diff --git a/docs/migrations/epc-building-part-fabric-overrides.md b/docs/migrations/epc-building-part-fabric-overrides.md
index a29a4d9f8..805265f90 100644
--- a/docs/migrations/epc-building-part-fabric-overrides.md
+++ b/docs/migrations/epc-building-part-fabric-overrides.md
@@ -1,4 +1,4 @@
-# Migration note — `epc_building_part.rafter_insulation_thickness`
+# Migration note — `epc_building_part` RdSAP construction inputs
 
 **For the team to action alongside the model-side change (Hestia-Homes/Model#1656).**
 The Python side is done and tested against the SQLModel-built test DB; the
@@ -45,6 +45,7 @@ Additive, nullable, **no backfill possible** (the value was never stored):
 | Column | Type | Null | Notes |
 |---|---|---|---|
 | `rafter_insulation_thickness` | `jsonb` | yes | `Union[str, int]` — `"250mm"` from the API, int mm from Site Notes. **JSONB**, matching the existing `roof_insulation_thickness` / `flat_roof_insulation_thickness` / `wall_insulation_thickness` columns, so `int` vs `str` survives the round-trip. |
+| `wall_is_basement` | `boolean` | **yes — load-bearing** | Must be nullable, **not** `NOT NULL DEFAULT false`. `null` = "not stated, fall back to the gov-API code-6 heuristic"; `false` = "explicitly system-built, do NOT apply the heuristic". Collapsing `false` to `null` inverts the result. |
 
 Generate with `npm run migration:generate`; run it **before** deploying the
 backend code.
@@ -57,18 +58,45 @@ moves; the migration alone changes no existing baseline. Re-run **Baseline
 (stage 2) before Modelling (stage 3)** so both stages score on one calculator
 version.
 
+## The second column — `wall_is_basement`
+
+RdSAP 10 §5.17 / Table 23. It is **not** a lodged U-value: it is a
+disambiguation flag selecting *which* RdSAP default applies, so it belongs with
+`rafter_insulation_thickness`, not with the U-values below.
+
+RdSAP code 6 is canonically **system-built**, and the gov-API path infers
+"basement" from a code-6 heuristic. The Elmhurst site-notes mapper sets
+`wall_is_basement=False` precisely to defeat that heuristic. Dropping the field
+does not merely lose data — it **inverts** the answer:
+
+```
+wall_construction = 6 (system-built)
+  Elmhurst sets wall_is_basement=False  -> main_wall_is_basement = False
+  after DB round-trip (dropped -> None) -> main_wall_is_basement = True   <- flipped
+```
+
+A system-built wall then bills as a basement wall, and via `has_basement` drags
+the whole ground floor onto the Table 23 basement-floor U-value.
+
+The field is declared on **no** RdSAP schema, so it never arrives via the gov
+API — but the Elmhurst site-notes path sets it, and site-notes EPCs **are**
+persisted (`scripts/wchg_elmhurst_ingest.py`). A corpus scan of gov-API certs
+shows 0 occurrences and is therefore the wrong population to measure it on.
+
+Rare but real: `main_wall_is_basement` is documented at 54 of 67k parts in the
+2026 sweep.
+
 ## Deliberately NOT persisted
 
-The same audit found four sibling `SapBuildingPart` fields with the same drop —
-`wall_u_value`, `roof_u_value`, `floor_u_value`, `wall_is_basement`. They are
+The same audit found three sibling `SapBuildingPart` fields with the same drop —
+`wall_u_value`, `roof_u_value`, `floor_u_value`. They are
 **intentionally left unpersisted**: they are full-SAP artefacts, and we re-model
 every dwelling *as* an RdSAP assessment, which derives U-values from the
 construction-default cascade (RdSAP 10 §5.6/§5.7/§5.11) rather than honouring an
 assessor's measured value. Carrying them would make the re-model a hybrid.
 
 Population on the committed RdSAP-21.0.1 corpus (1,000 certs) confirms it:
-`roof_u_value` **0**, `floor_u_value` **0**, `wall_is_basement` **0**,
-`wall_u_value` **4 (0.4%)**.
+`roof_u_value` **0**, `floor_u_value` **0**, `wall_u_value` **4 (0.4%)**.
 
 They stay on the `_UNPERSISTED_ALLOWLIST` in
 `tests/repositories/epc/test_epc_persistence_field_coverage.py`, but with a
@@ -97,7 +125,9 @@ under `domain/sap10_calculator/`.
 - `repositories/epc/epc_postgres_repository.py` — `_to_building_part`
   reconstructs it.
 - `tests/repositories/epc/test_epc_round_trip.py` —
-  `test_rafter_insulation_thickness_round_trips` pins it deep-equal.
-- `tests/repositories/epc/test_epc_persistence_field_coverage.py` — the four
-  U-value entries re-justified; the `rafter_insulation_thickness` entry removed
-  so the guard now *enforces* its reconstruction.
+  `test_rafter_insulation_thickness_round_trips` pins it deep-equal;
+  `test_wall_is_basement_round_trips` pins the code-6 inversion case.
+- `tests/repositories/epc/test_epc_persistence_field_coverage.py` — the three
+  U-value entries re-justified; the `rafter_insulation_thickness` and
+  `wall_is_basement` entries removed so the guard now *enforces* their
+  reconstruction.
diff --git a/infrastructure/postgres/epc_property_table.py b/infrastructure/postgres/epc_property_table.py
index d00ed184c..d2c2de654 100644
--- a/infrastructure/postgres/epc_property_table.py
+++ b/infrastructure/postgres/epc_property_table.py
@@ -660,6 +660,14 @@ class EpcBuildingPartModel(SQLModel, table=True):
     rafter_insulation_thickness: Optional[Union[str, int]] = Field(
         default=None, sa_column=Column(JSONB, nullable=True)
     )
+    # RdSAP 10 §5.17 / Table 23 — selects the basement-wall U-value column for
+    # the part's primary wall (`heat_transmission.py:955` via
+    # `SapBuildingPart.main_wall_is_basement`). NOT a lodged U-value: it picks
+    # WHICH RdSAP default applies, so unlike wall/roof/floor_u_value it must be
+    # persisted. Nullable is load-bearing — `None` means "not stated, fall back
+    # to the gov-API code-6 heuristic", `False` means "explicitly system-built,
+    # do NOT apply the heuristic". Collapsing False to None inverts the answer.
+    wall_is_basement: Optional[bool] = Field(default=None)
     room_in_roof_floor_area: Optional[float] = Field(default=None)
     room_in_roof_construction_age_band: Optional[str] = Field(default=None)
     alt_wall_1_area: Optional[float] = Field(default=None)
@@ -710,6 +718,7 @@ class EpcBuildingPartModel(SQLModel, table=True):
             roof_insulation_location=part.roof_insulation_location,
             roof_insulation_thickness=part.roof_insulation_thickness,
             rafter_insulation_thickness=part.rafter_insulation_thickness,
+            wall_is_basement=part.wall_is_basement,
             room_in_roof_floor_area=float(rir.floor_area) if rir else None,
             room_in_roof_construction_age_band=(
                 rir.construction_age_band if rir else None
diff --git a/repositories/epc/epc_postgres_repository.py b/repositories/epc/epc_postgres_repository.py
index 8afe946a7..caa038e4e 100644
--- a/repositories/epc/epc_postgres_repository.py
+++ b/repositories/epc/epc_postgres_repository.py
@@ -1004,6 +1004,7 @@ class EpcPostgresRepository(EpcRepository):
             roof_insulation_location=bp.roof_insulation_location,
             roof_insulation_thickness=bp.roof_insulation_thickness,
             rafter_insulation_thickness=bp.rafter_insulation_thickness,
+            wall_is_basement=bp.wall_is_basement,
             sap_room_in_roof=(
                 SapRoomInRoof(
                     floor_area=bp.room_in_roof_floor_area,
diff --git a/tests/repositories/epc/test_epc_persistence_field_coverage.py b/tests/repositories/epc/test_epc_persistence_field_coverage.py
index a44ee4fdf..a3f12bb12 100644
--- a/tests/repositories/epc/test_epc_persistence_field_coverage.py
+++ b/tests/repositories/epc/test_epc_persistence_field_coverage.py
@@ -72,11 +72,14 @@ _UNPERSISTED_ALLOWLIST: dict[str, str] = {
     # (RdSAP 10 §5.6/§5.7/§5.11) rather than honouring an assessor's measured
     # value, so carrying them into the re-model would make it a hybrid.
     # Measured on the committed RdSAP-21.0.1 corpus (1000 certs): roof_u_value
-    # 0, floor_u_value 0, wall_is_basement 0, wall_u_value 4 (0.4%).
+    # 0, floor_u_value 0, wall_u_value 4 (0.4%).
+    #
+    # `wall_is_basement` is deliberately NOT in this list — it is a
+    # disambiguation flag selecting which RdSAP default applies (§5.17/Table 23),
+    # not a measured U-value, and it IS persisted.
     "SapBuildingPart.wall_u_value": "deliberate — full-SAP only; RdSAP re-model uses default U-values (#1656)",
     "SapBuildingPart.roof_u_value": "deliberate — full-SAP only; RdSAP re-model uses default U-values (#1656)",
     "SapBuildingPart.floor_u_value": "deliberate — full-SAP only; RdSAP re-model uses default U-values (#1656)",
-    "SapBuildingPart.wall_is_basement": "deliberate — full-SAP only; RdSAP re-model uses default U-values (#1656)",
 }
 
 
diff --git a/tests/repositories/epc/test_epc_round_trip.py b/tests/repositories/epc/test_epc_round_trip.py
index d18a2bc0d..0cacee2aa 100644
--- a/tests/repositories/epc/test_epc_round_trip.py
+++ b/tests/repositories/epc/test_epc_round_trip.py
@@ -282,3 +282,50 @@ def test_rafter_insulation_thickness_round_trips(db_engine: Engine) -> None:
 
     # Assert — the measured rafter thickness survives, deep-equal.
     assert reloaded == original
+
+
+def test_wall_is_basement_round_trips(db_engine: Engine) -> None:
+    # RdSAP 10 §5.17 / Table 23 — `wall_is_basement` decides whether a part's
+    # primary wall bills on the basement-wall column (and, via `has_basement`,
+    # drags the whole ground floor onto the Table 23 basement-floor U-value).
+    #
+    # It is NOT a lodged U-value: it is a disambiguation flag that selects WHICH
+    # RdSAP default applies, so unlike wall/roof/floor_u_value it must survive
+    # the round-trip. The gov API never lodges it (declared on no RdSAP schema);
+    # the Elmhurst site-notes mapper sets it, and site-notes EPCs ARE persisted
+    # (scripts/wchg_elmhurst_ingest.py).
+    #
+    # Dropping it is worse than lossy, it INVERTS: RdSAP code 6 is canonically
+    # SYSTEM-BUILT, and the API path infers "basement" from a code-6 heuristic.
+    # Elmhurst sets `wall_is_basement=False` precisely to defeat that heuristic.
+    # Round-tripping False -> None silently re-enables it, so a system-built wall
+    # comes back as a basement wall.
+    from dataclasses import replace
+
+    from datatypes.epc.domain.epc_property_data import (
+        BASEMENT_WALL_CONSTRUCTION_CODE,
+    )
+
+    # Arrange — a system-built (code 6) main wall explicitly flagged NOT basement.
+    original = _load_epc("RdSAP-Schema-21.0.1")
+    assert original.sap_building_parts, "fixture must have a building part"
+    bp0 = replace(
+        original.sap_building_parts[0],
+        wall_construction=BASEMENT_WALL_CONSTRUCTION_CODE,
+        wall_is_basement=False,
+    )
+    original = replace(
+        original, sap_building_parts=[bp0, *original.sap_building_parts[1:]]
+    )
+    assert original.sap_building_parts[0].main_wall_is_basement is False
+
+    # Act
+    with Session(db_engine) as session:
+        epc_property_id = EpcPostgresRepository(session).save(original)
+        session.commit()
+    with Session(db_engine) as session:
+        reloaded = EpcPostgresRepository(session).get(epc_property_id)
+
+    # Assert — the wall is still system-built, not silently re-read as basement.
+    assert reloaded is not None
+    assert reloaded.sap_building_parts[0].main_wall_is_basement is False