diff --git a/tests/domain/epc/test_construction_age_band_overlay.py b/tests/domain/epc/test_construction_age_band_overlay.py
index 066244a6..bb66933e 100644
--- a/tests/domain/epc/test_construction_age_band_overlay.py
+++ b/tests/domain/epc/test_construction_age_band_overlay.py
@@ -7,6 +7,8 @@ the override's building part.
 
 from __future__ import annotations
 
+import pytest
+
 from datatypes.epc.domain.epc_property_data import BuildingPartIdentifier
 from domain.epc.property_overlays.construction_age_band_overlay import (
     age_band_overlay_for,
@@ -21,3 +23,38 @@ def test_age_band_overlays_the_main_building_part() -> None:
     assert simulation is not None
     overlay = simulation.building_parts[BuildingPartIdentifier.MAIN]
     assert overlay.construction_age_band == "B"
+
+
+def test_age_band_overlay_targets_the_extension_building_part() -> None:
+    # Act — building_part 1 is the first extension.
+    simulation = age_band_overlay_for("L", 1)
+
+    # Assert
+    assert simulation is not None
+    assert BuildingPartIdentifier.EXTENSION_1 in simulation.building_parts
+    assert (
+        simulation.building_parts[BuildingPartIdentifier.EXTENSION_1]
+        .construction_age_band
+        == "L"
+    )
+
+
+def test_lowercase_age_band_is_normalised_to_its_letter_code() -> None:
+    # Act
+    simulation = age_band_overlay_for("d", 0)
+
+    # Assert — the calculator upper-cases the band; the overlay stores it upper.
+    assert simulation is not None
+    assert (
+        simulation.building_parts[BuildingPartIdentifier.MAIN].construction_age_band
+        == "D"
+    )
+
+
+@pytest.mark.parametrize("age_band_value", ["Z", "", "1900-1929", "Unknown"])
+def test_unrecognised_age_band_produces_no_overlay(age_band_value: str) -> None:
+    # Act
+    simulation = age_band_overlay_for(age_band_value, 0)
+
+    # Assert
+    assert simulation is None