diff --git a/domain/epc/property_overlays/main_heating_system_overlay.py b/domain/epc/property_overlays/main_heating_system_overlay.py
index 957ce94da..5fb199407 100644
--- a/domain/epc/property_overlays/main_heating_system_overlay.py
+++ b/domain/epc/property_overlays/main_heating_system_overlay.py
@@ -159,6 +159,9 @@ _MAIN_HEATING_CODES: dict[str, int] = {
     # from the code (cert_to_inputs). A generic "Community heating" with no named
     # source stays unmapped (-> None) — the source can't be assumed (ADR-0041).
     "Community heating, boilers": 301,
+    # Modern standalone oil room heater — SAP Table 4a 623 ("Oil room heater,
+    # 2000 or later", no boiler). Natural fuel heating oil (ADR-0041).
+    "Oil room heater, 2000 or later": 623,
 }
 
 
diff --git a/domain/epc/property_overrides/main_heating_system_type.py b/domain/epc/property_overrides/main_heating_system_type.py
index eda1bef7a..c15afd602 100644
--- a/domain/epc/property_overrides/main_heating_system_type.py
+++ b/domain/epc/property_overrides/main_heating_system_type.py
@@ -28,4 +28,5 @@ class MainHeatingSystemType(Enum):
     SOLID_FUEL_ROOM_HEATER_CLOSED = "Solid fuel room heater, closed"
     AIR_SOURCE_HEAT_PUMP = "Air source heat pump"
     COMMUNITY_BOILERS = "Community heating, boilers"
+    OIL_ROOM_HEATER_POST_2000 = "Oil room heater, 2000 or later"
     UNKNOWN = "Unknown"