diff --git a/tests/domain/epc/test_water_heating_overlay.py b/tests/domain/epc/test_water_heating_overlay.py
index 49cfffbf2..7dd8c2d0a 100644
--- a/tests/domain/epc/test_water_heating_overlay.py
+++ b/tests/domain/epc/test_water_heating_overlay.py
@@ -11,6 +11,10 @@ import pytest
 from domain.epc.property_overlays.water_heating_overlay import (
     water_heating_overlay_for,
 )
+from domain.modelling.scoring.overlay_applicator import apply_simulations
+from tests.domain.sap10_calculator.worksheet._elmhurst_worksheet_000490 import (
+    build_epc,
+)
 
 
 def test_from_main_system_mains_gas_overlays_water_heating() -> None:
@@ -53,3 +57,17 @@ def test_unresolvable_water_heating_produces_no_overlay(
 
     # Assert
     assert simulation is None
+
+
+def test_water_heating_override_remaps_the_hot_water_arrangement() -> None:
+    # Arrange — landlord correction: HW is a separate electric immersion.
+    baseline = build_epc()
+    overlay = water_heating_overlay_for("Electric immersion, electricity", 0)
+    assert overlay is not None
+
+    # Act
+    result = apply_simulations(baseline, [overlay])
+
+    # Assert — the calculator reads these off sap_heating.
+    assert result.sap_heating.water_heating_code == 903
+    assert result.sap_heating.water_heating_fuel == 29