     async (showSpinner: boolean) => {
@@ -120,6 +133,15 @@ export default function DimensionsManager() {
   )
   const isProjectTab = activeDim?.sie_dim_no === PROJECT_DIM_NO
 
+  // Parent (#UNDERDIM) of the active tab's dimension, when it has one.
+  const parentDimName = useMemo(() => {
+    if (!activeDim || activeDim.parent_sie_dim_no == null) return null
+    const parent = dimensions.find(
+      (d) => d.sie_dim_no === activeDim.parent_sie_dim_no,
+    )
+    return parent?.name ?? `#${activeDim.parent_sie_dim_no}`
+  }, [dimensions, activeDim])
+
   const filteredValues = useMemo(() => {
     if (!activeDim) return []
     const term = searchTerm.trim().toLowerCase()
@@ -224,6 +246,45 @@ export default function DimensionsManager() {
     }
   }
 
+  async function handleCreateDimension(input: NewDimensionFormInput) {
+    setIsCreatingDimension(true)
+    try {
+      // Contract: sie_dim_no omitted → the server picks the next free ≥20;
+      // parent_sie_dim_no omitted → no #UNDERDIM hierarchy.
+      const body: Record<string, unknown> = {
+        name: input.name,
+        resets_annually: input.resets_annually,
+      }
+      if (input.sie_dim_no !== null) body.sie_dim_no = input.sie_dim_no
+      if (input.parent_sie_dim_no !== null) {
+        body.parent_sie_dim_no = input.parent_sie_dim_no
+      }
+      const res = await fetch('/api/dimensions', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify(body),
+      })
+      const json = await res.json().catch(() => null)
+      if (!res.ok) throw json ?? new Error()
+      const createdId = (json?.data?.dimension as { id?: string } | undefined)?.id
+      toast({ title: t('dim_created_title') })
+      setNewDimDialogOpen(false)
+      await loadDimensions(false)
+      if (createdId) {
+        setActiveDimId(createdId)
+        setSearchTerm('')
+      }
+    } catch (err) {
+      toast({
+        title: t('save_failed_title'),
+        description: getErrorMessage(err, { locale: errorLocale }),
+        variant: 'destructive',
+      })
+    } finally {
+      setIsCreatingDimension(false)
+    }
+  }
+
   async function handleDeleteValue() {
     if (!activeDim || dialog?.mode !== 'edit') return
     setIsSaving(true)
@@ -318,31 +379,49 @@ export default function DimensionsManager() {
 
   return (
     <div className="space-y-8">
-      {/* Segmented tabs — one per registry dimension (1 Kostnadsställe, 6 Projekt) */}
-      <div className="flex flex-col gap-4 sm:flex-row sm:items-center sm:justify-between">
-        <Tabs
-          value={activeDimId ?? undefined}
-          onValueChange={(id) => {
-            setActiveDimId(id)
-            setSearchTerm('')
-          }}
-        >
-          <TabsList>
-            {dimensions.map((dim) => (
-              <TabsTrigger key={dim.id} value={dim.id}>
-                {dim.name}
-              </TabsTrigger>
-            ))}
-          </TabsList>
-        </Tabs>
-        <Button
-          disabled={!canWrite}
-          title={!canWrite ? t('viewer_disabled_tooltip') : undefined}
-          onClick={() => setDialog({ mode: 'create' })}
-        >
-          {canWrite ? <Plus className="mr-2 h-4 w-4" /> : <Lock className="mr-2 h-4 w-4" />}
-          {t('new_value')}
-        </Button>
+      {/* Segmented tabs — one per registry dimension (1 Kostnadsställe,
+          6 Projekt, plus any custom 20+ dims) */}
+      <div className="space-y-2">
+        <div className="flex flex-col gap-4 sm:flex-row sm:items-center sm:justify-between">
+          <Tabs
+            value={activeDimId ?? undefined}
+            onValueChange={(id) => {
+              setActiveDimId(id)
+              setSearchTerm('')
+            }}
+          >
+            <TabsList>
+              {dimensions.map((dim) => (
+                <TabsTrigger key={dim.id} value={dim.id}>
+                  {dim.name}
+                </TabsTrigger>
+              ))}
+            </TabsList>
+          </Tabs>
+          <div className="flex items-center gap-3">
+            <Button
+              variant="outline"
+              disabled={!canWrite}
+              title={!canWrite ? t('viewer_disabled_tooltip') : undefined}
+              onClick={() => setNewDimDialogOpen(true)}
+            >
+              {t('new_dimension')}
+            </Button>
+            <Button
+              disabled={!canWrite}
+              title={!canWrite ? t('viewer_disabled_tooltip') : undefined}
+              onClick={() => setDialog({ mode: 'create' })}
+            >
+              {canWrite ? <Plus className="mr-2 h-4 w-4" /> : <Lock className="mr-2 h-4 w-4" />}
+              {t('new_value')}
+            </Button>
+          </div>
+        </div>
+        {parentDimName && (
+          <p className="text-xs text-muted-foreground">
+            {t('subdimension_of', { parent: parentDimName })}
+          </p>
+        )}
       </div>
 
       {/* Search */}
@@ -479,6 +558,181 @@ export default function DimensionsManager() {
           )}
         </DialogContent>
       </Dialog>
+
+      {/* New dimension dialog — the content (and thus the form state)
+          unmounts on close, so each open starts from a blank form. */}
+      <Dialog
+        open={newDimDialogOpen}
+        onOpenChange={(open) => !open && setNewDimDialogOpen(false)}
+      >
+        <DialogContent className="sm:max-w-lg max-h-[95dvh] sm:max-h-[90vh] overflow-y-auto">
+          <DialogHeader>
+            <DialogTitle>{t('new_dimension_title')}</DialogTitle>
+          </DialogHeader>
+          <NewDimensionForm
+            dimensions={dimensions}
+            isSaving={isCreatingDimension}
+            onSubmit={handleCreateDimension}
+          />
+        </DialogContent>
+      </Dialog>
     </div>
   )
 }
+
+interface NewDimensionFormInput {
+  name: string
+  /** null → omit from the POST; the server picks the next free number ≥20. */
+  sie_dim_no: number | null
+  resets_annually: boolean
+  /** null → omit from the POST; no #UNDERDIM hierarchy. */
+  parent_sie_dim_no: number | null
+}
+
+/**
+ * Create form for a custom registry dimension (#DIM 20+). The parent
+ * (#UNDERDIM) select is an advanced, rarely-used SIE concept — kept behind a
+ * quiet disclosure so the common path stays name + number.
+ */
+function NewDimensionForm({
+  dimensions,
+  isSaving,
+  onSubmit,
+}: {
+  /** Existing dims — active ones populate the parent select. */
+  dimensions: DimensionDto[]
+  isSaving: boolean
+  onSubmit: (input: NewDimensionFormInput) => void | Promise<void>
+}) {
+  const t = useTranslations('dimensions')
+  const [name, setName] = useState('')
+  const [numberStr, setNumberStr] = useState('')
+  const [resetsAnnually, setResetsAnnually] = useState(true)
+  const [showAdvanced, setShowAdvanced] = useState(false)
+  const [parent, setParent] = useState('none')
+  const [nameError, setNameError] = useState<string | null>(null)
+  const [numberError, setNumberError] = useState<string | null>(null)
+
+  const parentOptions = dimensions.filter((d) => d.is_active)
+
+  function handleSubmit(e: React.FormEvent<HTMLFormElement>) {
+    e.preventDefault()
+    const trimmedName = name.trim()
+    const trimmedNumber = numberStr.trim()
+    let valid = true
+    if (!trimmedName || trimmedName.length > 60) {
+      setNameError(t('dim_form_name_invalid'))
+      valid = false
+    } else {
+      setNameError(null)
+    }
+    let sieDimNo: number | null = null
+    if (trimmedNumber) {
+      const parsed = Number(trimmedNumber)
+      if (!Number.isInteger(parsed) || parsed < 20) {
+        setNumberError(t('dim_form_number_invalid'))
+        valid = false
+      } else {
+        setNumberError(null)
+        sieDimNo = parsed
+      }
+    } else {
+      setNumberError(null)
+    }
+    if (!valid) return
+    void onSubmit({
+      name: trimmedName,
+      sie_dim_no: sieDimNo,
+      resets_annually: resetsAnnually,
+      parent_sie_dim_no: parent === 'none' ? null : Number(parent),
+    })
+  }
+
+  return (
+    <form onSubmit={handleSubmit} className="space-y-6">
+      <div className="grid gap-4 sm:grid-cols-2">
+        <div className="space-y-2">
+          <Label htmlFor="new-dimension-name">{t('form_name_label')}</Label>
+          <Input
+            id="new-dimension-name"
+            value={name}
+            onChange={(e) => setName(e.target.value)}
+            autoComplete="off"
+            autoFocus
+          />
+          {nameError && <p className="text-xs text-destructive">{nameError}</p>}
+        </div>
+        <div className="space-y-2">
+          <Label htmlFor="new-dimension-number">{t('dim_form_number_label')}</Label>
+          <Input
+            id="new-dimension-number"
+            type="number"
+            inputMode="numeric"
+            min={20}
+            step={1}
+            value={numberStr}
+            onChange={(e) => setNumberStr(e.target.value)}
+            className="tabular-nums"
+          />
+          <p className="text-xs text-muted-foreground">{t('dim_form_number_help')}</p>
+          {numberError && <p className="text-xs text-destructive">{numberError}</p>}
+        </div>
+      </div>
+
+      <div className="flex items-start justify-between gap-4">
+        <div className="space-y-1">
+          <Label htmlFor="new-dimension-resets">{t('dim_form_resets_label')}</Label>
+          <p className="text-xs text-muted-foreground max-w-md">
+            {t('dim_form_resets_help')}
+          </p>
+        </div>
+        <Switch
+          id="new-dimension-resets"
+          checked={resetsAnnually}
+          onCheckedChange={setResetsAnnually}
+        />
+      </div>
+
+      <div className="space-y-3">
+        <button
+          type="button"
+          onClick={() => setShowAdvanced((prev) => !prev)}
+          className="inline-flex items-center gap-1 text-xs font-medium text-muted-foreground hover:text-foreground transition-colors"
+          aria-expanded={showAdvanced}
+        >
+          <ChevronRight
+            className={`h-3 w-3 transition-transform ${showAdvanced ? 'rotate-90' : ''}`}
+            aria-hidden="true"
+          />
+          {t('dim_form_advanced')}
+        </button>
+        {showAdvanced && (
+          <div className="space-y-2">
+            <Label>{t('dim_form_parent_label')}</Label>
+            <Select value={parent} onValueChange={setParent}>
+              <SelectTrigger>
+                <SelectValue />
+              </SelectTrigger>
+              <SelectContent>
+                <SelectItem value="none">{t('dim_form_parent_none')}</SelectItem>
+                {parentOptions.map((dim) => (
+                  <SelectItem key={dim.id} value={String(dim.sie_dim_no)}>
+                    {dim.name} ({dim.sie_dim_no})
+                  </SelectItem>
+                ))}
+              </SelectContent>
+            </Select>
+            <p className="text-xs text-muted-foreground">{t('dim_form_parent_help')}</p>
+          </div>
+        )}
+      </div>
+
+      <div className="flex justify-end pt-2">
+        <Button type="submit" disabled={isSaving}>
+          {isSaving && <Loader2 className="mr-2 h-4 w-4 animate-spin" />}
+          {t('form_create')}
+        </Button>
+      </div>
+    </form>
+  )
+}
diff --git a/components/dimensions/LineDimensionFields.tsx b/components/dimensions/LineDimensionFields.tsx
index 9e0b697f..20ef6d43 100644
--- a/components/dimensions/LineDimensionFields.tsx
+++ b/components/dimensions/LineDimensionFields.tsx
@@ -1,7 +1,12 @@
 'use client'
 
+import { useEffect, useMemo, useState } from 'react'
 import { Label } from '@/components/ui/label'
 import DimensionCombobox from '@/components/dimensions/DimensionCombobox'
+import {
+  fetchDimensionsCached,
+  type DimensionDto,
+} from '@/components/dimensions/types'
 
 interface LineDimensionFieldsProps {
   /** Current dimensions map ({sie_dim_no: object_code}) — a line's map or the header default. */
@@ -16,11 +21,23 @@ interface LineDimensionFieldsProps {
 }
 
 /**
- * The Kostnadsställe + Projekt combobox pair (SIE dims 1/6) used by the
- * voucher form's header default, the per-row tag popover, and the mobile line
- * cards. Labels are the seeded system-dimension names and hardcoded Swedish —
- * the component mounts on the voucher editor, a stays-Swedish surface per
- * .claude/rules/i18n.md (same convention as DimensionCombobox).
+ * While the registry loads (or if the fetch fails) we render the seeded
+ * system pair (SIE dims 1/6) so the tagging affordance never disappears —
+ * the registry always contains at least these two.
+ */
+const FALLBACK_FIELDS: { sieDimNo: string; label: string }[] = [
+  { sieDimNo: '1', label: 'Kostnadsställe' },
+  { sieDimNo: '6', label: 'Projekt' },
+]
+
+/**
+ * Registry-driven dimension comboboxes used by the voucher form's header
+ * default, the per-row tag popover, and the mobile line cards. One combobox
+ * per active registry dimension, ordered by sort_order then sie_dim_no (the
+ * seeded 1/6 pair sorts first). Labels are the registry dimension names and
+ * hardcoded-Swedish fallbacks — the component mounts on the voucher editor,
+ * a stays-Swedish surface per .claude/rules/i18n.md (same convention as
+ * DimensionCombobox).
  */
 export default function LineDimensionFields({
   dimensions,
@@ -29,32 +46,46 @@ export default function LineDimensionFields({
   stacked,
   inputClassName,
 }: LineDimensionFieldsProps) {
+  const [registry, setRegistry] = useState<DimensionDto[] | null>(null)
+
+  useEffect(() => {
+    let cancelled = false
+    fetchDimensionsCached()
+      .then((dims) => {
+        if (!cancelled) setRegistry(dims)
+      })
+      .catch(() => {
+        /* keep the hardcoded 1/6 fallback */
+      })
+    return () => {
+      cancelled = true
+    }
+  }, [])
+
+  const fields = useMemo(() => {
+    const active = registry?.filter((d) => d.is_active) ?? []
+    if (active.length === 0) return FALLBACK_FIELDS
+    return [...active]
+      .sort((a, b) => a.sort_order - b.sort_order || a.sie_dim_no - b.sie_dim_no)
+      .map((d) => ({ sieDimNo: String(d.sie_dim_no), label: d.name }))
+  }, [registry])
+
   return (
     <div className={stacked ? 'space-y-3' : 'grid grid-cols-2 gap-3'}>
-      <div>
-        <Label className="text-xs text-muted-foreground">Kostnadsställe</Label>
-        <div className="mt-1">
-          <DimensionCombobox
-            sieDimNo="1"
-            value={dimensions?.['1'] ?? null}
-            onChange={(code) => onChange('1', code)}
-            disabled={disabled}
-            className={inputClassName}
-          />
+      {fields.map((field) => (
+        <div key={field.sieDimNo}>
+          <Label className="text-xs text-muted-foreground">{field.label}</Label>
+          <div className="mt-1">
+            <DimensionCombobox
+              sieDimNo={field.sieDimNo}
+              value={dimensions?.[field.sieDimNo] ?? null}
+              onChange={(code) => onChange(field.sieDimNo, code)}
+              disabled={disabled}
+              className={inputClassName}
+            />
+          </div>
         </div>
-      </div>
-      <div>
-        <Label className="text-xs text-muted-foreground">Projekt</Label>
-        <div className="mt-1">
-          <DimensionCombobox
-            sieDimNo="6"
-            value={dimensions?.['6'] ?? null}
-            onChange={(code) => onChange('6', code)}
-            disabled={disabled}
-            className={inputClassName}
-          />
-        </div>
-      </div>
+      ))}
     </div>
   )
 }
diff --git a/components/dimensions/types.ts b/components/dimensions/types.ts
index 72ed43e5..51f203f6 100644
--- a/components/dimensions/types.ts
+++ b/components/dimensions/types.ts
@@ -19,17 +19,38 @@ export interface DimensionValueDto {
 
 export interface DimensionDto {
   id: string
-  /** SIE #DIM number (1 = Kostnadsställe, 6 = Projekt). */
+  /** SIE #DIM number (1 = Kostnadsställe, 6 = Projekt, 20+ = custom). */
   sie_dim_no: number
   name: string
   resets_annually: boolean
   is_system: boolean
   is_active: boolean
   sort_order: number
+  /** SIE #UNDERDIM parent — the sie_dim_no of the parent dimension, or null. */
+  parent_sie_dim_no: number | null
   /** Sorted by code by the API. */
   values: DimensionValueDto[]
 }
 
+export type DimensionRuleType = 'required' | 'default' | 'fixed'
+
+/**
+ * Per-account dimension rule as served by GET /api/dimensions/rules —
+ * a flattened join row (rule + dimension + optional pinned value).
+ */
+export interface AccountDimensionRuleDto {
+  account_dimension_rule_id: string
+  account_number: string
+  dimension_id: string
+  sie_dim_no: number
+  dimension_name: string
+  rule_type: DimensionRuleType
+  value_id: string | null
+  value_code: string | null
+  value_name: string | null
+  is_active: boolean
+}
+
 /** SIE dimension number whose values carry start/end dates (Projekt). */
 export const PROJECT_DIM_NO = 6
 
@@ -54,3 +75,22 @@ export async function fetchDimensions(): Promise<DimensionDto[]> {
   }
   return (json?.dimensions ?? []) as DimensionDto[]
 }
+
+let cachedDimensionsPromise: Promise<DimensionDto[]> | null = null
+
+/**
+ * Module-level cached variant of fetchDimensions for high-mount-count
+ * consumers (one registry fetch per page load instead of one per line
+ * picker). A failed fetch clears the cache so the next mount retries.
+ * Registry mutations are rare enough that staleness within a page visit
+ * is acceptable — the register UI uses the uncached fetch.
+ */
+export function fetchDimensionsCached(): Promise<DimensionDto[]> {
+  if (!cachedDimensionsPromise) {
+    cachedDimensionsPromise = fetchDimensions().catch((err) => {
+      cachedDimensionsPromise = null
+      throw err
+    })
+  }
+  return cachedDimensionsPromise
+}
diff --git a/extensions/general/mcp-server/__tests__/agent-briefing.test.ts b/extensions/general/mcp-server/__tests__/agent-briefing.test.ts
index 503b874c..b492c558 100644
--- a/extensions/general/mcp-server/__tests__/agent-briefing.test.ts
+++ b/extensions/general/mcp-server/__tests__/agent-briefing.test.ts
@@ -53,6 +53,7 @@ function mockSupabase(opts: {
   // Dimension registry (dimensions PR3). Default: empty → block omitted.
   dimensionRows?: Array<{ id: string; sie_dim_no: number; name: string }>
   dimensionValueRows?: Array<{ dimension_id: string; code: string; name: string }>
+  dimensionRuleRows?: Array<{ account_number: string; rule_type: string; dimension_id: string }>
   dimensionsEnabled?: boolean
   errors?: { profile?: string; memory?: string; atoms?: string }
 }) {
@@ -164,6 +165,10 @@ function mockSupabase(opts: {
       if (table === 'dimension_values') {
         return chainResolving(opts.dimensionValueRows ?? [])
       }
+      if (table === 'account_dimension_rules') {
+        // PR10: the briefing surfaces active rules; default none.
+        return chainResolving(opts.dimensionRuleRows ?? [])
+      }
       throw new Error(`Unexpected table in test mock: ${table}`)
     }),
   }
diff --git a/extensions/general/mcp-server/server.ts b/extensions/general/mcp-server/server.ts
index 17563f88..4af1089c 100644
--- a/extensions/general/mcp-server/server.ts
+++ b/extensions/general/mcp-server/server.ts
@@ -2253,6 +2253,16 @@ export const tools: McpTool[] = [
                   sie_dim_no: { type: 'number' },
                   name: { type: 'string' },
                   active_value_count: { type: 'number' },
+                  required_on_accounts: {
+                    type: 'array',
+                    description: 'BAS accounts with an active required-rule: postings there are refused without a value for this dimension.',
+                    items: { type: 'string' },
+                  },
+                  default_on_accounts: {
+                    type: 'array',
+                    description: 'BAS accounts where a default/fixed rule auto-applies a value at draft creation.',
+                    items: { type: 'string' },
+                  },
                   top_values: {
                     type: 'array',
                     description: 'Up to 10 active values; full list via gnubok_list_dimension_values.',
@@ -2267,7 +2277,7 @@ export const tools: McpTool[] = [
                     },
                   },
                 },
-                required: ['sie_dim_no', 'name', 'active_value_count', 'top_values'],
+                required: ['sie_dim_no', 'name', 'active_value_count', 'required_on_accounts', 'default_on_accounts', 'top_values'],
               },
             },
           },
@@ -2402,6 +2412,8 @@ export const tools: McpTool[] = [
               sie_dim_no: number
               name: string
               active_value_count: number
+              required_on_accounts: string[]
+              default_on_accounts: string[]
               top_values: Array<{ code: string; name: string }>
             }>
           }
@@ -2421,6 +2433,25 @@ export const tools: McpTool[] = [
               bucket.push({ code: v.code, name: v.name })
               byDimension.set(v.dimension_id, bucket)
             }
+            // Account dimension rules (PR10): tell the agent up front which
+            // accounts refuse postings without a value (required) and which
+            // auto-apply one (default/fixed) — so gnubok_create_voucher calls
+            // self-correct instead of bouncing off MANDATORY_DIMENSION_MISSING.
+            const requiredByDimension = new Map<string, string[]>()
+            const defaultByDimension = new Map<string, string[]>()
+            const { data: ruleRows, error: ruleErr } = await supabase
+              .from('account_dimension_rules')
+              .select('account_number, rule_type, dimension_id')
+              .eq('company_id', companyId)
+              .eq('is_active', true)
+            if (!ruleErr) {
+              for (const r of (ruleRows ?? []) as Array<{ account_number: string; rule_type: string; dimension_id: string }>) {
+                const target = r.rule_type === 'required' ? requiredByDimension : defaultByDimension
+                const bucket = target.get(r.dimension_id) ?? []
+                bucket.push(r.account_number)
+                target.set(r.dimension_id, bucket)
+              }
+            }
             dimensionsBlock = {
               enabled: settingsRow?.dimensions_enabled === true,
               dimensions: dimensionRows.map((d) => {
@@ -2429,6 +2460,8 @@ export const tools: McpTool[] = [
                   sie_dim_no: d.sie_dim_no,
                   name: d.name,
                   active_value_count: values.length,
+                  required_on_accounts: (requiredByDimension.get(d.id) ?? []).sort(),
+                  default_on_accounts: (defaultByDimension.get(d.id) ?? []).sort(),
                   top_values: values.slice(0, 10),
                 }
               }),
diff --git a/lib/api/schemas.ts b/lib/api/schemas.ts
index f693cee9..cf008075 100644
--- a/lib/api/schemas.ts
+++ b/lib/api/schemas.ts
@@ -841,6 +841,62 @@ export const CreateDimensionValueSchema = z
  * open period, lock date, active registry values); this schema only shapes
  * the request. An empty bag {} untags the line.
  */
+/**
+ * POST /api/dimensions — create a custom dimension (dimensions PR10).
+ * sie_dim_no omitted → server picks the next free number >= 20 (SIE leaves
+ * 20+ unreserved). parent_sie_dim_no declares an #UNDERDIM hierarchy.
+ */
+export const CreateDimensionSchema = z.object({
+  name: z.string().trim().min(1).max(60),
+  sie_dim_no: z.coerce.number().int().min(1).max(9999).optional(),
+  resets_annually: z.boolean().optional(),
+  parent_sie_dim_no: z.coerce.number().int().min(1).max(9999).nullable().optional(),
+})
+
+const AccountDimensionRuleTypeSchema = z.enum(['required', 'default', 'fixed'])
+
+/** GET /api/dimensions/rules query — optional exact-account filter. */
+export const ListDimensionRulesQuerySchema = z.object({
+  account_number: accountNumber.optional(),
+})
+
+/**
+ * POST /api/dimensions/rules — per-account dimension policy (dimensions
+ * PR10). 'required' carries no value; 'default'/'fixed' must carry the value
+ * to apply. One rule per (account, dimension) — enforced by the DB UNIQUE.
+ */
+export const CreateAccountDimensionRuleSchema = z
+  .object({
+    account_number: accountNumber,
+    dimension_id: uuid,
+    rule_type: AccountDimensionRuleTypeSchema,
+    value_id: uuid.optional(),
+    is_active: z.boolean().optional(),
+  })
+  .superRefine((rule, ctx) => {
+    if (rule.rule_type === 'required' && rule.value_id) {
+      ctx.addIssue({
+        code: z.ZodIssueCode.custom,
+        path: ['value_id'],
+        message: 'En obligatorisk regel har inget värde — värden hör till Förval/Låst.',
+      })
+    }
+    if (rule.rule_type !== 'required' && !rule.value_id) {
+      ctx.addIssue({
+        code: z.ZodIssueCode.custom,
+        path: ['value_id'],
+        message: 'Välj vilket värde regeln ska använda.',
+      })
+    }
+  })
+
+/** PATCH /api/dimensions/rules/[id] — the value-presence rule re-checks in the route (partial update). */
+export const UpdateAccountDimensionRuleSchema = z.object({
+  rule_type: AccountDimensionRuleTypeSchema.optional(),
+  value_id: uuid.nullable().optional(),
+  is_active: z.boolean().optional(),
+})
+
 export const RetagLineDimensionsSchema = z.object({
   // {} passes (no entries to validate) = UNTAG. Intentional divergence from
   // the MCP staged path (RetagLineDimensionsParamsSchema), which rejects an
diff --git a/lib/bookkeeping/__tests__/dimension-rules.test.ts b/lib/bookkeeping/__tests__/dimension-rules.test.ts
new file mode 100644
index 00000000..a0ed740c
--- /dev/null
+++ b/lib/bookkeeping/__tests__/dimension-rules.test.ts
@@ -0,0 +1,255 @@
+/**
+ * Pure-function tests for the account dimension rule layer (dimensions PR10).
+ *
+ * applyDimensionRules: 'default' fills only absent bag keys, 'fixed' always
+ * overwrites (including alias-sourced values, with the deprecated aliases
+ * cleared on changed lines), and the zero-effect paths preserve array/line
+ * identity so the common rule-less booking allocates nothing.
+ *
+ * assertMandatoryDimensions: throws MandatoryDimensionMissingError with one
+ * violation per (account, dimension) regardless of line count, treats
+ * alias-sourced values as satisfying (normalize folds them into the bag),
+ * and no-ops when no 'required' rule exists.
+ */
+import { describe, it, expect } from 'vitest'
+import {
+  applyDimensionRules,
+  assertMandatoryDimensions,
+  type AccountDimensionRule,
+} from '../dimension-rules'
+import {
+  MANDATORY_DIMENSION_MISSING,
+  MandatoryDimensionMissingError,
+} from '../dimension-errors'
+
+interface TestLine {
+  account_number: string
+  dimensions?: Record<string, string> | null
+  cost_center?: string | null
+  project?: string | null
+}
+
+function makeRule(overrides: Partial<AccountDimensionRule> = {}): AccountDimensionRule {
+  return {
+    account_number: '4010',
+    rule_type: 'default',
+    sie_dim_no: '6',
+    dimension_name: 'Projekt',
+    value_code: 'P001',
+    ...overrides,
+  }
+}
+
+describe('applyDimensionRules', () => {
+  it('default fills only absent keys — caller-set keys win', () => {
+    const lines: TestLine[] = [
+      { account_number: '4010', dimensions: { '6': 'CALLER' } },
+    ]
+    const rules = [
+      makeRule({ rule_type: 'default', sie_dim_no: '6', value_code: 'PDEF' }),
+      makeRule({
+        rule_type: 'default',
+        sie_dim_no: '1',
+        dimension_name: 'Kostnadsställe',
+        value_code: 'KS01',
+      }),
+    ]
+
+    const out = applyDimensionRules(lines, rules)
+
+    // Absent key '1' filled; present key '6' untouched.
+    expect(out[0].dimensions).toEqual({ '6': 'CALLER', '1': 'KS01' })
+    // The input line object was not mutated.
+    expect(lines[0].dimensions).toEqual({ '6': 'CALLER' })
+  })
+
+  it('default does not override an alias-sourced value (line identity preserved)', () => {
+    const lines: TestLine[] = [{ account_number: '4010', cost_center: 'KS-ALIAS' }]
+    const rules = [
+      makeRule({
+        rule_type: 'default',
+        sie_dim_no: '1',
+        dimension_name: 'Kostnadsställe',
+        value_code: 'KS99',
+      }),
+    ]
+
+    // normalize folds cost_center into key '1', so the default has nothing to
+    // fill — nothing applies and the SAME array comes back.
+    expect(applyDimensionRules(lines, rules)).toBe(lines)
+    expect(lines[0].cost_center).toBe('KS-ALIAS')
+  })
+
+  it('fixed overwrites an alias-sourced value and clears the aliases', () => {
+    const lines: TestLine[] = [{ account_number: '4010', cost_center: 'OLD' }]
+    const rules = [
+      makeRule({
+        rule_type: 'fixed',
+        sie_dim_no: '1',
+        dimension_name: 'Kostnadsställe',
+        value_code: 'KS99',
+      }),
+    ]
+
+    const out = applyDimensionRules(lines, rules)
+
+    expect(out[0].dimensions).toEqual({ '1': 'KS99' })
+    // Aliases nulled so downstream normalization cannot resurrect 'OLD'.
+    expect(out[0].cost_center).toBeNull()
+    expect(out[0].project).toBeNull()
+  })
+
+  it('fixed overwrites a caller-supplied bag value', () => {
+    const lines: TestLine[] = [{ account_number: '4010', dimensions: { '6': 'CALLER' } }]
+    const rules = [makeRule({ rule_type: 'fixed', sie_dim_no: '6', value_code: 'PLOCK' })]
+
+    const out = applyDimensionRules(lines, rules)
+
+    expect(out[0].dimensions).toEqual({ '6': 'PLOCK' })
+  })
+
+  it('a fixed rule already satisfied is a no-op — same array identity', () => {
+    const lines: TestLine[] = [{ account_number: '4010', dimensions: { '6': 'P001' } }]
+    const rules = [makeRule({ rule_type: 'fixed', sie_dim_no: '6', value_code: 'P001' })]
+
+    expect(applyDimensionRules(lines, rules)).toBe(lines)
+  })
+
+  it('untouched lines keep identity while changed lines are copied', () => {
+    const lines: TestLine[] = [
+      { account_number: '4010' },
+      { account_number: '1930', dimensions: { '1': 'KS01' } },
+    ]
+    const rules = [makeRule({ rule_type: 'fixed', sie_dim_no: '6', value_code: 'P001' })]
+
+    const out = applyDimensionRules(lines, rules)
+
+    expect(out).not.toBe(lines)
+    expect(out[0]).not.toBe(lines[0])
+    expect(out[0].dimensions).toEqual({ '6': 'P001' })
+    // The 1930 line has no rule — the exact same object rides through.
+    expect(out[1]).toBe(lines[1])
+  })
+
+  it('returns the same array for zero rules and for required-only rules', () => {
+    const lines: TestLine[] = [{ account_number: '4010' }]
+
+    expect(applyDimensionRules(lines, [])).toBe(lines)
+    // 'required' rules carry no value — they never apply at draft time.
+    expect(
+      applyDimensionRules(lines, [
+        makeRule({ rule_type: 'required', value_code: null }),
+      ]),
+    ).toBe(lines)
+  })
+
+  it('rules for other accounts do not leak onto unrelated lines', () => {
+    const lines: TestLine[] = [{ account_number: '4010' }]
+    const rules = [
+      makeRule({ account_number: '5010', rule_type: 'fixed', value_code: 'P001' }),
+      makeRule({ account_number: '5010', rule_type: 'default', value_code: 'P002' }),
+    ]
+
+    expect(applyDimensionRules(lines, rules)).toBe(lines)
+    expect(lines[0].dimensions).toBeUndefined()
+  })
+})
+
+describe('assertMandatoryDimensions', () => {
+  const requiredProjekt = makeRule({ rule_type: 'required', value_code: null })
+
+  it('throws with one deduped violation across multiple missing lines', () => {
+    const lines: TestLine[] = [
+      { account_number: '4010', dimensions: {} },
+      { account_number: '4010' },
+      { account_number: '1930' },
+    ]
+
+    let caught: unknown
+    try {
+      assertMandatoryDimensions(lines, [requiredProjekt])
+    } catch (err) {
+      caught = err
+    }
+
+    expect(caught).toBeInstanceOf(MandatoryDimensionMissingError)
+    const error = caught as MandatoryDimensionMissingError
+    expect(error.code).toBe(MANDATORY_DIMENSION_MISSING)
+    // Two 4010 lines miss the same rule → ONE violation, not two.
+    expect(error.violations).toEqual([
+      { account_number: '4010', sie_dim_no: '6', dimension_name: 'Projekt' },
+    ])
+  })
+
+  it('reports one violation per (account, dimension) pair', () => {
+    const lines: TestLine[] = [
+      { account_number: '4010' },
+      { account_number: '5010' },
+    ]
+    const rules = [
+      requiredProjekt,
+      makeRule({
+        account_number: '5010',
+        rule_type: 'required',
+        sie_dim_no: '1',
+        dimension_name: 'Kostnadsställe',
+        value_code: null,
+      }),
+    ]
+
+    let caught: unknown
+    try {
+      assertMandatoryDimensions(lines, rules)
+    } catch (err) {
+      caught = err
+    }
+
+    const error = caught as MandatoryDimensionMissingError
+    expect(error.violations).toEqual([
+      { account_number: '4010', sie_dim_no: '6', dimension_name: 'Projekt' },
+      { account_number: '5010', sie_dim_no: '1', dimension_name: 'Kostnadsställe' },
+    ])
+  })
+
+  it('uses the Swedish message format naming account and dimension', () => {
+    expect(() =>
+      assertMandatoryDimensions([{ account_number: '4010' }], [requiredProjekt]),
+    ).toThrow('Konto 4010 kräver Projekt — välj ett värde innan bokföring.')
+  })
+
+  it('is satisfied via the deprecated cost_center alias through normalize', () => {
+    const requiredKostnadsstalle = makeRule({
+      rule_type: 'required',
+      sie_dim_no: '1',
+      dimension_name: 'Kostnadsställe',
+      value_code: null,
+    })
+    const lines: TestLine[] = [{ account_number: '4010', cost_center: 'KS01' }]
+
+    expect(() => assertMandatoryDimensions(lines, [requiredKostnadsstalle])).not.toThrow()
+  })
+
+  it('is satisfied by a bag value on the required key', () => {
+    const lines: TestLine[] = [{ account_number: '4010', dimensions: { '6': 'P001' } }]
+
+    expect(() => assertMandatoryDimensions(lines, [requiredProjekt])).not.toThrow()
+  })
+
+  it('never throws when no required rule exists (default/fixed only)', () => {
+    const lines: TestLine[] = [{ account_number: '4010' }]
+    const rules = [
+      makeRule({ rule_type: 'default' }),
+      makeRule({ rule_type: 'fixed', sie_dim_no: '1', value_code: 'KS01' }),
+    ]
+
+    expect(() => assertMandatoryDimensions(lines, rules)).not.toThrow()
+    expect(() => assertMandatoryDimensions(lines, [])).not.toThrow()
+  })
+
+  it('required rules on other accounts do not fire', () => {
+    const lines: TestLine[] = [{ account_number: '4010' }]
+    const rules = [makeRule({ account_number: '5010', rule_type: 'required', value_code: null })]
+
+    expect(() => assertMandatoryDimensions(lines, rules)).not.toThrow()
+  })
+})
diff --git a/lib/bookkeeping/__tests__/engine-dimension-validation.test.ts b/lib/bookkeeping/__tests__/engine-dimension-validation.test.ts
index 31573bfb..98175b34 100644
--- a/lib/bookkeeping/__tests__/engine-dimension-validation.test.ts
+++ b/lib/bookkeeping/__tests__/engine-dimension-validation.test.ts
@@ -1,14 +1,20 @@
 /**
- * Engine wiring of validateEntryDimensions (dimensions plan PR3).
+ * Engine wiring of validateEntryDimensions (dimensions plan PR3) and of the
+ * account dimension rules (dimensions PR10).
  *
  * createDraftEntry and updateDraftEntry must run the soft dimension
  * validation AFTER balance validation and BEFORE any insert/update, so a
  * rejection leaves no orphan rows. Untagged entries must not even fetch
  * company_settings; companies without the toggle keep free-text passthrough.
+ *
+ * PR10: createDraftEntry applies default/fixed rules onto the line bags
+ * before validation + insert; commitEntry asserts 'required' rules against
+ * the entry's stored lines BEFORE the commit_journal_entry RPC, and skips
+ * the line fetch entirely when no required rule exists.
  */
 import { describe, it, expect, vi, beforeEach } from 'vitest'
-import { createDraftEntry, updateDraftEntry } from '../engine'
-import { DimensionValidationError } from '../errors'
+import { commitEntry, createDraftEntry, updateDraftEntry } from '../engine'
+import { DimensionValidationError, MandatoryDimensionMissingError } from '../errors'
 import type { CreateJournalEntryInput } from '@/types'
 
 vi.mock('@/lib/events', () => ({
@@ -252,3 +258,136 @@ describe('updateDraftEntry — dimension validation wiring', () => {
     expect(lineRows[0].dimensions).toEqual({ '6': 'P001' })
   })
 })
+
+/**
+ * Raw account_dimension_rules row exactly as fetchActiveDimensionRules
+ * selects it: joined registry rows ride along nested (dimensions,
+ * dimension_values).
+ */
+function makeRuleRow(overrides: Record<string, unknown> = {}) {
+  return {
+    account_number: '4010',
+    rule_type: 'default',
+    dimensions: { sie_dim_no: 6, name: 'Projekt' },
+    dimension_values: { code: 'P001' },
+    ...overrides,
+  }
+}
+
+describe('createDraftEntry — account dimension rules (PR10)', () => {
+  it('applies a default rule onto the inserted line bag when the key is absent', async () => {
+    const { supabase, inserts } = buildSupabase({
+      ...BASE_TABLES,
+      account_dimension_rules: { data: [makeRuleRow()] },
+    })
+
+    const entry = await createDraftEntry(supabase as never, 'company-1', 'user-1', makeInput())
+
+    expect(entry.id).toBe('entry-1')
+    const lineRows = inserts.journal_entry_lines[0] as Array<Record<string, unknown>>
+    // The 4010 line got the default; the 1930 line has no rule and stays bare.
+    expect(lineRows[0].dimensions).toEqual({ '6': 'P001' })
+    expect(lineRows[1].dimensions).toEqual({})
+    // PR9 cutover: generated mirror columns must never appear in the payload.
+    expect('cost_center' in lineRows[0]).toBe(false)
+    expect('project' in lineRows[0]).toBe(false)
+  })
+
+  it('fixed rule overwrites the caller-supplied bag value', async () => {
+    const { supabase, inserts } = buildSupabase({
+      ...BASE_TABLES,
+      account_dimension_rules: {
+        data: [makeRuleRow({ rule_type: 'fixed', dimension_values: { code: 'PLOCK' } })],
+      },
+    })
+
+    const entry = await createDraftEntry(
+      supabase as never,
+      'company-1',
+      'user-1',
+      makeInput({ '6': 'CALLER' })
+    )
+
+    expect(entry.id).toBe('entry-1')
+    const lineRows = inserts.journal_entry_lines[0] as Array<Record<string, unknown>>
+    // Rule pinned the 4010 line; the rule-less 1930 line keeps the caller tag.
+    expect(lineRows[0].dimensions).toEqual({ '6': 'PLOCK' })
+    expect(lineRows[1].dimensions).toEqual({ '6': 'CALLER' })
+  })
+})
+
+describe('commitEntry — mandatory dimension enforcement (PR10)', () => {
+  const requiredRule = makeRuleRow({ rule_type: 'required', dimension_values: null })
+
+  it('rejects an untagged line BEFORE the commit_journal_entry RPC fires', async () => {
+    const { supabase } = buildSupabase({
+      ...BASE_TABLES,
+      account_dimension_rules: { data: [requiredRule] },
+      journal_entry_lines: {
+        data: [
+          { account_number: '4010', dimensions: {} },
+          { account_number: '1930', dimensions: {} },
+        ],
+      },
+    })
+
+    await expect(
+      commitEntry(supabase as never, 'company-1', 'user-1', 'entry-1')
+    ).rejects.toBeInstanceOf(MandatoryDimensionMissingError)
+    await expect(
+      commitEntry(supabase as never, 'company-1', 'user-1', 'entry-1')
+    ).rejects.toThrow('Konto 4010 kräver Projekt — välj ett värde innan bokföring.')
+
+    // The verifikat must never have been posted.
+    expect(supabase.rpc).not.toHaveBeenCalled()
+  })
+
+  it('commits when every required dimension is satisfied on the stored lines', async () => {
+    const { supabase } = buildSupabase({
+      ...BASE_TABLES,
+      account_dimension_rules: { data: [requiredRule] },
+      journal_entry_lines: {
+        data: [
+          { account_number: '4010', dimensions: { '6': 'P001' } },
+          { account_number: '1930', dimensions: {} },
+        ],
+      },
+    })
+
+    const entry = await commitEntry(supabase as never, 'company-1', 'user-1', 'entry-1')
+
+    expect(entry.id).toBe('entry-1')
+    expect(supabase.rpc).toHaveBeenCalledWith(
+      'commit_journal_entry',
+      expect.objectContaining({ p_company_id: 'company-1', p_entry_id: 'entry-1' })
+    )
+  })
+
+  it('skips the line fetch entirely when the company has zero rules', async () => {
+    const { supabase, queriedTables } = buildSupabase(BASE_TABLES)
+
+    const entry = await commitEntry(supabase as never, 'company-1', 'user-1', 'entry-1')
+
+    expect(entry.id).toBe('entry-1')
+    // Rules were checked, but no required rule exists → no line fetch.
+    expect(queriedTables()).toContain('account_dimension_rules')
+    expect(queriedTables()).not.toContain('journal_entry_lines')
+    expect(supabase.rpc).toHaveBeenCalledWith(
+      'commit_journal_entry',
+      expect.objectContaining({ p_entry_id: 'entry-1' })
+    )
+  })
+
+  it('skips the line fetch when the only rules are default/fixed', async () => {
+    const { supabase, queriedTables } = buildSupabase({
+      ...BASE_TABLES,
+      account_dimension_rules: {
+        data: [makeRuleRow(), makeRuleRow({ rule_type: 'fixed', account_number: '5010' })],
+      },
+    })
+
+    await commitEntry(supabase as never, 'company-1', 'user-1', 'entry-1')
+
+    expect(queriedTables()).not.toContain('journal_entry_lines')
+  })
+})
diff --git a/lib/bookkeeping/__tests__/voucher-atomicity.test.ts b/lib/bookkeeping/__tests__/voucher-atomicity.test.ts
index c502e988..63c3f71b 100644
--- a/lib/bookkeeping/__tests__/voucher-atomicity.test.ts
+++ b/lib/bookkeeping/__tests__/voucher-atomicity.test.ts
@@ -83,8 +83,11 @@ describe('voucher number atomicity', () => {
       p_actor_label: null,
     })
 
-    // from() was never called — the RPC handles everything atomically
-    expect(supabase.from).not.toHaveBeenCalled()
+    // No line/entry fetch happened — the RPC handles everything atomically.
+    // (PR10: commitEntry now also probes account_dimension_rules first; the
+    // bare mock makes that probe fail open, which is exactly the posture.)
+    expect(supabase.from).not.toHaveBeenCalledWith('journal_entries')
+    expect(supabase.from).not.toHaveBeenCalledWith('journal_entry_lines')
   })
 
   it('commitEntry succeeds via atomic RPC and returns posted entry', async () => {
diff --git a/lib/bookkeeping/dimension-errors.ts b/lib/bookkeeping/dimension-errors.ts
index 3858cc78..ad19460b 100644
--- a/lib/bookkeeping/dimension-errors.ts
+++ b/lib/bookkeeping/dimension-errors.ts
@@ -97,3 +97,44 @@ export class DimensionValidationError extends Error {
 export function isDimensionValidationError(err: unknown): err is DimensionValidationError {
   return err instanceof DimensionValidationError
 }
+
+// ============================================================================
+// Mandatory dimension enforcement (dimensions PR10)
+// ============================================================================
+
+export const MANDATORY_DIMENSION_MISSING = 'MANDATORY_DIMENSION_MISSING' as const
+
+export interface MandatoryDimensionViolation {
+  account_number: string
+  /** SIE dimension number the rule requires, e.g. '6'. */
+  sie_dim_no: string
+  /** Registry display name for the dimension, e.g. 'Projekt'. */
+  dimension_name: string
+}
+
+/** Swedish user-facing sentence for a single missing-dimension violation. */
+export function formatMandatoryDimensionViolation(v: MandatoryDimensionViolation): string {
+  return `Konto ${v.account_number} kräver ${v.dimension_name} — välj ett värde innan bokföring.`
+}
+
+/**
+ * Raised at COMMIT time (commitEntry / the bulk-book pre-check) when an
+ * active 'required' rule in account_dimension_rules is unsatisfied by a
+ * line's dimensions bag. Drafts may be incomplete by design — the rule bites
+ * when the verifikat is about to become immutable. Companies without rules
+ * (every company by default) never reach this error.
+ */
+export class MandatoryDimensionMissingError extends Error {
+  readonly code = MANDATORY_DIMENSION_MISSING
+
+  constructor(public readonly violations: MandatoryDimensionViolation[]) {
+    super(violations.map(formatMandatoryDimensionViolation).join(' '))
+    this.name = 'MandatoryDimensionMissingError'
+  }
+}
+
+export function isMandatoryDimensionMissingError(
+  err: unknown,
+): err is MandatoryDimensionMissingError {
+  return err instanceof MandatoryDimensionMissingError
+}
diff --git a/lib/bookkeeping/dimension-rules.ts b/lib/bookkeeping/dimension-rules.ts
new file mode 100644
index 00000000..65101ac0
--- /dev/null
+++ b/lib/bookkeeping/dimension-rules.ts
@@ -0,0 +1,176 @@
+/**
+ * Account dimension rules (dimensions PR10) — the policy layer over the
+ * dimensions substrate. Rules live in account_dimension_rules
+ * (20260703200000), one per (account, dimension):
+ *
+ *   'required'  the account cannot be POSTED without a value → enforced by
+ *               assertMandatoryDimensions at commitEntry and the bulk-book
+ *               route pre-check. Drafts may be incomplete by design; storno/
+ *               correction paths never pass through commitEntry, so history
+ *               always reverses regardless of policy.
+ *   'default'   pre-applied to the line bag at draft creation when the key
+ *               is absent (user-overridable).
+ *   'fixed'     ALWAYS applied at draft creation (overwrites the caller's
+ *               key) — the account is pinned to one value.
+ *
+ * Zero rules (every company by default) short-circuits everything — the
+ * engine behaves exactly as before PR10. Rule fetches FAIL OPEN like the
+ * soft registry validation: a transient DB error must not block bookkeeping,
+ * and the write hits the same database anyway.
+ *
+ * Pure of next/server so it stays importable from anywhere the resolver is.
+ */
+
+import type { SupabaseClient } from '@supabase/supabase-js'
+import {
+  MandatoryDimensionMissingError,
+  type MandatoryDimensionViolation,
+} from './dimension-errors'
+import {
+  normalizeLineDimensions,
+  type DimensionAliasInput,
+  type LineDimensions,
+} from './dimension-resolver'
+
+export interface AccountDimensionRule {
+  account_number: string
+  rule_type: 'required' | 'default' | 'fixed'
+  /** Canonical SIE dimension number as a string key, e.g. '6'. */
+  sie_dim_no: string
+  dimension_name: string
+  /** Value code for default/fixed rules; null for required. */
+  value_code: string | null
+}
+
+interface RawRuleRow {
+  account_number: string
+  rule_type: 'required' | 'default' | 'fixed'
+  dimensions: { sie_dim_no: number; name: string }
+  dimension_values: { code: string } | null
+}
+
+/**
+ * All ACTIVE rules for the company. Returns null on query failure (callers
+ * fail open — same posture as validateEntryDimensions). The table is tiny
+ * and indexed on (company_id, account_number); one fetch per booking is the
+ * whole cost for rule-less companies.
+ */
+export async function fetchActiveDimensionRules(
+  supabase: SupabaseClient,
+  companyId: string
+): Promise<AccountDimensionRule[] | null> {
+  // try/catch on top of the error-result check: fail-open must also cover
+  // thrown exceptions (broken client, network throw) — policy lookups are
+  // never allowed to take bookkeeping down with them.
+  try {
+    const { data, error } = await supabase
+      .from('account_dimension_rules')
+      .select(
+        'account_number, rule_type, dimensions!account_dimension_rules_dimension_id_company_id_fkey(sie_dim_no, name), dimension_values!account_dimension_rules_value_id_fkey(code)'
+      )
+      .eq('company_id', companyId)
+      .eq('is_active', true)
+
+    if (error) return null
+
+    return ((data ?? []) as unknown as RawRuleRow[]).map((row) => ({
+      account_number: row.account_number,
+      rule_type: row.rule_type,
+      sie_dim_no: String(row.dimensions.sie_dim_no),
+      dimension_name: row.dimensions.name,
+      value_code: row.dimension_values?.code ?? null,
+    }))
+  } catch {
+    return null
+  }
+}
+
+/**
+ * Apply default/fixed rules onto entry lines before validation + insert.
+ * Returns the same array when nothing applies (zero allocation on the
+ * common path); otherwise a copy where affected lines carry the augmented
+ * bag (aliases folded in first, so the returned lines are bag-authoritative).
+ */
+export function applyDimensionRules<
+  T extends DimensionAliasInput & { account_number: string },
+>(lines: T[], rules: AccountDimensionRule[]): T[] {
+  const applicable = rules.filter(
+    (r) => (r.rule_type === 'default' || r.rule_type === 'fixed') && r.value_code
+  )
+  if (applicable.length === 0) return lines
+
+  const byAccount = new Map<string, AccountDimensionRule[]>()
+  for (const rule of applicable) {
+    const bucket = byAccount.get(rule.account_number) ?? []
+    bucket.push(rule)
+    byAccount.set(rule.account_number, bucket)
+  }
+
+  let anyChanged = false
+  const result = lines.map((line) => {
+    const forAccount = byAccount.get(line.account_number)
+    if (!forAccount) return line
+
+    const bag: LineDimensions = normalizeLineDimensions(line)
+    let changed = false
+    for (const rule of forAccount) {
+      if (rule.rule_type === 'fixed') {
+        if (bag[rule.sie_dim_no] !== rule.value_code) {
+          bag[rule.sie_dim_no] = rule.value_code as string
+          changed = true
+        }
+      } else if (!(rule.sie_dim_no in bag)) {
+        bag[rule.sie_dim_no] = rule.value_code as string
+        changed = true
+      }
+    }
+    if (!changed) return line
+    anyChanged = true
+    // The bag now carries everything (aliases folded by normalize) — clear
+    // the deprecated aliases so downstream normalization can't resurrect a
+    // value a fixed rule just overwrote.
+    return { ...line, dimensions: bag, cost_center: null, project: null }
+  })
+
+  return anyChanged ? result : lines
+}
+
+/**
+ * Throw MandatoryDimensionMissingError when any ACTIVE 'required' rule is
+ * unsatisfied. One violation per (account, dimension) regardless of how many
+ * lines miss it — the Swedish message stays readable for multi-line entries.
+ */
+export function assertMandatoryDimensions(
+  lines: Array<DimensionAliasInput & { account_number: string }>,
+  rules: AccountDimensionRule[]
+): void {
+  const required = rules.filter((r) => r.rule_type === 'required')
+  if (required.length === 0) return
+
+  const byAccount = new Map<string, AccountDimensionRule[]>()
+  for (const rule of required) {
+    const bucket = byAccount.get(rule.account_number) ?? []
+    bucket.push(rule)
+    byAccount.set(rule.account_number, bucket)
+  }
+
+  const violations = new Map<string, MandatoryDimensionViolation>()
+  for (const line of lines) {
+    const forAccount = byAccount.get(line.account_number)
+    if (!forAccount) continue
+    const bag = normalizeLineDimensions(line)
+    for (const rule of forAccount) {
+      if (!bag[rule.sie_dim_no]) {
+        violations.set(`${line.account_number} ${rule.sie_dim_no}`, {
+          account_number: line.account_number,
+          sie_dim_no: rule.sie_dim_no,
+          dimension_name: rule.dimension_name,
+        })
+      }
+    }
+  }
+
+  if (violations.size > 0) {
+    throw new MandatoryDimensionMissingError([...violations.values()])
+  }
+}
diff --git a/lib/bookkeeping/engine.ts b/lib/bookkeeping/engine.ts
index c45c684a..c8e2641a 100644
--- a/lib/bookkeeping/engine.ts
+++ b/lib/bookkeeping/engine.ts
@@ -18,6 +18,11 @@ import {
   normalizeLineDimensions,
   validateEntryDimensions,
 } from '@/lib/bookkeeping/dimension-resolver'
+import {
+  applyDimensionRules,
+  assertMandatoryDimensions,
+  fetchActiveDimensionRules,
+} from '@/lib/bookkeeping/dimension-rules'
 import { backfillStandardBASAccounts } from '@/lib/bookkeeping/account-backfill'
 import { syncInvoiceStatusFromPaymentEntry, isPaymentSourceType } from '@/lib/bookkeeping/payment-sync'
 import { getActor } from '@/lib/bookkeeping/actor-context'
@@ -227,12 +232,22 @@ export async function createDraftEntry(
     throw new JournalEntryNotBalancedError(balance.totalDebit, balance.totalCredit, 'draft')
   }
 
+  // Account dimension rules (dimensions PR10): apply 'default'/'fixed'
+  // values onto the line bags before validation + insert. Zero rules —
+  // every company by default — returns the input untouched; a failed rule
+  // fetch fails open like the soft validation below.
+  const rules = await fetchActiveDimensionRules(supabase, companyId)
+  if (rules === null) {
+    log.warn('dimension rule fetch failed — defaults/fixed skipped (fail-open)', { companyId })
+  }
+  const lines = rules ? applyDimensionRules(input.lines, rules) : input.lines
+
   // Soft dimension validation (dimensions plan PR3): free for untagged
   // entries; free-text passthrough unless company_settings.dimensions_enabled;
   // enabled companies get registry validation with a typed Swedish rejection.
   // Runs before any insert so a rejection leaves no orphan rows. Reversal/
   // storno/correction paths bypass this — they copy posted data verbatim.
-  await validateEntryDimensions(supabase, companyId, input.lines)
+  await validateEntryDimensions(supabase, companyId, lines)
 
   // Validate that entry_date falls within the selected fiscal period
   const { data: period, error: periodError } = await supabase
@@ -319,7 +334,7 @@ export async function createDraftEntry(
   }
 
   // Insert journal entry lines with dimensions
-  const lineInserts = buildLineInserts(entry.id, input.lines, accountIdMap)
+  const lineInserts = buildLineInserts(entry.id, lines, accountIdMap)
 
   const { error: linesError } = await supabase
     .from('journal_entry_lines')
@@ -409,7 +424,13 @@ export async function updateDraftEntry(
 
   // Same soft dimension validation as createDraftEntry — before any write, so
   // a rejection leaves both the header and the existing lines untouched.
-  await validateEntryDimensions(supabase, companyId, input.lines)
+  // Account dimension rules (PR10) apply first — same as create.
+  const rules = await fetchActiveDimensionRules(supabase, companyId)
+  if (rules === null) {
+    log.warn('dimension rule fetch failed — defaults/fixed skipped (fail-open)', { companyId })
+  }
+  const lines = rules ? applyDimensionRules(input.lines, rules) : input.lines
+  await validateEntryDimensions(supabase, companyId, lines)
 
   // Entry date must fall within the selected fiscal period.
   const { data: period, error: periodError } = await supabase
@@ -479,7 +500,7 @@ export async function updateDraftEntry(
     throw new BookkeepingDatabaseError('create_entry_lines', deleteError.message)
   }
 
-  const lineInserts = buildLineInserts(entryId, input.lines, accountIdMap)
+  const lineInserts = buildLineInserts(entryId, lines, accountIdMap)
   const { error: linesError } = await supabase
     .from('journal_entry_lines')
     .insert(lineInserts)
@@ -528,6 +549,39 @@ export async function commitEntry(
 ): Promise<JournalEntry> {
   const actor = getActor()
 
+  // Mandatory dimension rules (dimensions PR10): 'required' rules bite when
+  // the verifikat is about to become immutable — drafts may be incomplete,
+  // posting may not. Zero active rules (the default) skips the line fetch
+  // entirely; a failed rule fetch fails open (transient DB errors must not
+  // block bookkeeping). Reversal/correction paths never pass through
+  // commitEntry, so history always reverses regardless of policy.
+  const rules = await fetchActiveDimensionRules(supabase, companyId)
+  if (rules === null) {
+    // Deliberate fail-open, but LOUD: a transient policy-table error must not
+    // block month-end bookings company-wide, yet a silently skipped control
+    // is invisible — the warning makes the degradation observable.
+    log.warn('dimension rule fetch failed — mandatory enforcement skipped (fail-open)', {
+      companyId,
+      entityId: entryId,
+    })
+  } else if (rules.some((r) => r.rule_type === 'required')) {
+    const { data: ruleLines, error: ruleLinesError } = await supabase
+      .from('journal_entry_lines')
+      .select('account_number, dimensions')
+      .eq('journal_entry_id', entryId)
+    if (ruleLinesError || !ruleLines) {
+      log.warn('line fetch for mandatory dimension check failed — enforcement skipped (fail-open)', {
+        companyId,
+        entityId: entryId,
+      })
+    } else {
+      assertMandatoryDimensions(
+        ruleLines as Array<{ account_number: string; dimensions: Record<string, string> }>,
+        rules
+      )
+    }
+  }
+
   // Atomic: increment voucher sequence + update status in one transaction.
   // Rolls back the sequence if the balance trigger or any constraint fails.
   const { data: rpcResult, error: commitError } = await supabase.rpc('commit_journal_entry', {
diff --git a/lib/bookkeeping/errors.ts b/lib/bookkeeping/errors.ts
index 00d5cb8e..ce0e0f26 100644
--- a/lib/bookkeeping/errors.ts
+++ b/lib/bookkeeping/errors.ts
@@ -1,5 +1,5 @@
 import { NextResponse } from 'next/server'
-import { DimensionValidationError } from './dimension-errors'
+import { DimensionValidationError, MandatoryDimensionMissingError } from './dimension-errors'
 
 // ============================================================================
 // Dimension validation error — class lives in ./dimension-errors.ts (pure
@@ -14,10 +14,15 @@ export {
   formatDimensionValidationIssue,
   formatDimensionValidationIssues,
   isDimensionValidationError,
+  MANDATORY_DIMENSION_MISSING,
+  MandatoryDimensionMissingError,
+  formatMandatoryDimensionViolation,
+  isMandatoryDimensionMissingError,
 } from './dimension-errors'
 export type {
   DimensionValidationIssue,
   DimensionValidationReason,
+  MandatoryDimensionViolation,
 } from './dimension-errors'
 
 // ============================================================================
@@ -331,7 +336,8 @@ export function isBookkeepingError(err: unknown): boolean {
     err instanceof NoOpenPeriodForDateError ||
     err instanceof TargetPeriodClosedError ||
     err instanceof TargetPeriodLockedError ||
-    err instanceof DimensionValidationError
+    err instanceof DimensionValidationError ||
+    err instanceof MandatoryDimensionMissingError
   )
 }
 
@@ -574,6 +580,22 @@ export function bookkeepingErrorResponse(err: unknown): NextResponse | null {
     )
   }
 
+  if (err instanceof MandatoryDimensionMissingError) {
+    // Policy rejection at commit (dimensions PR10): the message names every
+    // account + required dimension so a user or agent self-corrects in one
+    // pass; details.violations is the machine-readable list.
+    return NextResponse.json(
+      {
+        error: {
+          code: err.code,
+          message: err.message,
+          details: { violations: err.violations },
+        },
+      },
+      { status: 400 }
+    )
+  }
+
   if (err instanceof BookkeepingDatabaseError) {
     return NextResponse.json(
       {
diff --git a/messages/en.json b/messages/en.json
index 057bd766..abb46cf4 100644
--- a/messages/en.json
+++ b/messages/en.json
@@ -3477,7 +3477,21 @@
     "settings_imported_toast": "{count, plural, one {1 existing code was imported} other {# existing codes were imported}}",
     "settings_import_failed_title": "Could not import existing codes",
     "settings_save_failed_title": "Could not save the setting",
-    "settings_open_register": "Open cost centres & projects"
+    "settings_open_register": "Open cost centres & projects",
+    "new_dimension": "New dimension",
+    "new_dimension_title": "New dimension",
+    "dim_form_name_invalid": "Enter a name (1–60 characters).",
+    "dim_form_number_label": "Number",
+    "dim_form_number_help": "Leave empty for the next free number (20+). SIE reserves 1–19.",
+    "dim_form_number_invalid": "Enter a whole number of 20 or higher.",
+    "dim_form_resets_label": "Resets every fiscal year",
+    "dim_form_resets_help": "Balances per value start over at a new fiscal year.",
+    "dim_form_advanced": "Advanced",
+    "dim_form_parent_label": "Parent dimension",
+    "dim_form_parent_none": "None",
+    "dim_form_parent_help": "Makes the new dimension a subdimension (SIE #UNDERDIM) of the selected one.",
+    "dim_created_title": "Dimension created",
+    "subdimension_of": "Subdimension of {parent}"
   },
   "tic_workspace": {
     "toast_settings_failed": "Could not fetch settings",
diff --git a/messages/sv.json b/messages/sv.json
index d2da8108..e8759bf9 100644
--- a/messages/sv.json
+++ b/messages/sv.json
@@ -3477,7 +3477,21 @@
     "settings_imported_toast": "{count, plural, one {1 befintlig kod importerades} other {# befintliga koder importerades}}",
     "settings_import_failed_title": "Kunde inte importera befintliga koder",
     "settings_save_failed_title": "Kunde inte spara inställningen",
-    "settings_open_register": "Öppna kostnadsställen & projekt"
+    "settings_open_register": "Öppna kostnadsställen & projekt",
+    "new_dimension": "Ny dimension",
+    "new_dimension_title": "Ny dimension",
+    "dim_form_name_invalid": "Ange ett namn (1–60 tecken).",
+    "dim_form_number_label": "Nummer",
+    "dim_form_number_help": "Lämna tomt för nästa lediga (20+). SIE reserverar 1–19.",
+    "dim_form_number_invalid": "Ange ett heltal, 20 eller högre.",
+    "dim_form_resets_label": "Nollställs varje räkenskapsår",
+    "dim_form_resets_help": "Saldon per värde börjar om vid nytt räkenskapsår.",
+    "dim_form_advanced": "Avancerat",
+    "dim_form_parent_label": "Överordnad dimension",
+    "dim_form_parent_none": "Ingen",
+    "dim_form_parent_help": "Gör den nya dimensionen till en underdimension (SIE #UNDERDIM) av den valda.",
+    "dim_created_title": "Dimension skapad",
+    "subdimension_of": "Underdimension till {parent}"
   },
   "tic_workspace": {
     "toast_settings_failed": "Kunde inte hämta inställningar",
diff --git a/supabase/migrations/20260703200000_account_dimension_rules.sql b/supabase/migrations/20260703200000_account_dimension_rules.sql
new file mode 100644
index 00000000..0162a32a
--- /dev/null
+++ b/supabase/migrations/20260703200000_account_dimension_rules.sql
@@ -0,0 +1,83 @@
+-- Dimensions PR10 (advanced): per-account dimension policy.
+--
+--   account_dimension_rules — one rule per (account, dimension):
+--     'required'  the account cannot be POSTED without a value for the
+--                 dimension (enforced TS-side at commitEntry + the bulk-book
+--                 route pre-check; drafts may be incomplete)
+--     'default'   the value is pre-applied to the line's bag at draft
+--                 creation when the key is absent (user-overridable)
+--     'fixed'     the value is ALWAYS applied at draft creation (overwrites
+--                 whatever the caller sent for that key)
+--
+-- Opt-in by construction: zero rows (every company's default) = the engine
+-- behaves exactly as before. There is deliberately NO settings toggle for
+-- enforcement — a rule that exists but is silently ignored is worse than
+-- either extreme; pausing one rule is what is_active is for.
+--
+-- Shape follows the dimensions registry (20260702084500): company_id-native,
+-- no user_id (rules are company policy, not personal data), RLS via
+-- user_company_ids(). value_id's dimension/company consistency is validated
+-- at the API layer (and re-checked by engine-side registry validation at
+-- booking); a composite FK is deliberately skipped — dimension_values has no
+-- (id, dimension_id) unique pair and adding one for this is not worth the
+-- churn.
+--
+-- pg-test: tests/pg/account-dimension-rules.pg.test.ts (RLS + CHECKs +
+-- cascade behavior).
+
+CREATE TABLE public.account_dimension_rules (
+  id             uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  company_id     uuid NOT NULL REFERENCES public.companies(id) ON DELETE CASCADE,
+  -- Exact BAS account; ranges can layer on later without schema change.
+  account_number text NOT NULL CHECK (account_number ~ '^[0-9]{4}$'),
+  dimension_id   uuid NOT NULL,
+  rule_type      text NOT NULL CHECK (rule_type IN ('required', 'default', 'fixed')),
+  -- required → no value; default/fixed → the value to apply.
+  value_id       uuid REFERENCES public.dimension_values(id) ON DELETE CASCADE,
+  is_active      boolean NOT NULL DEFAULT true,
+  created_at     timestamptz NOT NULL DEFAULT now(),
+  updated_at     timestamptz NOT NULL DEFAULT now(),
+
+  -- Composite FK: the dimension must belong to the same company (the
+  -- registry's UNIQUE (id, company_id) exists exactly for this pattern).
+  FOREIGN KEY (dimension_id, company_id)
+    REFERENCES public.dimensions(id, company_id) ON DELETE CASCADE,
+  -- One rule per (account, dimension) — 'required'+'default' combos et al.
+  -- are a later refinement; three clean types for v1.
+  UNIQUE (company_id, account_number, dimension_id),
+  CONSTRAINT adr_value_presence CHECK (
+    (rule_type = 'required' AND value_id IS NULL)
+    OR (rule_type IN ('default', 'fixed') AND value_id IS NOT NULL)
+  )
+);
+
+ALTER TABLE public.account_dimension_rules ENABLE ROW LEVEL SECURITY;
+
+CREATE POLICY "view own-company account_dimension_rules"
+  ON public.account_dimension_rules FOR SELECT
+  USING (company_id IN (SELECT user_company_ids()));
+CREATE POLICY "insert own-company account_dimension_rules"
+  ON public.account_dimension_rules FOR INSERT
+  WITH CHECK (company_id IN (SELECT user_company_ids()));
+CREATE POLICY "update own-company account_dimension_rules"
+  ON public.account_dimension_rules FOR UPDATE
+  USING (company_id IN (SELECT user_company_ids()));
+CREATE POLICY "delete own-company account_dimension_rules"
+  ON public.account_dimension_rules FOR DELETE
+  USING (company_id IN (SELECT user_company_ids()));
+
+CREATE INDEX idx_adr_company_account
+  ON public.account_dimension_rules (company_id, account_number);
+
+CREATE TRIGGER set_updated_at_account_dimension_rules
+  BEFORE UPDATE ON public.account_dimension_rules
+  FOR EACH ROW EXECUTE FUNCTION public.update_updated_at_column();
+
+CREATE TRIGGER audit_account_dimension_rules
+  AFTER INSERT OR UPDATE OR DELETE ON public.account_dimension_rules
+  FOR EACH ROW EXECUTE FUNCTION public.write_audit_log();
+
+COMMENT ON TABLE public.account_dimension_rules IS
+  'Per-account dimension policy (dimensions PR10): required blocks posting without a value (TS-side, commitEntry), default pre-fills, fixed always applies. Zero rows = no behavior change.';
+
+NOTIFY pgrst, 'reload schema';
diff --git a/tests/pg/account-dimension-rules.pg.test.ts b/tests/pg/account-dimension-rules.pg.test.ts
new file mode 100644
index 00000000..c2d9fd27
--- /dev/null
+++ b/tests/pg/account-dimension-rules.pg.test.ts
@@ -0,0 +1,178 @@
+import { describe, it, expect } from 'vitest'
+import { randomUUID } from 'node:crypto'
+import { getPool, withUserContext } from './setup'
+import { seedCompany } from './fixtures'
+
+// PR10 account_dimension_rules (20260703200000_account_dimension_rules.sql):
+// RLS via user_company_ids() on all four operations, the adr_value_presence
+// CHECK (required ⇔ no value), UNIQUE (company_id, account_number,
+// dimension_id), value_id ON DELETE CASCADE, and the composite
+// (dimension_id, company_id) FK that pins a rule's dimension to the same
+// company.
+
+async function seedWithDimensions() {
+  const seeded = await seedCompany()
+  await getPool().query(`SELECT public.ensure_company_dimensions($1)`, [seeded.companyId])
+  return seeded
+}
+
+async function getDimensionId(companyId: string, sieDimNo: number): Promise<string> {
+  const { rows } = await getPool().query(
+    `SELECT id FROM public.dimensions WHERE company_id = $1 AND sie_dim_no = $2`,
+    [companyId, sieDimNo],
+  )
+  return rows[0].id
+}
+
+async function insertValue(params: {
+  companyId: string
+  dimensionId: string
+  code: string
+  name?: string
+}): Promise<string> {
+  const id = randomUUID()
+  await getPool().query(
+    `INSERT INTO public.dimension_values (id, company_id, dimension_id, code, name)
+     VALUES ($1, $2, $3, $4, $5)`,
+    [id, params.companyId, params.dimensionId, params.code, params.name ?? params.code],
+  )
+  return id
+}
+
+async function insertRule(params: {
+  companyId: string
+  dimensionId: string
+  ruleType: 'required' | 'default' | 'fixed'
+  accountNumber?: string
+  valueId?: string | null
+}): Promise<string> {
+  const id = randomUUID()
+  await getPool().query(
+    `INSERT INTO public.account_dimension_rules
+       (id, company_id, account_number, dimension_id, rule_type, value_id)
+     VALUES ($1, $2, $3, $4, $5, $6)`,
+    [
+      id,
+      params.companyId,
+      params.accountNumber ?? '4010',
+      params.dimensionId,
+      params.ruleType,
+      params.valueId ?? null,
+    ],
+  )
+  return id
+}
+
+describe('account_dimension_rules RLS', () => {
+  it('lets a member insert and read an own-company rule', async () => {
+    const { userId, companyId } = await seedWithDimensions()
+    const dimId = await getDimensionId(companyId, 6)
+
+    await withUserContext(userId, async (client) => {
+      await client.query(
+        `INSERT INTO public.account_dimension_rules
+           (company_id, account_number, dimension_id, rule_type)
+         VALUES ($1, '4010', $2, 'required')`,
+        [companyId, dimId],
+      )
+      const { rows } = await client.query(
+        `SELECT account_number, rule_type, value_id, is_active
+         FROM public.account_dimension_rules WHERE company_id = $1`,
+        [companyId],
+      )
+      expect(rows).toEqual([
+        { account_number: '4010', rule_type: 'required', value_id: null, is_active: true },
+      ])
+    })
+  })
+
+  it('hides other companies rules and blocks cross-company inserts', async () => {
+    const a = await seedWithDimensions()
+    const b = await seedWithDimensions()
+    const aDimId = await getDimensionId(a.companyId, 6)
+    await insertRule({ companyId: a.companyId, dimensionId: aDimId, ruleType: 'required' })
+
+    await withUserContext(b.userId, async (client) => {
+      // The outsider sees none of A's rules (and has none of their own).
+      const { rows } = await client.query(
+        `SELECT id FROM public.account_dimension_rules`,
+      )
+      expect(rows).toEqual([])
+
+      await expect(
+        client.query(
+          `INSERT INTO public.account_dimension_rules
+             (company_id, account_number, dimension_id, rule_type)
+           VALUES ($1, '5010', $2, 'required')`,
+          [a.companyId, aDimId],
+        ),
+      ).rejects.toThrow(/row-level security/)
+    })
+  })
+})
+
+describe('adr_value_presence CHECK', () => {
+  it('rejects a required rule that carries a value', async () => {
+    const { companyId } = await seedWithDimensions()
+    const dimId = await getDimensionId(companyId, 6)
+    const valueId = await insertValue({ companyId, dimensionId: dimId, code: 'P001' })
+
+    await expect(
+      insertRule({ companyId, dimensionId: dimId, ruleType: 'required', valueId }),
+    ).rejects.toThrow(/adr_value_presence/)
+  })
+
+  it('rejects a default rule without a value', async () => {
+    const { companyId } = await seedWithDimensions()
+    const dimId = await getDimensionId(companyId, 6)
+
+    await expect(
+      insertRule({ companyId, dimensionId: dimId, ruleType: 'default', valueId: null }),
+    ).rejects.toThrow(/adr_value_presence/)
+  })
+})
+
+describe('UNIQUE (company_id, account_number, dimension_id)', () => {
+  it('rejects a second rule for the same account and dimension', async () => {
+    const { companyId } = await seedWithDimensions()
+    const dimId = await getDimensionId(companyId, 6)
+    const valueId = await insertValue({ companyId, dimensionId: dimId, code: 'P001' })
+    await insertRule({ companyId, dimensionId: dimId, ruleType: 'default', valueId })
+
+    // Different rule_type, same (company, account, dimension) — still one slot.
+    await expect(
+      insertRule({ companyId, dimensionId: dimId, ruleType: 'required' }),
+    ).rejects.toThrow(/duplicate|unique/)
+  })
+})
+
+describe('cascade behavior', () => {
+  it('deleting the dimension_value removes rules pinned to it (ON DELETE CASCADE)', async () => {
+    const { companyId } = await seedWithDimensions()
+    const dimId = await getDimensionId(companyId, 6)
+    const valueId = await insertValue({ companyId, dimensionId: dimId, code: 'P001' })
+    const ruleId = await insertRule({ companyId, dimensionId: dimId, ruleType: 'fixed', valueId })
+
+    // The value is unreferenced by posted lines, so the retention trigger
+    // allows the delete — and the rule must ride the cascade.
+    await getPool().query(`DELETE FROM public.dimension_values WHERE id = $1`, [valueId])
+
+    const { rows } = await getPool().query(
+      `SELECT id FROM public.account_dimension_rules WHERE id = $1`,
+      [ruleId],
+    )
+    expect(rows).toEqual([])
+  })
+})
+
+describe('composite (dimension_id, company_id) FK', () => {
+  it('rejects a rule whose dimension belongs to another company', async () => {
+    const a = await seedWithDimensions()
+    const b = await seedWithDimensions()
+    const aDimId = await getDimensionId(a.companyId, 6)
+
+    await expect(
+      insertRule({ companyId: b.companyId, dimensionId: aDimId, ruleType: 'required' }),
+    ).rejects.toThrow(/foreign key/)
+  })
+})