diff --git a/docs/adr/0046-electric-underfloor-scored-by-its-own-codes-meter-deferred-when-tariff-ambiguous.md b/docs/adr/0046-electric-underfloor-scored-by-its-own-codes-meter-deferred-when-tariff-ambiguous.md
new file mode 100644
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@@ -0,0 +1,92 @@
+# Electric underfloor is scored by its own SAP codes; the meter defers to the cert when the tariff is ambiguous
+
+## Status
+
+accepted
+
+## Context
+
+Electric-underfloor `main_heating_system` overrides had no archetype, so the LLM
+classifier funnelled them into the nearest wrong bucket (#1376):
+
+| description | rows | mis-mapped to |
+| --- | --- | --- |
+| Electric Underfloor Heating: In concrete slab (off-peak only) | 15 | Direct-acting electric (191), **Single** meter |
+| Electric Underfloor Heating: In screed above insulation (standard or off peak) | 63 | Direct-acting electric (191), **Single** meter |
+| Electric Underfloor Heating: Integrated (storage+direct-acting) (off peak) | 12 | Electric storage heaters, old (401) |
+
+The **meter is the crux**. A Landlord heating override drags an assumed meter from
+its SAP code (ADR-0035), and the `keep_existing_off_peak_meter` mitigation
+([overlay_applicator.py](../../domain/modelling/scoring/overlay_applicator.py))
+protects a cert's lodged off-peak meter — but *only when the overlay's own desired
+meter is off-peak*. Direct-acting (191) is a **Single**-meter system, so the
+mitigation never fires and the overlay **forces Single**, overwriting the cert's
+meter. For off-peak underfloor that bills overnight heat at the peak rate and
+craters SAP — the exact Class-A meter defect.
+
+SAP Table 4a has **dedicated electric-underfloor codes** (labels from
+`cert_to_inputs.py`): **421** in concrete slab (off-peak only), **422** integrated
+(storage+direct-acting), **424** in screed above insulation (423 integrated-low-
+off-peak and 425 timber-floor are not in the corpus). Per SAP 10.2 §12, **421/422
+are off-peak (Rule 2, 7-hour → Dual)**; **424 is not off-peak-implying (Single)**.
+
+A lodged-meter audit is decisive for the screed case: **51 of 63 (81%)** of the
+`"…in screed (standard or off peak)"` dwellings lodge an **off-peak** meter (24-hour
+dual or 7/10-hour), the rest single. The description itself says "standard **or**
+off peak" — so the archetype genuinely cannot know the tariff; the cert does.
+Forcing 424/Single would downgrade 81%; forcing Dual would over-credit the ~19% on
+single.
+
+## Decision
+
+Add three electric-underfloor archetypes mapping to their own SAP codes, and treat
+the meter per what the description actually asserts.
+
+1. **New archetypes** in `_MAIN_HEATING_CODES`:
+   `"Electric underfloor, in concrete slab (off-peak)"` → **421**,
+   `"Electric underfloor, integrated storage and direct-acting"` → **422**,
+   `"Electric underfloor, in screed above insulation"` → **424**.
+2. **Electricity fuel (29)** drags for 421/422/424 via `_natural_fuel_for` — electric
+   underfloor is unambiguously electric (ADR-0041), closing the prior no-fuel gap.
+3. **421 and 422 assert an off-peak Dual meter.** Their descriptions positively
+   claim off-peak ("off-peak only" / "off peak"), and both are already in
+   `_ASSUMED_DUAL_METER_CODES`; the `keep_existing_off_peak_meter` mitigation keeps
+   a more-specific cert off-peak meter and applies Dual to a single-rate cert.
+4. **424 (screed) defers the meter to the cert.** Because "standard or off peak" is
+   tariff-agnostic and 81% lodge off-peak, `_meter_for` returns **`None`** for 424
+   (a new meter-agnostic set), so the overlay leaves `meter_type` unset and the
+   lodged meter stands — each dwelling scores on its actual tariff. This is a
+   **targeted exception to the "always set the meter, never let it bleed"
+   invariant**: that invariant guards against a switched-*off* storage system
+   leaving a stale Dual meter, but an underfloor override *confirms* underfloor —
+   there is no switch-off, so deferring is faithful, not a bleed.
+5. **Deterministic `main_heating_guard` + reclassify** (TEXT-first, pgEnum-deferred),
+   reusing the generalized `reclassify_main_heating.py`. All three underfloor
+   descriptions are description-determined, so the guard resolves them directly (no
+   fuel-awareness needed) and the reclassify follows.
+
+## Consequences
+
+- The 15 concrete-slab-off-peak rows stop being billed at the peak rate (191/Single
+  → 421/Dual); the 12 integrated rows get the correct underfloor code (401 → 422,
+  meter already Dual); the 63 screed rows keep their lodged (81% off-peak) meter and
+  score on code 424 with correct screed responsiveness (Table 4d R=0.75). Correct
+  Rebaselining (ADR-0039).
+- **FE-owned pgEnum additions (Dan):** three new `main_heating_system` values —
+  `"Electric underfloor, in concrete slab (off-peak)"`, `"Electric underfloor,
+  integrated storage and direct-acting"`, `"Electric underfloor, in screed above
+  insulation"`. TEXT read path fixed on merge; cache writes defer to the migration.
+- `_meter_for` becomes `Optional[str]` (a code may defer the meter). Every existing
+  code keeps its current Single/Dual answer; only 424 returns `None`.
+- Extends [ADR-0041](0041-landlord-heating-classification-targets-a-complete-modellable-taxonomy.md)
+  and [ADR-0035](0035-coherent-heating-system-synthesis.md); refines the
+  `keep_existing_off_peak_meter` behaviour ADR-0035 introduced.
+
+### Alternatives rejected
+
+- **Force Single (424 per spec).** Rejected: downgrades the 81% lodged off-peak,
+  reproducing the meter bug.
+- **Force Dual for screed.** Rejected: over-credits the ~19% genuinely on single and
+  asserts a tariff the "standard or off peak" description does not claim.
+- **Map underfloor to Direct-acting (191) / old storage (401) as today.** Rejected:
+  wrong code (responsiveness/meter) and the source of the mis-score.
diff --git a/tests/domain/epc/test_main_heating_system_overlay.py b/tests/domain/epc/test_main_heating_system_overlay.py
index 4b4558718..ad6944255 100644
--- a/tests/domain/epc/test_main_heating_system_overlay.py
+++ b/tests/domain/epc/test_main_heating_system_overlay.py
@@ -127,6 +127,29 @@ def test_electric_cpsu_decodes_to_192_electric_on_an_off_peak_meter() -> None:
     assert simulation.heating.meter_type == "Dual"
 
 
+@pytest.mark.parametrize(
+    ("main_heating_value", "code"),
+    [
+        # Electric underfloor funnelled into Direct-acting (191) / old storage (401)
+        # for want of the dedicated SAP Table 4a underfloor codes (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),
+    ],
+)
+def test_electric_underfloor_decodes_to_its_own_code_dragging_electricity(
+    main_heating_value: str, code: int
+) -> None:
+    # Act
+    simulation = main_heating_overlay_for(main_heating_value, 0)
+
+    # Assert — its own underfloor code, electric (29).
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.sap_main_heating_code == code
+    assert simulation.heating.main_fuel_type == 29
+
+
 @pytest.mark.parametrize(
     ("main_heating_value", "code"),
     [