Resolve a landlord age-band override onto the main building part 🟥

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

domain/epc/property_overlays/construction_age_band_overlay.py

@@ -0,0 +1,22 @@
+"""Map a Landlord-Override construction-age-band value to a fabric Simulation
+Overlay.
+
+A construction-age-band value is the RdSAP England-&-Wales letter code (A..M)
+the calculator's U-value cascades key on (`SapBuildingPart.construction_age_band`,
+read via `.strip().upper()` against the letter-code bands). The overlay targets
+the override's building part and sets the band; an unrecognised code produces no
+overlay. Re-dating a part re-derives its construction-default U-values, so this
+is the highest-leverage fabric override.
+"""
+
+from __future__ import annotations
+
+from typing import Optional
+
+from domain.modelling.simulation import EpcSimulation
+
+
+def age_band_overlay_for(
+    age_band_value: str, building_part: int
+) -> Optional[EpcSimulation]:
+    raise NotImplementedError

domain/modelling/simulation.py

@@ -28,6 +28,12 @@ class BuildingPartOverlay:
     # The wall material (RdSAP `wall_construction` code). Left `None` by Measures
     # — insulating a wall doesn't change its material — but set by a Landlord
     # Override that corrects the construction itself (ADR-0032).
+    # RdSAP England-&-Wales construction age band — the letter code A..M the
+    # calculator's U-value cascades key on (`SapBuildingPart.construction_age_band`).
+    # Left `None` by Measures (retrofits don't change build era); set by a Landlord
+    # Override that corrects the lodged age band, which re-derives this part's
+    # fabric U-value defaults. Folds onto the part via the generic field loop.
+    construction_age_band: Optional[str] = None
     wall_construction: Optional[int] = None
     wall_insulation_type: Optional[int] = None
     # Added solid-wall insulation depth (mm) — drives the calculator's Table 6

tests/domain/epc/test_construction_age_band_overlay.py

@@ -0,0 +1,23 @@
+"""The Landlord-Override construction-age-band → fabric Simulation Overlay.
+
+An age-band value resolves to the RdSAP letter code the calculator's U-value
+cascades read from `SapBuildingPart.construction_age_band`; the overlay targets
+the override's building part.
+"""
+
+from __future__ import annotations
+
+from datatypes.epc.domain.epc_property_data import BuildingPartIdentifier
+from domain.epc.property_overlays.construction_age_band_overlay import (
+    age_band_overlay_for,
+)
+
+
+def test_age_band_overlays_the_main_building_part() -> None:
+    # Act — band B (1900-1929) on the main building part.
+    simulation = age_band_overlay_for("B", 0)
+
+    # Assert
+    assert simulation is not None
+    overlay = simulation.building_parts[BuildingPartIdentifier.MAIN]
+    assert overlay.construction_age_band == "B"