diff --git a/tradein-mvp/backend/app/services/matching/houses.py b/tradein-mvp/backend/app/services/matching/houses.py
index 87d78c84..64a1fa67 100644
--- a/tradein-mvp/backend/app/services/matching/houses.py
+++ b/tradein-mvp/backend/app/services/matching/houses.py
@@ -34,11 +34,11 @@ def match_or_create_house(
 ) -> tuple[int, float, str]:
     """Match existing house or create new canonical record.
 
-    ⚠️ KNOWN LIMITATION: Concurrent calls for the same address can race —
-    two scrapers calling simultaneously for an unknown address can both
-    miss Tier 0-3 and each INSERT a new house row. Mitigation (Stage 8.x):
-    gate scraper concurrency to 1 per fingerprint OR add
-    pg_advisory_xact_lock(hashtext(normalized_address)) at function entry.
+    Concurrency-safe: serializes concurrent calls for the same address fingerprint
+    via pg_advisory_xact_lock(42, hashtext(fp)). The lock is transaction-scoped,
+    released on the caller's COMMIT/ROLLBACK. Without this, two scrapers calling
+    for an unknown address could both miss Tier 0-3 and each INSERT a duplicate
+    house row. Closes finding #1 from 2026-05-24 audit.
 
     Returns:
         (house_id, confidence ∈ [0.0, 1.0], method ∈ {
@@ -54,6 +54,18 @@ def match_or_create_house(
     """
     cad = building_cadastral_number or cadastral_number
 
+    # Compute fingerprint early so we can acquire the advisory lock before any tier reads.
+    fp = address_fingerprint(address, lat, lon)
+
+    # Serialize concurrent calls for the same address fingerprint to prevent
+    # racing INSERTs into houses (finding #1 from 2026-05-24 audit).
+    # pg_advisory_xact_lock takes (int4, int4) — namespace 42 = "tradein house matching".
+    # Lock is released automatically on caller's COMMIT/ROLLBACK.
+    db.execute(
+        text('SELECT pg_advisory_xact_lock(42, hashtext(:fp))'),
+        {'fp': fp},
+    )
+
     # Tier 0: cadastral number exact match
     if cad:
         row = db.execute(
@@ -83,9 +95,6 @@ def match_or_create_house(
                     ext_id)
         return (house_id, 1.0, 'source_exact')
 
-    # Compute fingerprint once for Tier 2 and reuse for new house registration
-    fp = address_fingerprint(address, lat, lon)
-
     # Tier 2: address fingerprint lookup
     row = db.execute(
         text(