diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index 12f5b17e..d60daefd 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -2393,6 +2393,15 @@ _TARIFF_HIGH_LOW_FUEL_CODES_TABLE_12: Final[dict[Tariff, tuple[int, int]]] = {
 }
 
 
+# SAP Table 4a electric room-heater codes (panel/convector/radiant 691,
+# fan 692, portable 693, water-/oil-filled 694, "electric heaters assumed"
+# 699) — the same set RdSAP 10 §12 Rule 3 (PDF p.62) routes to the 10-hour
+# tariff. They are direct-acting electric for the Table 12a Grid 1 SH split.
+_ELECTRIC_ROOM_HEATER_SAP_CODES: Final[frozenset[int]] = frozenset(
+    {691, 692, 693, 694, 699}
+)
+
+
 def _table_12a_system_for_main(
     main: Optional[MainHeatingDetail],
 ) -> Optional[Table12aSystem]:
@@ -2421,15 +2430,26 @@ def _table_12a_system_for_main(
         main.main_heating_index_number is not None
         and heat_pump_record(main.main_heating_index_number) is not None
     )
-    # Electric room heaters (RdSAP main_heating_category 10) are direct-
-    # acting electric → SAP 10.2 Table 12a Grid 1 (PDF p.191) "Other
-    # systems including direct-acting electric" row (7-hour high-rate
-    # fraction 1.00, 10-hour 0.50). Distinct from electric STORAGE
-    # heaters (category 7), which charge off-peak and correctly fall
-    # through to None here (→ 100% low rate). Gated on `_is_electric_main`
-    # so a non-electric room heater (gas / solid-fuel cat 10) is excluded;
-    # all callers already pre-gate on electric, this is belt-and-braces.
-    if main.main_heating_category == 10 and _is_electric_main(main):
+    # Electric room heaters are direct-acting electric → SAP 10.2 Table
+    # 12a Grid 1 (PDF p.191) "Other systems including direct-acting
+    # electric" row (7-hour high-rate fraction 1.00, 10-hour 0.50).
+    # Identified EITHER by RdSAP main_heating_category 10 OR by a Table 4a
+    # electric room-heater SAP code (691-694 panel/fan/portable/water-
+    # filled, 699 "electric heaters assumed" — the SAME set RdSAP 10 §12
+    # Rule 3 (PDF p.62) routes to the 10-hour tariff). The "No system
+    # present: electric heaters assumed" lodging (code 699) carries
+    # main_heating_category 1, NOT 10, so the category-only gate missed it
+    # and it fell through to None → 100% off-peak LOW rate, billing
+    # direct-acting heaters as if they charged overnight like storage
+    # (cert 2958 +32.2 SAP, the worst over-rate in the sample). Distinct
+    # from electric STORAGE heaters (category 7), which DO charge off-peak
+    # and correctly fall through to None here (→ 100% low rate). Gated on
+    # `_is_electric_main` so a non-electric room heater (gas / solid-fuel
+    # cat 10) is excluded; all callers already pre-gate on electric.
+    if _is_electric_main(main) and (
+        main.main_heating_category == 10
+        or (code is not None and code in _ELECTRIC_ROOM_HEATER_SAP_CODES)
+    ):
         return Table12aSystem.OTHER_DIRECT_ACTING_ELECTRIC
     # A PCDB Table 362 record IS a heat pump by definition (the Appendix-N
     # efficiency cascade keys off it), whether or not a Table-4a SAP code
diff --git a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
index dbb817bb..4531c08d 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -715,6 +715,47 @@ def test_non_integrated_storage_heater_bills_100_percent_low_rate() -> None:
     assert abs(inputs.space_heating_fuel_cost_gbp_per_kwh - 0.0550) <= 1e-9
 
 
+def test_no_system_electric_heaters_assumed_code_699_bills_direct_acting_split() -> None:
+    # Arrange — "No system present: electric heaters assumed" lodges SAP
+    # Table 4a code 699 (electric room heaters) but RdSAP main_heating_
+    # category 1, NOT 10, on a Dual (Economy-7) meter. RdSAP 10 §12 Rule 3
+    # (PDF p.62) routes electric room heaters (691-694, 699) to the 10-hour
+    # tariff, and SAP 10.2 Table 12a Grid 1 (PDF p.191) gives the "other
+    # direct-acting electric" row a 0.50 high-rate fraction at 10-hour.
+    # The scalar SH rate is therefore the blend 0.50 × high (14.68) + 0.50
+    # × low (7.50) = 11.09 p/kWh — NOT the 7.50 p/kWh of 100% off-peak that
+    # the category-10-only gate produced when it missed this category-1
+    # lodging.
+    from dataclasses import replace
+
+    main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=29,  # electricity
+        heat_emitter_type=0,
+        emitter_temperature=1,
+        main_heating_control=2699,
+        main_heating_category=1,  # NOT 10 — the "electric heaters assumed" form
+        sap_main_heating_code=699,
+    )
+    base = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=4,
+        country_code="ENG",
+        sap_building_parts=[make_building_part(construction_age_band="E")],
+        sap_heating=make_sap_heating(main_heating_details=[main]),
+    )
+    epc = replace(
+        base,
+        sap_energy_source=replace(base.sap_energy_source, meter_type="1"),
+    )
+
+    # Act
+    inputs = cert_to_inputs(epc)
+
+    # Assert — blended 0.50×14.68 + 0.50×7.50 = 11.09 p/kWh.
+    assert abs(inputs.space_heating_fuel_cost_gbp_per_kwh - 0.1109) <= 1e-9
+
+
 def test_heat_network_distribution_electricity_per_sap_10_2_appendix_c_3_2() -> None:
     # Arrange — heat-network main (Table 4a code 301 = community heating,
     # category 6). SAP 10.2 Appendix C §C3.2 (PDF p.51): distribution