mapper: disambiguate SY system-built from B basement wall (both share code 6)

RdSAP10 `wall_construction == 6` is canonically WALL_SYSTEM_BUILT, but
the gov-EPC basement heuristic hijacked it: Elmhurst lodges both "SY
System build" and "B Basement wall" as code 6, so a system-built wall
was mis-flagged `main_wall_is_basement` and routed to the RdSAP §5.17
`u_basement_wall` override instead of the system-built U-value table.

System-built stays on its canonical code 6; the basement signal moves
to an explicit `is_basement` (SapAlternativeWall) / `wall_is_basement`
(SapBuildingPart) Optional[bool] flag, set by the Elmhurst mapper from
the distinct "SY"/"B" codes via `_elmhurst_wall_is_basement` (True for
B, False for SY, None otherwise). The `main_wall_is_basement` /
`is_basement_wall` properties honour the flag when set and fall back to
the gov-EPC API code-6 heuristic when None — so the API path (basement
lodged as integer 6, no flag) and the cert 000565 "B" cohort are
unchanged.

Acceptance (a recommendation-summary generator depends on it): a
system-built MAIN wall reports wall_construction == 6 AND
main_wall_is_basement is False; a genuine basement main wall still
reports main_wall_is_basement is True.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/documents_parser/tests/test_summary_pdf_mapper_chain.py

@@ -4359,3 +4359,24 @@ def test_elmhurst_foam_cylinder_insulation_still_maps_to_factory_code_1() -> Non
 
     # Assert
     assert code == 1
+
+
+def test_elmhurst_wall_is_basement_disambiguates_system_built_from_basement() -> None:
+    # Arrange — "SY System build" and "B Basement wall" both map to SAP10
+    # wall_construction=6 (canonical WALL_SYSTEM_BUILT). The explicit
+    # basement flag separates them: only "B" is a basement wall (drives
+    # RdSAP §5.17 u_basement_wall); "SY" is False so it routes through the
+    # normal system-built U-value table; any other code → None (the
+    # gov-EPC API code-6 heuristic still applies).
+    from datatypes.epc.domain.mapper import _elmhurst_wall_construction_int  # pyright: ignore[reportPrivateUsage]
+    from datatypes.epc.domain.mapper import _elmhurst_wall_is_basement  # pyright: ignore[reportPrivateUsage]
+
+    # Act / Assert — system-built keeps code 6 but is NOT basement.
+    assert _elmhurst_wall_construction_int("SY System build") == 6
+    assert _elmhurst_wall_is_basement("SY System build") is False
+    # Genuine basement: code 6 AND flagged basement.
+    assert _elmhurst_wall_construction_int("B Basement wall") == 6
+    assert _elmhurst_wall_is_basement("B Basement wall") is True
+    # Other constructions defer to the API code-6 heuristic.
+    assert _elmhurst_wall_is_basement("CA Cavity") is None
+    assert _elmhurst_wall_is_basement("") is None

datatypes/epc/domain/epc_property_data.py

@@ -435,13 +435,24 @@ class SapAlternativeWall:
     # Mirrors `SapBuildingPart.wall_thickness_mm` per the
     # [[feedback-no-misleading-insulation-type]] convention.
     wall_thickness_mm: Optional[int] = None
+    # Explicit basement determination. RdSAP10 `wall_construction == 6` is
+    # canonically SYSTEM-BUILT (`WALL_SYSTEM_BUILT`) — the basement
+    # heuristic hijacked it because Elmhurst lodges both "SY System build"
+    # and "B Basement wall" as code 6. When the source can tell them apart
+    # (the Elmhurst mapper, from the distinct "SY"/"B" codes) it sets this
+    # flag; None falls back to the gov-EPC API code-6 heuristic so the API
+    # path (basement lodged as integer 6, no flag) is unchanged.
+    is_basement: Optional[bool] = None
 
     @property
     def is_basement_wall(self) -> bool:
-        """True iff this alt sub-area is the dwelling's basement wall —
-        identified by RdSAP10 wall_construction code = 6 (see module
-        constant `BASEMENT_WALL_CONSTRUCTION_CODE`). RdSAP §5.17 / Table 23
-        applies a special U-value lookup to basement walls."""
+        """True iff this alt sub-area is the dwelling's basement wall.
+        Honours the explicit `is_basement` flag when set; otherwise falls
+        back to the gov-EPC API basement sentinel `wall_construction == 6`
+        (`BASEMENT_WALL_CONSTRUCTION_CODE`). RdSAP §5.17 / Table 23 applies
+        a special U-value lookup to basement walls."""
+        if self.is_basement is not None:
+            return self.is_basement
         return self.wall_construction == BASEMENT_WALL_CONSTRUCTION_CODE
 
 
@@ -514,12 +525,23 @@ class SapBuildingPart:
     # The dwelling-wide `construction_age_band` does NOT govern curtain
     # walls; this field decouples them per spec.
     curtain_wall_age: Optional[str] = None
+    # Explicit basement determination for the primary wall. See
+    # `SapAlternativeWall.is_basement` — RdSAP10 code 6 is canonically
+    # SYSTEM-BUILT, so the Elmhurst mapper sets this flag from the distinct
+    # "SY"/"B" codes (False for system-built, True for basement); None
+    # preserves the gov-EPC API code-6 heuristic.
+    wall_is_basement: Optional[bool] = None
 
     @property
     def main_wall_is_basement(self) -> bool:
         """True iff this part's primary wall (not an alt sub-area) is the
         basement wall — happens when the whole part sits below grade.
-        Empirically 54 of 67k parts in the 2026 sweep; rare but real."""
+        Empirically 54 of 67k parts in the 2026 sweep; rare but real.
+        Honours the explicit `wall_is_basement` flag when set (so a
+        SYSTEM-BUILT wall, which shares code 6, is not mis-flagged);
+        otherwise falls back to the gov-EPC API code-6 heuristic."""
+        if self.wall_is_basement is not None:
+            return self.wall_is_basement
         return self.wall_construction == BASEMENT_WALL_CONSTRUCTION_CODE
 
     @property

datatypes/epc/domain/mapper.py

@@ -2172,16 +2172,16 @@ _ELMHURST_WALL_CODE_TO_SAP10: Dict[str, int] = {
     "CA": 4,   # Cavity
     "TF": 5,   # Timber frame
     "TI": 5,   # Timber frame (Elmhurst's alt-wall code; same SAP10 mapping)
-    "SY": 6,   # System build
-    "B":  6,   # Basement wall (cert 000565 Ext3+Ext4) — routes to the
-               # `BASEMENT_WALL_CONSTRUCTION_CODE=6` canonical signal so
-               # the cascade's `part.main_wall_is_basement` triggers the
-               # RdSAP 10 §5.17 / Table 23 `u_basement_wall` override
-               # (heat_transmission.py:640). Collides numerically with
-               # "SY" System build — the cascade's basement check
-               # precedes `u_wall(construction=6)` so SY would be
-               # silently mis-routed to u_basement_wall today; no cohort
-               # fixture exercises SY yet so the conflict is dormant.
+    "SY": 6,   # System build — canonical RdSAP10 WALL_SYSTEM_BUILT=6.
+    "B":  6,   # Basement wall (cert 000565 Ext3+Ext4). Numerically
+               # collides with "SY" System build on code 6, so the
+               # basement vs system-built distinction is carried by the
+               # explicit `is_basement` / `wall_is_basement` flag (set
+               # via `_elmhurst_wall_is_basement`) rather than the code:
+               # only "B" triggers the cascade's `main_wall_is_basement`
+               # → RdSAP 10 §5.17 / Table 23 `u_basement_wall` override.
+               # "SY" sets the flag False so it routes through the normal
+               # `u_wall(construction=6)` system-built table instead.
     "CO": 7,   # Cob
     "PH": 8,   # Park home
     "CW": 9,   # Curtain wall
@@ -2263,6 +2263,32 @@ def _elmhurst_wall_construction_int(coded: str) -> Optional[int]:
     return _ELMHURST_WALL_CODE_TO_SAP10[code]
 
 
+# Elmhurst wall codes that both resolve to SAP10 wall_construction=6 but
+# carry opposite basement meaning: "B" Basement wall vs "SY" System build
+# (see `_ELMHURST_WALL_CODE_TO_SAP10`). RdSAP10 code 6 is canonically
+# WALL_SYSTEM_BUILT; the explicit basement flag lets the cascade route a
+# genuine basement to RdSAP §5.17 `u_basement_wall` without mis-flagging
+# a system-built wall.
+_ELMHURST_BASEMENT_WALL_CODE: Final[str] = "B"
+_ELMHURST_SYSTEM_BUILT_WALL_CODE: Final[str] = "SY"
+
+
+def _elmhurst_wall_is_basement(coded: str) -> Optional[bool]:
+    """Disambiguate the SAP10 code-6 collision from the Elmhurst wall_type
+    string. Returns True for "B Basement wall", False for "SY System
+    build", and None for every other code (so the SapBuildingPart /
+    SapAlternativeWall properties fall back to the gov-EPC API code-6
+    heuristic — unchanged for the API path). Empty lodging → None."""
+    code = _leading_code(coded)
+    if not code:
+        return None
+    if code == _ELMHURST_BASEMENT_WALL_CODE:
+        return True
+    if code == _ELMHURST_SYSTEM_BUILT_WALL_CODE:
+        return False
+    return None
+
+
 # Elmhurst Party Wall Type codes — distinct category-set from the Wall
 # Type field; the codes describe construction class for `u_party_wall`
 # (Table 4 / RdSAP §S.3.2) rather than a specific SAP10 wall-type. Maps
@@ -3385,6 +3411,7 @@ def _map_elmhurst_building_part(
         identifier=identifier,
         construction_age_band=age_code,
         wall_construction=_elmhurst_wall_construction_int(walls.wall_type),
+        wall_is_basement=_elmhurst_wall_is_basement(walls.wall_type),
         wall_insulation_type=_elmhurst_wall_insulation_int(walls.insulation),
         wall_thickness_measured=not walls.thickness_unknown,
         party_wall_construction=_elmhurst_party_wall_construction_int(walls.party_wall_type),
@@ -3463,6 +3490,7 @@ def _map_elmhurst_alternative_wall(
         wall_thickness_measured="Y" if not a.thickness_unknown else "N",
         wall_insulation_thickness=None,
         wall_thickness_mm=measured_thickness_mm,
+        is_basement=_elmhurst_wall_is_basement(a.wall_type),
     )
 
 

tests/domain/sap10_calculator/worksheet/test_heat_transmission.py

@@ -1222,6 +1222,58 @@ def test_sap_building_part_has_basement_detects_main_wall_and_alt_wall_codes() -
     assert alt_is_basement.main_wall_is_basement is False  # main is still wc=4
 
 
+def test_explicit_wall_is_basement_flag_disambiguates_system_built_from_basement() -> None:
+    """RdSAP10 `wall_construction == 6` is canonically SYSTEM-BUILT
+    (`WALL_SYSTEM_BUILT`), but the gov-EPC basement heuristic hijacked it
+    (Elmhurst lodges both "SY System build" and "B Basement wall" as
+    code 6). The explicit `wall_is_basement` flag — set by the Elmhurst
+    mapper from the distinct "SY"/"B" codes — disambiguates:
+        - flag True  → basement (drives §5.17 u_basement_wall)
+        - flag False → system-built (drives the u_wall code-6 table)
+        - flag None  → fall back to the gov-EPC API code-6 heuristic
+    so the API path (which lodges basement as integer 6 with no flag) is
+    unchanged."""
+    from dataclasses import replace
+    # Arrange — three parts, all wall_construction=6, differing only in flag.
+    plain = make_building_part(
+        identifier=BuildingPartIdentifier.MAIN,
+        construction_age_band="G",
+        wall_construction=6, wall_insulation_type=4,
+        party_wall_construction=1, roof_construction=4,
+        floor_dimensions=[
+            make_floor_dimension(
+                total_floor_area_m2=80.0, room_height_m=2.5,
+                party_wall_length_m=0.0, heat_loss_perimeter_m=35.0, floor=0,
+            ),
+        ],
+    )
+    system_built = replace(plain, wall_is_basement=False)
+    basement = replace(plain, wall_is_basement=True)
+    api_code_6 = replace(plain, wall_is_basement=None)
+
+    # Act / Assert
+    assert system_built.main_wall_is_basement is False
+    assert basement.main_wall_is_basement is True
+    assert api_code_6.main_wall_is_basement is True  # API heuristic preserved
+
+    # Alt-wall mirror — same Optional disambiguation on SapAlternativeWall.
+    alt_system_built = SapAlternativeWall(
+        wall_area=14.24, wall_dry_lined="N", wall_construction=6,
+        wall_insulation_type=4, wall_thickness_measured="Y", is_basement=False,
+    )
+    alt_basement = SapAlternativeWall(
+        wall_area=14.24, wall_dry_lined="N", wall_construction=6,
+        wall_insulation_type=4, wall_thickness_measured="Y", is_basement=True,
+    )
+    alt_api_code_6 = SapAlternativeWall(
+        wall_area=14.24, wall_dry_lined="N", wall_construction=6,
+        wall_insulation_type=4, wall_thickness_measured="Y",
+    )
+    assert alt_system_built.is_basement_wall is False
+    assert alt_basement.is_basement_wall is True
+    assert alt_api_code_6.is_basement_wall is True
+
+
 def test_basement_alt_wall_uses_table_23_u_value_not_cascade() -> None:
     """RdSAP §5.17 / Table 23 governs basement-wall U-values: 0.7 for age
     A-F, 0.6 for G-H, 0.45 for I, 0.35 for J, ..., 0.26 for M. The