diff --git a/lib/import/__tests__/sie-import.test.ts b/lib/import/__tests__/sie-import.test.ts
index 1f07aa1f..352d87a7 100644
--- a/lib/import/__tests__/sie-import.test.ts
+++ b/lib/import/__tests__/sie-import.test.ts
@@ -323,12 +323,12 @@ describe('ensureFiscalPeriod validation', () => {
   // actionable Swedish error instead of a raw Postgres message.
   type Supabase = Parameters<typeof ensureFiscalPeriod>[0]
 
-  it('rejects mid-month start when another period exists for the company', async () => {
+  it('rejects mid-month start when an earlier period already exists', async () => {
     const { supabase, enqueueMany } = createQueuedMockSupabase()
     enqueueMany([
       { data: null, error: null }, // containing check — no match
       { data: [], error: null },   // overlapping check — none
-      { data: null, error: null, count: 1 }, // count existing — 1 period already
+      { data: [{ id: 'earlier' }], error: null }, // earlier period exists
     ])
 
     await expect(
@@ -338,7 +338,7 @@ describe('ensureFiscalPeriod validation', () => {
         '2026-04-16',
         '2026-12-31',
       ),
-    ).rejects.toThrow(/endast företagets första räkenskapsår får börja mitt i månaden/)
+    ).rejects.toThrow(/kronologiskt första räkenskapsår får börja mitt i månaden/)
   })
 
   it('rejects end date that is not the last day of the month', async () => {
@@ -346,7 +346,7 @@ describe('ensureFiscalPeriod validation', () => {
     enqueueMany([
       { data: null, error: null },
       { data: [], error: null },
-      { data: null, error: null, count: 0 }, // first fiscal period
+      { data: [], error: null }, // no earlier period
     ])
 
     await expect(
@@ -364,7 +364,7 @@ describe('ensureFiscalPeriod validation', () => {
     enqueueMany([
       { data: null, error: null },
       { data: [], error: null },
-      { data: null, error: null, count: 0 }, // no existing periods
+      { data: [], error: null }, // no earlier period
       { data: { id: 'new-period-id' }, error: null }, // insert result
     ])
 
@@ -378,6 +378,29 @@ describe('ensureFiscalPeriod validation', () => {
     expect(id).toBe('new-period-id')
   })
 
+  it('allows mid-month start when importing a retroactive earliest period', async () => {
+    // Scenario: onboarding created a 2026 fiscal period, user now imports
+    // an SIE for their förlängt första räkenskapsår 2017-07-28 – 2018-12-31.
+    // The 2017 period is chronologically earliest, so mid-month start is
+    // legal under BFL 3 kap.
+    const { supabase, enqueueMany } = createQueuedMockSupabase()
+    enqueueMany([
+      { data: null, error: null }, // containing check — no match
+      { data: [], error: null },   // overlapping check — none (2017 vs 2026)
+      { data: [], error: null },   // no earlier period than 2017-07-28
+      { data: { id: 'retro-first-year-id' }, error: null }, // insert
+    ])
+
+    const id = await ensureFiscalPeriod(
+      supabase as unknown as Supabase,
+      'company-id',
+      '2017-07-28',
+      '2018-12-31',
+    )
+
+    expect(id).toBe('retro-first-year-id')
+  })
+
   it('reuses an existing period that contains the range (no validation needed)', async () => {
     const { supabase, enqueueMany } = createQueuedMockSupabase()
     enqueueMany([
diff --git a/lib/import/sie-import.ts b/lib/import/sie-import.ts
index 42e41a2e..f4f7e350 100644
--- a/lib/import/sie-import.ts
+++ b/lib/import/sie-import.ts
@@ -252,22 +252,28 @@ export async function ensureFiscalPeriod(
 
   // Pre-validate against the DB-side enforce_period_start_day trigger so the
   // user gets an actionable Swedish error instead of a raw Postgres message.
-  // Only the company's FIRST fiscal period may start mid-month (BFL 3 kap.);
-  // any subsequent period must start on day 1. The trigger queries the same
-  // table, so "another period exists" == the overlap check above missed it,
-  // i.e. this would be a non-contiguous second period.
-  const { count: existingCount } = await supabase
-    .from('fiscal_periods')
-    .select('id', { count: 'exact', head: true })
-    .eq('company_id', companyId)
-
+  // Per BFL 3 kap., only the company's chronologically FIRST fiscal year may
+  // start mid-month (förlängt första räkenskapsår). Any period that comes
+  // after an earlier one must start on day 1. We check "is there a period
+  // that starts earlier?" rather than "does any period exist?" so a user can
+  // retroactively import an old first fiscal year via SIE even after an
+  // onboarding-created period already exists later in time.
   const startParts = parseDateParts(startDate)
   const endParts = parseDateParts(endDate)
 
-  if ((existingCount ?? 0) > 0 && startParts.day !== 1) {
-    throw new Error(
-      `SIE-filens räkenskapsår börjar ${startDate} — endast företagets första räkenskapsår får börja mitt i månaden. Efterföljande räkenskapsår måste börja den 1:a i en månad (BFL 3 kap.). Kontrollera datumen i #RAR-raden eller importera filen innan du skapar fler perioder.`
-    )
+  if (startParts.day !== 1) {
+    const { data: earlier } = await supabase
+      .from('fiscal_periods')
+      .select('id')
+      .eq('company_id', companyId)
+      .lt('period_start', startDate)
+      .limit(1)
+
+    if (earlier && earlier.length > 0) {
+      throw new Error(
+        `SIE-filens räkenskapsår börjar ${startDate} — endast företagets kronologiskt första räkenskapsår får börja mitt i månaden. Efterföljande räkenskapsår måste börja den 1:a i en månad (BFL 3 kap.). Kontrollera datumen i #RAR-raden.`
+      )
+    }
   }
 
   // Matches the fiscal_period_end_last_of_month CHECK constraint on prod;
diff --git a/supabase/migrations/20260418120000_allow_retroactive_first_fiscal_year.sql b/supabase/migrations/20260418120000_allow_retroactive_first_fiscal_year.sql
new file mode 100644
index 00000000..dd7c9816
--- /dev/null
+++ b/supabase/migrations/20260418120000_allow_retroactive_first_fiscal_year.sql
@@ -0,0 +1,38 @@
+-- Allow retroactive first fiscal year via SIE import.
+--
+-- The previous trigger (20260416120000) rejected any non-first-of-month
+-- period_start when *any other* fiscal period existed for the company.
+-- That blocked a common flow: user onboards and gets a default period
+-- (e.g. the current year, starting on day 1), then imports an SIE file
+-- covering an older, förlängt första räkenskapsår that starts mid-month
+-- (e.g. 2017-07-28 – 2018-12-31).
+--
+-- Per BFL 3 kap., the forst-in-time (chronologically earliest) fiscal year
+-- is the one that may be 6–18 months and start mid-month. That property is
+-- about *when* the period starts relative to other periods, not about the
+-- order rows were inserted. This migration changes the trigger to reflect
+-- that: mid-month start is allowed iff no existing period starts earlier.
+
+CREATE OR REPLACE FUNCTION public.enforce_first_of_month_for_subsequent_periods()
+RETURNS trigger
+LANGUAGE plpgsql
+AS $$
+BEGIN
+  IF EXTRACT(DAY FROM NEW.period_start) = 1 THEN
+    RETURN NEW;
+  END IF;
+
+  IF EXISTS (
+    SELECT 1 FROM public.fiscal_periods
+    WHERE company_id = NEW.company_id
+      AND id IS DISTINCT FROM NEW.id
+      AND period_start < NEW.period_start
+  ) THEN
+    RAISE EXCEPTION 'Non-first fiscal period must start on the 1st of a month';
+  END IF;
+
+  RETURN NEW;
+END;
+$$;
+
+NOTIFY pgrst, 'reload schema';