diff --git a/src/lib/chartOfAccountsSource.ts b/src/lib/chartOfAccountsSource.ts
index bfc31ef..42777d8 100644
--- a/src/lib/chartOfAccountsSource.ts
+++ b/src/lib/chartOfAccountsSource.ts
@@ -54,7 +54,10 @@ function fromPlatform(row: any, associationId: string): NormalizedAccount {
  * - `platform`  → the Accounting module's `accounting.accounts` (single source of
  *   truth once an association is on the platform). Returns [] if the association
  *   has no `accounting.companies` row yet.
- * - `zoho` / `buildium` → the public `chart_of_accounts`, scoped by system.
+ * - `zoho` / `buildium` → the public `chart_of_accounts`, scoped by system AND, when
+ *   an association is given, by that association. Each association owns an independent
+ *   set of accounts (one row per association — see the per-association COA migration),
+ *   so two associations can both have a "5000" meaning different things.
  *
  * Returned rows are normalized to {@link NormalizedAccount} so callers never
  * branch on the source.
@@ -81,11 +84,14 @@ export async function fetchChartOfAccounts(
     return (data ?? []).map((row) => fromPlatform(row, associationId));
   }
 
-  const { data, error } = await supabase
+  let query = supabase
     .from("chart_of_accounts")
     .select("*")
-    .eq("accounting_system", system)
-    .order("account_number", { ascending: true });
+    .eq("accounting_system", system);
+  // Scope to the current association so each one sees only its own accounts. When no
+  // association is supplied, fall back to the whole system set (back-compat).
+  if (associationId) query = query.eq("association_id", associationId);
+  const { data, error } = await query.order("account_number", { ascending: true });
   if (error) throw error;
   return (data ?? []).map(fromPublic);
 }
diff --git a/supabase/migrations/20260604140000_per_association_chart_of_accounts.sql b/supabase/migrations/20260604140000_per_association_chart_of_accounts.sql
new file mode 100644
index 0000000..bd7bcc4
--- /dev/null
+++ b/supabase/migrations/20260604140000_per_association_chart_of_accounts.sql
@@ -0,0 +1,140 @@
+-- Per-association Chart of Accounts.
+--
+-- Today public.chart_of_accounts shares one row across many associations via the
+-- association_ids[] array (buildium system: 94 rows shared by up to 13 associations).
+-- Editing a number for one association edits the row the others use. This migration
+-- gives every association its own row: it splits each shared buildium row into one
+-- row per association in its association_ids, repoints all references by each
+-- referencing record's association, and swaps the uniqueness to be per-association.
+--
+-- Nothing is deleted: the original shared row simply becomes the per-association row
+-- for its own association_id; clones are added for the other associations. So no
+-- foreign key can dangle. association_ids is kept as a single-element mirror of
+-- association_id during the transition (existing `association_ids @> [assoc]`
+-- callers keep working); drop it in a later cleanup pass.
+--
+-- Pre-audited on live data: association_id is always in association_ids; account
+-- numbers are globally unique within buildium (so no per-association collisions);
+-- all buildium parents are buildium rows; every reference's association is genuinely
+-- in its target row's array (zero mismatches). One row carries a stray NULL array
+-- element (excluded). Two child accounts (4000, 4004) are shared with associations
+-- that don't own their parent (4999) — those clones become top-level.
+
+-- Pristine snapshot for rollback / original-parent lookups.
+drop table if exists public._coa_perassoc_backup;
+create table public._coa_perassoc_backup as table public.chart_of_accounts;
+
+-- Drop the old per-system uniqueness up front: clones reuse the same account_number
+-- within buildium, which the old (account_number, accounting_system) index forbids.
+-- The new per-association index is created at the end, after the data is consistent.
+drop index if exists public.chart_of_accounts_number_per_system_unique;
+
+-- (old_id, association) -> new_id. The owning association keeps the original row id;
+-- every other (non-null) association in the array gets a fresh clone id.
+-- Only map to associations that still exist — some arrays carry ids of deleted
+-- associations (stale memberships); those simply don't get a row.
+create temp table coa_map as
+select c.id as old_id, u.assoc as association_id,
+       case when u.assoc = c.association_id then c.id else gen_random_uuid() end as new_id,
+       (u.assoc = c.association_id) as is_keep
+from public.chart_of_accounts c
+cross join lateral (
+  select distinct e as assoc from unnest(c.association_ids) e
+  where e is not null and exists (select 1 from public.associations a where a.id = e)
+) u
+where c.accounting_system = 'buildium';
+
+-- Clone rows for the non-owning associations.
+insert into public.chart_of_accounts
+  (id, association_id, account_number, account_name, account_type, parent_account_id,
+   is_active, description, association_ids, accounting_system, created_at, updated_at)
+select m.new_id, m.association_id, c.account_number, c.account_name, c.account_type,
+       c.parent_account_id, c.is_active, c.description, array[m.association_id],
+       c.accounting_system, c.created_at, now()
+from coa_map m
+join public.chart_of_accounts c on c.id = m.old_id
+where not m.is_keep;
+
+-- Collapse the kept rows' arrays to their single association.
+update public.chart_of_accounts c
+set association_ids = array[c.association_id], updated_at = now()
+where c.accounting_system = 'buildium' and association_ids <> array[c.association_id];
+
+-- Remap parent_account_id to the same-association copy of the parent (original
+-- parents read from the pristine snapshot so in-place updates don't interfere).
+update public.chart_of_accounts r
+set parent_account_id = pm.new_id, updated_at = now()
+from coa_map rm
+join public._coa_perassoc_backup b on b.id = rm.old_id
+join coa_map pm on pm.old_id = b.parent_account_id and pm.association_id = rm.association_id
+where r.id = rm.new_id and b.parent_account_id is not null
+  and r.parent_account_id is distinct from pm.new_id;
+
+-- Any parent that still isn't owned by the row's association (a child shared with an
+-- association that doesn't own its parent) can't nest there — make it top-level.
+update public.chart_of_accounts c
+set parent_account_id = null, updated_at = now()
+from public.chart_of_accounts p
+where c.parent_account_id = p.id and c.accounting_system = 'buildium'
+  and p.association_id is distinct from c.association_id;
+
+-- Repoint every reference to the association-specific row, by the record's association.
+update public.bills x set expense_account_id = m.new_id
+from coa_map m where m.old_id = x.expense_account_id and m.association_id = x.association_id and m.new_id <> x.expense_account_id;
+
+update public.budgets x set gl_account_id = m.new_id
+from coa_map m where m.old_id = x.gl_account_id and m.association_id = x.association_id and m.new_id <> x.gl_account_id;
+
+update public.owner_ledger_entries x set gl_account_id = m.new_id
+from coa_map m where m.old_id = x.gl_account_id and m.association_id = x.association_id and m.new_id <> x.gl_account_id;
+
+-- budget_actuals_monthly is a VIEW derived from the base tables above; it recomputes
+-- and needs no repointing (it is still read in verification below).
+
+update public.vendor_coa_mappings x set chart_of_accounts_id = m.new_id
+from coa_map m where m.old_id = x.chart_of_accounts_id and m.association_id = x.association_id and m.new_id <> x.chart_of_accounts_id;
+
+update public.units x set assessment_account_id = m.new_id
+from coa_map m where m.old_id = x.assessment_account_id and m.association_id = x.association_id and m.new_id <> x.assessment_account_id;
+
+update public.vendors x set default_expense_account_id = m.new_id
+from coa_map m where m.old_id = x.default_expense_account_id and m.association_id = x.association_id and m.new_id <> x.default_expense_account_id;
+
+update public.journal_entries x set chart_of_account_id = m.new_id
+from coa_map m where m.old_id = x.chart_of_account_id and m.association_id = x.association_id and m.new_id <> x.chart_of_account_id;
+
+-- Add per-association uniqueness. Excludes 'platform' (whose rows are id-keyed
+-- mirrors of accounting.accounts and may carry blank/duplicate codes), matching the
+-- old index's exclusion. Also validates the importer's existing onConflict target.
+create unique index chart_of_accounts_assoc_number_unique
+  on public.chart_of_accounts (association_id, account_number)
+  where accounting_system <> 'platform';
+
+-- Verify (raises -> rolls back the whole migration on any inconsistency).
+do $$
+declare _n int; _bad int;
+begin
+  select count(*) into _n from public.chart_of_accounts where accounting_system='buildium';
+  if _n <> 370 then raise exception 'buildium row count % <> expected 370', _n; end if;
+
+  if exists (select 1 from public.chart_of_accounts where accounting_system='buildium' and array_length(association_ids,1) <> 1) then
+    raise exception 'buildium rows with non-singleton association_ids remain';
+  end if;
+
+  select count(*) into _bad from (
+    select x.association_id ra, c.association_id ca from public.bills x join public.chart_of_accounts c on c.id=x.expense_account_id where c.accounting_system='buildium'
+    union all select x.association_id, c.association_id from public.budgets x join public.chart_of_accounts c on c.id=x.gl_account_id where c.accounting_system='buildium'
+    union all select x.association_id, c.association_id from public.owner_ledger_entries x join public.chart_of_accounts c on c.id=x.gl_account_id where c.accounting_system='buildium'
+    union all select x.association_id, c.association_id from public.budget_actuals_monthly x join public.chart_of_accounts c on c.id=x.gl_account_id where c.accounting_system='buildium'
+    union all select x.association_id, c.association_id from public.vendor_coa_mappings x join public.chart_of_accounts c on c.id=x.chart_of_accounts_id where c.accounting_system='buildium'
+    union all select x.association_id, c.association_id from public.units x join public.chart_of_accounts c on c.id=x.assessment_account_id where c.accounting_system='buildium'
+    union all select x.association_id, c.association_id from public.vendors x join public.chart_of_accounts c on c.id=x.default_expense_account_id where c.accounting_system='buildium'
+    union all select x.association_id, c.association_id from public.journal_entries x join public.chart_of_accounts c on c.id=x.chart_of_account_id where c.accounting_system='buildium'
+  ) t where ra is distinct from ca;
+  if _bad > 0 then raise exception '% references point to a different-association COA row', _bad; end if;
+
+  select count(*) into _bad from public.chart_of_accounts c
+    join public.chart_of_accounts p on p.id=c.parent_account_id
+    where c.accounting_system='buildium' and p.association_id is distinct from c.association_id;
+  if _bad > 0 then raise exception '% buildium rows have a cross-association parent', _bad; end if;
+end $$;