diff --git a/domain/epc/property_overlays/main_heating_system_overlay.py b/domain/epc/property_overlays/main_heating_system_overlay.py
index 5d7c3b836..12d695b18 100644
--- a/domain/epc/property_overlays/main_heating_system_overlay.py
+++ b/domain/epc/property_overlays/main_heating_system_overlay.py
@@ -116,6 +116,9 @@ _ELECTRIC_ROOM_HEATER_CODES = frozenset(range(691, 702))
 # unambiguously electric, so they drag electricity like the electric room heaters
 # (ADR-0045, closing the prior no-fuel gap on 191/192).
 _ELECTRIC_BOILER_CODES = frozenset({191, 192})
+# Electric underfloor — SAP Table 4a 421/422 (off-peak) and 424 (screed) — also
+# unambiguously electric (ADR-0046).
+_ELECTRIC_UNDERFLOOR_CODES = frozenset({421, 422, 424})
 
 # 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
@@ -184,6 +187,13 @@ _MAIN_HEATING_CODES: dict[str, int] = {
     # Electric CPSU — SAP Table 4a 192. Electric (drags 29), on an off-peak Dual
     # meter (already in _ASSUMED_DUAL_METER_CODES; §12 Rule 1 10-hour) — ADR-0045.
     "Electric CPSU": 192,
+    # Electric underfloor — dedicated SAP Table 4a codes, all electric (drag 29).
+    # 421 concrete slab / 422 integrated are off-peak (Dual, §12 Rule 2); 424
+    # screed is tariff-ambiguous ("standard or off peak") so it DEFERS the meter to
+    # the cert (ADR-0046).
+    "Electric underfloor, in concrete slab (off-peak)": 421,
+    "Electric underfloor, integrated storage and direct-acting": 422,
+    "Electric underfloor, in screed above insulation": 424,
     "Electric room heaters": 691,
     "Solid fuel room heater, closed": 633,
     # Solid-fuel room heaters off the Gas CPSU dumping ground (ADR-0045). SAP
@@ -264,6 +274,7 @@ def _natural_fuel_for(code: int) -> Optional[int]:
         or code in _HEAT_PUMP_CODES
         or code == _HHR_STORAGE_CODE
         or code in _ELECTRIC_BOILER_CODES
+        or code in _ELECTRIC_UNDERFLOOR_CODES
     ):
         return _ELECTRICITY_FUEL
     if code in _SOLID_FUEL_ROOM_HEATER_CODES:
diff --git a/domain/epc/property_overrides/main_heating_system_type.py b/domain/epc/property_overrides/main_heating_system_type.py
index 866767f4f..f8426fdcf 100644
--- a/domain/epc/property_overrides/main_heating_system_type.py
+++ b/domain/epc/property_overrides/main_heating_system_type.py
@@ -29,6 +29,13 @@ class MainHeatingSystemType(Enum):
     DIRECT_ELECTRIC = "Direct-acting electric"
     ELECTRIC_BOILER = "Electric boiler"
     ELECTRIC_CPSU = "Electric CPSU"
+    ELECTRIC_UNDERFLOOR_SLAB_OFF_PEAK = (
+        "Electric underfloor, in concrete slab (off-peak)"
+    )
+    ELECTRIC_UNDERFLOOR_INTEGRATED = (
+        "Electric underfloor, integrated storage and direct-acting"
+    )
+    ELECTRIC_UNDERFLOOR_SCREED = "Electric underfloor, in screed above insulation"
     ELECTRIC_ROOM_HEATERS = "Electric room heaters"
     SOLID_FUEL_ROOM_HEATER_CLOSED = "Solid fuel room heater, closed"
     SOLID_FUEL_ROOM_HEATER_OPEN_FIRE = "Solid fuel room heater, open fire"