diff --git a/datatypes/epc/domain/tests/test_from_site_notes.py b/datatypes/epc/domain/tests/test_from_site_notes.py
index 2631a9973..7d8dedfc6 100644
--- a/datatypes/epc/domain/tests/test_from_site_notes.py
+++ b/datatypes/epc/domain/tests/test_from_site_notes.py
@@ -2769,6 +2769,34 @@ class TestPasHubMainHeatingControlCoding:
         assert main.main_heating_control == 2601
         assert _control_type(main) == 2
 
+    def test_gas_room_heater_type_maps_to_table_4a_code_603(self) -> None:
+        # Arrange — a gas room-heater main system (verbatim fixture
+        # 500797805807 lodgement): open flue, 1980+, open fronted, sealed to
+        # fireplace → Table 4a code 603 (PCDB "sitting proud of, and sealed
+        # to, fireplace opening", eff 63%). 604 (with back boiler) and 605
+        # (flush-fitting live fuel effect) rejected — neither qualifier is in
+        # the label.
+        raw = load("pashub_rdsap_site_notes_example1.json")
+        mh = raw["heating_and_hot_water"]["main_heating"]
+        mh["system_type"] = "Room heaters"
+        mh["community_heat_source"] = (
+            "Gas fire, Open flue, 1980 or later (Open fronted), "
+            "Sealed to fireplace"
+        )
+        mh["fuel"] = "Mains Gas"
+        mh["product_id"] = 0
+        mh["controls"] = "No thermostatic control of room temperature"
+        survey = from_dict(PasHubRdSapSiteNotes, raw)
+
+        # Act
+        main = EpcPropertyDataMapper.from_site_notes(
+            survey
+        ).sap_heating.main_heating_details[0]
+
+        # Assert — the gas Table 4a row and the mains-gas fuel code.
+        assert main.sap_main_heating_code == 603
+        assert main.main_fuel_type == 26
+
     @pytest.mark.parametrize(
         "heater_type, expected_system_code",
         [