diff --git a/tests/domain/epc/test_construction_age_band_overlay.py b/tests/domain/epc/test_construction_age_band_overlay.py
index bb66933e..7527b9ea 100644
--- a/tests/domain/epc/test_construction_age_band_overlay.py
+++ b/tests/domain/epc/test_construction_age_band_overlay.py
@@ -13,6 +13,14 @@ from datatypes.epc.domain.epc_property_data import BuildingPartIdentifier
 from domain.epc.property_overlays.construction_age_band_overlay import (
     age_band_overlay_for,
 )
+from domain.modelling.scoring.overlay_applicator import apply_simulations
+from tests.domain.sap10_calculator.worksheet._elmhurst_worksheet_000490 import (
+    build_epc,
+)
+
+
+def _part(epc, identifier):  # type: ignore[no-untyped-def]
+    return next(p for p in epc.sap_building_parts if p.identifier is identifier)
 
 
 def test_age_band_overlays_the_main_building_part() -> None:
@@ -58,3 +66,21 @@ def test_unrecognised_age_band_produces_no_overlay(age_band_value: str) -> None:
 
     # Assert
     assert simulation is None
+
+
+def test_age_band_override_re_dates_the_main_part_only() -> None:
+    # Arrange — baseline main + extension are both band B; the landlord corrects
+    # the main building's age band to F (1976-1982).
+    baseline = build_epc()
+    overlay = age_band_overlay_for("F", 0)
+    assert overlay is not None
+
+    # Act
+    result = apply_simulations(baseline, [overlay])
+
+    # Assert — the main part is re-dated (its U-value cascade now keys on F); the
+    # extension is left untouched.
+    assert _part(result, BuildingPartIdentifier.MAIN).construction_age_band == "F"
+    assert (
+        _part(result, BuildingPartIdentifier.EXTENSION_1).construction_age_band == "B"
+    )