diff --git a/domain/epc/property_overlays/main_heating_system_overlay.py b/domain/epc/property_overlays/main_heating_system_overlay.py
index d0c498320..1110b1f68 100644
--- a/domain/epc/property_overlays/main_heating_system_overlay.py
+++ b/domain/epc/property_overlays/main_heating_system_overlay.py
@@ -134,6 +134,13 @@ _ELECTRIC_BOILER_CODES = frozenset({191, 192})
 # unambiguously electric (ADR-0046).
 _ELECTRIC_UNDERFLOOR_CODES = frozenset({421, 422, 424})
 
+# SAP Table 4a "electric underfloor heating" category. Stamped so an underfloor
+# override never leaves the replaced system's category behind — property 711795
+# (PRD #1435 WS2) kept a replaced heat network's category 6, which the heating
+# generator's `is_heat_network_main` gate read as community heating: no heating
+# upgrade was offered at all and the baseline mis-rated 51 vs 67 (ADR-0050).
+_UNDERFLOOR_CATEGORY = 8
+
 # Heat pumps (SAP Table 4a 211-224 wet, 521-527 warm-air) are category 4 and
 # unambiguously electric (natural fuel 29). Modellable on the default code's SPF
 # without a PCDB index (ADR-0041).
@@ -287,6 +294,8 @@ def _category_for(code: int) -> Optional[int]:
         return _ROOM_HEATER_CATEGORY
     if code in _STORAGE_HEATER_CODES:
         return _STORAGE_HEATER_CATEGORY
+    if code in _ELECTRIC_UNDERFLOOR_CODES:
+        return _UNDERFLOOR_CATEGORY
     if code in _HEAT_PUMP_CODES:
         return _HEAT_PUMP_CATEGORY
     if code in _HEAT_NETWORK_CODES: