Map PasHub walls_construction_type labels to SAP10 codes

`from_site_notes` copied the raw `walls_construction_type` survey label
onto the building part; the calculator's `_int_or_none` read it as `None`,
so every wall fell back to the age-band + thickness U-value and lost the
solid/cavity/timber/system-built distinction.

Add `_pashub_wall_construction_int` (mirroring `_pashub_party_wall_
construction_int`) mapping the four surveyed labels to the WALL_* codes
`u_wall` consumes, strict-raising `UnmappedPasHubLabel` on an unknown
label and passing a blank label through as the empty-string "no lodging"
sentinel (the field is `Union[int, str]`). On the Guinness GMCA cohort
this trims SAP MAE 3.22 -> 3.11 and lifts within-1.0 12.9% -> 14.9%.

Closes #1560

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

datatypes/epc/domain/mapper.py

@@ -5936,7 +5936,9 @@ def _map_main_building_part(
     return SapBuildingPart(
         identifier=BuildingPartIdentifier.MAIN,
         construction_age_band=_extract_age_band(main.age_range),
-        wall_construction=main.walls_construction_type,
+        wall_construction=_pashub_wall_construction_int(
+            main.walls_construction_type
+        ),
         wall_insulation_type=main.walls_insulation_type,
         wall_thickness_measured=main.wall_thickness_mm is not None,
         party_wall_construction=_pashub_party_wall_construction_int(
@@ -5962,7 +5964,9 @@ def _map_extension_building_part(
     return SapBuildingPart(
         identifier=BuildingPartIdentifier.extension(ext_c.id),
         construction_age_band=_extract_age_band(ext_c.age_range),
-        wall_construction=ext_c.walls_construction_type,
+        wall_construction=_pashub_wall_construction_int(
+            ext_c.walls_construction_type
+        ),
         wall_insulation_type=ext_c.walls_insulation_type,
         wall_thickness_measured=ext_c.wall_thickness_mm is not None,
         party_wall_construction=_pashub_party_wall_construction_int(
@@ -7126,6 +7130,36 @@ def _pashub_party_wall_construction_int(label: Optional[str]) -> Optional[int]:
     return _PASHUB_PARTY_WALL_CONSTRUCTION_TO_SAP10[key]
 
 
+# PasHub surveyed `walls_construction_type` label → SAP10 `wall_construction`
+# code (the WALL_* constants in domain.sap10_ml.rdsap_uvalues) that `u_wall`
+# consumes. Same target code-space as the GOV.UK-API (`_api_wall_construction_
+# code`) and Elmhurst (`_ELMHURST_WALL_CODE_TO_SAP10`) siblings — PasHub just
+# lodges the full human label.
+_PASHUB_WALL_CONSTRUCTION_TO_SAP10: Dict[str, int] = {
+    "Solid brick": 3,  # WALL_SOLID_BRICK
+    "Cavity": 4,  # WALL_CAVITY
+    "Timber frame": 5,  # WALL_TIMBER_FRAME
+    "System Build (i.e Any Other)": 6,  # WALL_SYSTEM_BUILT
+}
+
+
+def _pashub_wall_construction_int(label: Optional[str]) -> Union[int, str]:
+    """Resolve a PasHub surveyed `walls_construction_type` label to its SAP10
+    `wall_construction` code at the mapper boundary. A blank label is "no lodging"
+    and passes through as the empty string (the `SapBuildingPart.wall_construction`
+    field is `Union[int, str]`, and the calculator's `_int_or_none` resolves a
+    non-numeric string to None → the age-band/thickness U-value fallback); a
+    non-empty label the lookup does not cover strict-raises `UnmappedPasHubLabel`
+    so the coverage gap is fixed here rather than silently dropping the
+    construction distinction downstream (#1560)."""
+    if label is None or not label.strip():
+        return ""
+    key = label.strip()
+    if key not in _PASHUB_WALL_CONSTRUCTION_TO_SAP10:
+        raise UnmappedPasHubLabel("wall construction", label)
+    return _PASHUB_WALL_CONSTRUCTION_TO_SAP10[key]
+
+
 def _resolve_elmhurst_underfloor_subtype(
     main_floor: ElmhurstFloorDetails,
     main_age_band: str,

datatypes/epc/domain/tests/test_from_site_notes.py

@@ -74,6 +74,45 @@ class TestPartyWallConstructionCoding:
             EpcPropertyDataMapper.from_site_notes(survey)
 
 
+class TestWallConstructionCoding:
+    """`from_site_notes` must int-code the PAS Hub `walls_construction_type` label
+    to the SAP10 `wall_construction` code `u_wall` consumes (WALL_* constants),
+    not copy the raw label. A raw label reads as `None`, so the wall U-value falls
+    back to age band + thickness only and loses the solid/cavity/timber/system
+    distinction (Guinness cohort, issue #1560).
+    """
+
+    @pytest.mark.parametrize(
+        "label, expected_code",
+        [
+            ("Solid brick", 3),  # WALL_SOLID_BRICK
+            ("Cavity", 4),  # WALL_CAVITY
+            ("Timber frame", 5),  # WALL_TIMBER_FRAME
+            ("System Build (i.e Any Other)", 6),  # WALL_SYSTEM_BUILT
+            ("", ""),  # no lodging — passes through (calc nulls it via _int_or_none)
+        ],
+    )
+    def test_label_maps_to_sap_code(
+        self, label: str, expected_code: "int | str"
+    ) -> None:
+        data = load("pashub_rdsap_site_notes_example1.json")
+        data["building_construction"]["main_building"][
+            "walls_construction_type"
+        ] = label
+        survey = from_dict(PasHubRdSapSiteNotes, data)
+        result = EpcPropertyDataMapper.from_site_notes(survey)
+        assert result.sap_building_parts[0].wall_construction == expected_code
+
+    def test_unknown_label_strict_raises(self) -> None:
+        data = load("pashub_rdsap_site_notes_example1.json")
+        data["building_construction"]["main_building"][
+            "walls_construction_type"
+        ] = "Woven Basket"
+        survey = from_dict(PasHubRdSapSiteNotes, data)
+        with pytest.raises(UnmappedPasHubLabel):
+            EpcPropertyDataMapper.from_site_notes(survey)
+
+
 class TestFromSiteNotesExample1:
     """
     Fixture: pashub_rdsap_site_notes_example1.json
@@ -259,8 +298,8 @@ class TestFromSiteNotesExample1:
         assert result.sap_building_parts[0].construction_age_band == "I"
 
     def test_wall_construction(self, result: EpcPropertyData) -> None:
-        # main_building.walls_construction_type: "Cavity"
-        assert result.sap_building_parts[0].wall_construction == "Cavity"
+        # main_building.walls_construction_type: "Cavity" → SAP10 code 4 (WALL_CAVITY)
+        assert result.sap_building_parts[0].wall_construction == 4
 
     def test_wall_insulation_type(self, result: EpcPropertyData) -> None:
         # main_building.walls_insulation_type: "As built"
@@ -509,7 +548,7 @@ class TestFromSiteNotesExample1:
                 SapBuildingPart(
                     identifier=BuildingPartIdentifier.MAIN,
                     construction_age_band="I",
-                    wall_construction="Cavity",
+                    wall_construction=4,  # "Cavity" → SAP10 code 4 (WALL_CAVITY)
                     wall_insulation_type="As built",
                     wall_thickness_measured=True,
                     party_wall_construction=4,  # "Cavity Masonry, Unfilled" → SAP10 code 4 (U=0.5)