diff --git a/data/CONTRIBUTING.md b/data/CONTRIBUTING.md
index d7e3585..d7363e1 100644
--- a/data/CONTRIBUTING.md
+++ b/data/CONTRIBUTING.md
@@ -89,3 +89,15 @@ One place per commit is fine. Multiple places of the same type in one commit is
 
 - Run `npm run dev` and verify the pin lands on the correct spot on the map before opening a PR.
 - If adding many places at once, batch by type (one commit per file).
+
+## Adding a new place type (maintainers)
+
+The list of valid place types lives in more than one place. Adding a type #7
+(e.g. a new `<type>.json`) also requires:
+
+1. `src/lib/types.ts` — extend `PLACE_TYPES` (labels/colors derive from it)
+2. `data/places.schema.json` — extend the `type` enum
+3. `.github/ISSUE_TEMPLATE/add-place.yml` — extend the dropdown
+4. A DB migration for the `user_places.type` CHECK constraint
+   (`supabase/migrations/20260822_harden_user_places.sql`) so saved places of
+   the new type pass validation
diff --git a/supabase/migrations/20260822_harden_user_places.sql b/supabase/migrations/20260822_harden_user_places.sql
new file mode 100644
index 0000000..e5e7115
--- /dev/null
+++ b/supabase/migrations/20260822_harden_user_places.sql
@@ -0,0 +1,78 @@
+-- Harden user_places / user_home to match the app contract (#170).
+--
+-- Follows 20260701_create_user_places.sql. Ships as a separate migration so
+-- the checksum of the original file stays stable for databases where it was
+-- already applied.
+--
+-- Notes on the UPDATE policy: `using (auth.uid() = user_id)` with no explicit
+-- WITH CHECK already blocks reassigning a row to another user, because
+-- Postgres reuses the USING expression as the check and evaluates it against
+-- the NEW row. The explicit WITH CHECK below is documentation + insurance:
+-- it keeps that invariant true even if someone later adds a second update
+-- policy or edits this one without re-deriving the semantics.
+--
+-- Constraint deployment (review feedback): ADD CONSTRAINT ... CHECK validates
+-- every existing row under an ACCESS EXCLUSIVE lock, so a single legacy row
+-- with a retired `type` (the imp_locations -> other_places rename predates
+-- this) or an out-of-range coordinate would fail mid-deploy and block
+-- everything behind it. Two mitigations:
+--   1. unknown types are first normalized to 'other_places' (the bucket the
+--      app already uses for anything unmapped);
+--   2. constraints ship as NOT VALID + a separate VALIDATE step: the table is
+--      only briefly locked for catalog changes, and any surviving bad row
+--      surfaces as a clean validation failure instead of a locked-table
+--      timeout.
+
+update user_places
+  set type = 'other_places'
+  where type not in (
+    'library',
+    'other_places',
+    'airport',
+    'sat_centre',
+    'foreign_lang_exam_centre',
+    'gov_offices'
+  );
+
+alter table user_places
+  add constraint user_places_type_check
+  check (type in (
+    'library',
+    'other_places',
+    'airport',
+    'sat_centre',
+    'foreign_lang_exam_centre',
+    'gov_offices'
+  )) not valid;
+
+alter table user_places validate constraint user_places_type_check;
+
+alter table user_places
+  add constraint user_places_lat_check
+  check (lat between -90 and 90) not valid;
+alter table user_places validate constraint user_places_lat_check;
+alter table user_places
+  add constraint user_places_lng_check
+  check (lng between -180 and 180) not valid;
+alter table user_places validate constraint user_places_lng_check;
+
+alter table user_home
+  add constraint user_home_lat_check
+  check (lat between -90 and 90) not valid;
+alter table user_home validate constraint user_home_lat_check;
+alter table user_home
+  add constraint user_home_lng_check
+  check (lng between -180 and 180) not valid;
+alter table user_home validate constraint user_home_lng_check;
+
+drop policy if exists "Users can update their own places" on user_places;
+create policy "Users can update their own places"
+  on user_places for update
+  using (auth.uid() = user_id)
+  with check (auth.uid() = user_id);
+
+drop policy if exists "Users can update their own home" on user_home;
+create policy "Users can update their own home"
+  on user_home for update
+  using (auth.uid() = user_id)
+  with check (auth.uid() = user_id);