diff --git a/domain/epc/property_overlays/water_heating_overlay.py b/domain/epc/property_overlays/water_heating_overlay.py
index b65db1b1..d75ea81c 100644
--- a/domain/epc/property_overlays/water_heating_overlay.py
+++ b/domain/epc/property_overlays/water_heating_overlay.py
@@ -13,10 +13,27 @@ from __future__ import annotations
 
 from typing import Optional
 
-from domain.modelling.simulation import EpcSimulation
+from domain.modelling.simulation import EpcSimulation, HeatingOverlay
+
+# Canonical "<system>, <fuel>" description → (water_heating_code, water_heating_fuel).
+# water_heating_code: 901 "from main system" (SAP Table 4a inherit-from-main),
+# 903 "electric immersion". Fuel codes are the modern RdSAP "(not community)"
+# family (26 mains gas, 29 electricity), matching the main_fuel overlay.
+_WATER_HEATING_CODES: dict[str, tuple[int, int]] = {
+    "From main system, mains gas": (901, 26),
+}
 
 
 def water_heating_overlay_for(
     water_heating_value: str, building_part: int
 ) -> Optional[EpcSimulation]:
-    raise NotImplementedError
+    codes = _WATER_HEATING_CODES.get(water_heating_value)
+    if codes is None:
+        return None
+    water_heating_code, water_heating_fuel = codes
+    return EpcSimulation(
+        heating=HeatingOverlay(
+            water_heating_code=water_heating_code,
+            water_heating_fuel=water_heating_fuel,
+        )
+    )