diff --git a/domain/epc/property_overlays/main_heating_system_overlay.py b/domain/epc/property_overlays/main_heating_system_overlay.py
index 12d695b18..904a774bf 100644
--- a/domain/epc/property_overlays/main_heating_system_overlay.py
+++ b/domain/epc/property_overlays/main_heating_system_overlay.py
@@ -45,6 +45,10 @@ _OFF_PEAK_METER = "Dual"
 # meter bleed through and bill the new gas/direct-acting system on an Economy-7
 # split (the mirror of the storage→Dual drag, ADR-0035).
 _SINGLE_RATE_METER = "Single"
+# Meter-agnostic codes: the archetype cannot determine the tariff, so the overlay
+# defers `meter_type` to the cert (`_meter_for` → None). Screed underfloor (424) is
+# "standard or off peak" — 81% of the corpus lodge off-peak (ADR-0046).
+_METER_AGNOSTIC_CODES = frozenset({424})
 
 # Electric room heaters (SAP Table 4a 691). They don't *require* off-peak the way
 # storage/CPSU do, so they're absent from the calculator's §12
@@ -228,11 +232,19 @@ _MAIN_HEATING_CODES: dict[str, int] = {
 }
 
 
-def _meter_for(code: int) -> str:
+def _meter_for(code: int) -> Optional[str]:
     """The coherent meter a heating code implies: an off-peak ("Dual") meter for
     the §12 off-peak systems and all-electric room-heater dwellings
     (`_ASSUMED_DUAL_METER_CODES`), an explicit single-rate ("Single") meter for
-    every other system. Always set — never left to bleed."""
+    every other system — set explicitly so it never bleeds.
+
+    Exception: a **meter-agnostic** code (`_METER_AGNOSTIC_CODES` — screed
+    underfloor 424, "standard or off peak") returns ``None`` so the overlay leaves
+    `meter_type` unset and the cert's lodged tariff stands. The archetype genuinely
+    cannot know the tariff, and unlike a switched-off storage system it is not a
+    re-metering, so deferring is faithful, not a bleed (ADR-0046)."""
+    if code in _METER_AGNOSTIC_CODES:
+        return None
     return _OFF_PEAK_METER if code in _ASSUMED_DUAL_METER_CODES else _SINGLE_RATE_METER
 
 
diff --git a/tests/domain/epc/test_main_heating_system_overlay.py b/tests/domain/epc/test_main_heating_system_overlay.py
index c477bcee2..9f83c9b3f 100644
--- a/tests/domain/epc/test_main_heating_system_overlay.py
+++ b/tests/domain/epc/test_main_heating_system_overlay.py
@@ -13,6 +13,7 @@ from domain.epc.property_overlays.main_fuel_overlay import fuel_overlay_for
 from domain.epc.property_overlays.main_heating_system_overlay import (
     _ASSUMED_DUAL_METER_CODES,
     _MAIN_HEATING_CODES,
+    _METER_AGNOSTIC_CODES,
     main_heating_overlay_for,
 )
 from domain.epc.property_overlays.water_heating_overlay import (
@@ -367,11 +368,18 @@ def test_off_peak_archetypes_drag_dual_others_drag_single() -> None:
     # the calculator's single off-peak classification, so any archetype whose
     # code implies off-peak MUST synthesise a Dual meter and every other code
     # MUST synthesise a Single meter — a system switch can never silently leave
-    # the previous system's meter in place.
+    # the previous system's meter in place. The one exception is a meter-agnostic
+    # code (screed underfloor, tariff-ambiguous), which defers to the cert's
+    # lodged meter (None) rather than assert one (ADR-0046).
     for value, code in _MAIN_HEATING_CODES.items():
         simulation = main_heating_overlay_for(value, 0)
         assert simulation is not None and simulation.heating is not None
-        expected = "Dual" if code in _ASSUMED_DUAL_METER_CODES else "Single"
+        if code in _METER_AGNOSTIC_CODES:
+            expected = None
+        elif code in _ASSUMED_DUAL_METER_CODES:
+            expected = "Dual"
+        else:
+            expected = "Single"
         assert simulation.heating.meter_type == expected, value