diff --git a/infrastructure/postgres/epc_property_table.py b/infrastructure/postgres/epc_property_table.py
index f77128799..c5f20269f 100644
--- a/infrastructure/postgres/epc_property_table.py
+++ b/infrastructure/postgres/epc_property_table.py
@@ -686,6 +686,12 @@ class EpcBuildingPartModel(SQLModel, table=True):
     alt_wall_1_thickness_measured: Optional[str] = Field(default=None)
     alt_wall_1_insulation_thickness: Optional[str] = Field(default=None)
     alt_wall_1_is_sheltered: Optional[bool] = Field(default=None)
+    # #1665: calculator-read alt-wall fields (heat_transmission.py) with no column.
+    alt_wall_1_u_value: Optional[float] = Field(default=None)
+    alt_wall_1_thickness_mm: Optional[int] = Field(default=None)
+    # Nullable is load-bearing: None ("not stated") != False ("not a basement").
+    # A NOT NULL DEFAULT false re-enables the code-6 basement heuristic (cf #1661).
+    alt_wall_1_is_basement: Optional[bool] = Field(default=None)
     alt_wall_2_area: Optional[float] = Field(default=None)
     alt_wall_2_dry_lined: Optional[str] = Field(default=None)
     alt_wall_2_construction: Optional[int] = Field(default=None)
@@ -693,6 +699,9 @@ class EpcBuildingPartModel(SQLModel, table=True):
     alt_wall_2_thickness_measured: Optional[str] = Field(default=None)
     alt_wall_2_insulation_thickness: Optional[str] = Field(default=None)
     alt_wall_2_is_sheltered: Optional[bool] = Field(default=None)
+    alt_wall_2_u_value: Optional[float] = Field(default=None)
+    alt_wall_2_thickness_mm: Optional[int] = Field(default=None)
+    alt_wall_2_is_basement: Optional[bool] = Field(default=None)
 
     @classmethod
     def from_domain(
@@ -741,6 +750,9 @@ class EpcBuildingPartModel(SQLModel, table=True):
                 aw1.wall_insulation_thickness if aw1 else None
             ),
             alt_wall_1_is_sheltered=aw1.is_sheltered if aw1 else None,
+            alt_wall_1_u_value=aw1.u_value if aw1 else None,
+            alt_wall_1_thickness_mm=aw1.wall_thickness_mm if aw1 else None,
+            alt_wall_1_is_basement=aw1.is_basement if aw1 else None,
             alt_wall_2_area=aw2.wall_area if aw2 else None,
             alt_wall_2_dry_lined=aw2.wall_dry_lined if aw2 else None,
             alt_wall_2_construction=aw2.wall_construction if aw2 else None,
@@ -750,6 +762,9 @@ class EpcBuildingPartModel(SQLModel, table=True):
                 aw2.wall_insulation_thickness if aw2 else None
             ),
             alt_wall_2_is_sheltered=aw2.is_sheltered if aw2 else None,
+            alt_wall_2_u_value=aw2.u_value if aw2 else None,
+            alt_wall_2_thickness_mm=aw2.wall_thickness_mm if aw2 else None,
+            alt_wall_2_is_basement=aw2.is_basement if aw2 else None,
         )
 
 
diff --git a/repositories/epc/epc_postgres_repository.py b/repositories/epc/epc_postgres_repository.py
index 81620b069..c633cc542 100644
--- a/repositories/epc/epc_postgres_repository.py
+++ b/repositories/epc/epc_postgres_repository.py
@@ -1044,6 +1044,13 @@ class EpcPostgresRepository(EpcRepository):
             else bp.alt_wall_2_insulation_thickness
         )
         sheltered = bp.alt_wall_1_is_sheltered if n == 1 else bp.alt_wall_2_is_sheltered
+        u_value = bp.alt_wall_1_u_value if n == 1 else bp.alt_wall_2_u_value
+        thickness_mm = (
+            bp.alt_wall_1_thickness_mm if n == 1 else bp.alt_wall_2_thickness_mm
+        )
+        is_basement = (
+            bp.alt_wall_1_is_basement if n == 1 else bp.alt_wall_2_is_basement
+        )
         return SapAlternativeWall(
             wall_area=area,
             wall_dry_lined=_require(dry_lined, f"alt_wall_{n}_dry_lined"),
@@ -1058,6 +1065,9 @@ class EpcPostgresRepository(EpcRepository):
             # Nullable column (added later than the alt wall itself); a row from
             # before the column existed reads None → the domain default False.
             is_sheltered=sheltered if sheltered is not None else False,
+            u_value=u_value,
+            wall_thickness_mm=thickness_mm,
+            is_basement=is_basement,
         )
 
     @private
diff --git a/tests/repositories/epc/test_epc_persistence_field_coverage.py b/tests/repositories/epc/test_epc_persistence_field_coverage.py
index af416983a..631d69707 100644
--- a/tests/repositories/epc/test_epc_persistence_field_coverage.py
+++ b/tests/repositories/epc/test_epc_persistence_field_coverage.py
@@ -45,8 +45,6 @@ _UNPERSISTED_ALLOWLIST: dict[str, str] = {
     "EpcPropertyData.solar_hw_collector_orientation": "FE column pending — tracked round-trip gap",
     "EpcPropertyData.solar_hw_collector_pitch_deg": "FE column pending — tracked round-trip gap",
     "EpcPropertyData.solar_hw_overshading": "FE column pending — tracked round-trip gap",
-    "SapAlternativeWall.u_value": "FE column pending — tracked round-trip gap",
-    "SapAlternativeWall.wall_thickness_mm": "FE column pending — tracked round-trip gap",
     # Room-in-roof detail: only floor_area + age_band reconstruct from the
     # inlined columns; the geometry / surface detail awaits an FE child table
     # (tracked round-trip gap, docs/migrations §2).
@@ -58,8 +56,6 @@ _UNPERSISTED_ALLOWLIST: dict[str, str] = {
     "SapRoomInRoof.gable_2_height_m": "FE child table pending — room-in-roof detail",
     "SapRoomInRoof.gable_2_length_m": "FE child table pending — room-in-roof detail",
     "SapRoomInRoofSurface": "FE child table pending — room-in-roof surface detail",
-    # --- Nested gaps the calculator does NOT read (dormant); no FE column.
-    "SapAlternativeWall.is_basement": "dormant — not read by the calculator; no FE column",
     # --- DELIBERATELY not persisted (#1656). These four ARE read by the
     # calculator (`heat_transmission.py:994` wall U, `:1068` roof U, `:1118`
     # floor U, `:955` basement wall) — do NOT re-label them "not read by the