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

@@ -13,10 +13,27 @@ from __future__ import annotations
 
 from typing import Optional
 
-from domain.modelling.simulation import EpcSimulation
+from datatypes.epc.domain.epc_property_data import BuildingPartIdentifier
+from domain.modelling.simulation import BuildingPartOverlay, EpcSimulation
+
+# RdSAP England-&-Wales construction age bands (letter codes A..M).
+_VALID_AGE_BANDS: frozenset[str] = frozenset("ABCDEFGHIJKLM")
 
 
 def age_band_overlay_for(
     age_band_value: str, building_part: int
 ) -> Optional[EpcSimulation]:
-    raise NotImplementedError
+    band = age_band_value.strip().upper()
+    if band not in _VALID_AGE_BANDS:
+        return None
+
+    identifier = (
+        BuildingPartIdentifier.MAIN
+        if building_part == 0
+        else BuildingPartIdentifier.extension(building_part)
+    )
+    return EpcSimulation(
+        building_parts={
+            identifier: BuildingPartOverlay(construction_age_band=band)
+        }
+    )