diff --git a/tests/domain/epc/test_boiler_efficiency_band_overlay.py b/tests/domain/epc/test_boiler_efficiency_band_overlay.py
new file mode 100644
index 000000000..bc4dd7b72
--- /dev/null
+++ b/tests/domain/epc/test_boiler_efficiency_band_overlay.py
@@ -0,0 +1,124 @@
+from __future__ import annotations
+
+from typing import Optional
+
+import pytest
+
+from domain.epc.property_overrides.boiler_efficiency_band import BoilerEfficiencyBand
+from domain.epc.property_overlays.main_heating_system_overlay import (
+    band_seasonal_efficiency_pct,
+    main_heating_overlay_for,
+)
+
+
+@pytest.mark.parametrize(
+    ("base_code", "band", "expected"),
+    [
+        # Gas regular (base 102) — C-and-below reuse the accredited Table 4b
+        # (winter, summer) of the code the band maps to (ADR-0068).
+        (102, BoilerEfficiencyBand.C, (84.0, 74.0)),  # code 102
+        (102, BoilerEfficiencyBand.D, (80.0, 70.0)),  # code 106
+        (102, BoilerEfficiencyBand.E, (74.0, 64.0)),  # code 101
+        (102, BoilerEfficiencyBand.F, (70.0, 60.0)),  # code 105
+        (102, BoilerEfficiencyBand.G, (66.0, 56.0)),  # code 115
+        # Gas combi (base 104).
+        (104, BoilerEfficiencyBand.C, (84.0, 75.0)),  # code 104
+        (104, BoilerEfficiencyBand.D, (80.0, 71.0)),  # code 108
+        (104, BoilerEfficiencyBand.G, (66.0, 57.0)),  # code 118
+        # Oil regular (base 127) — E/F nearest-fit to code 125 (⚠ ADR-0068).
+        (127, BoilerEfficiencyBand.C, (84.0, 72.0)),  # code 127
+        (127, BoilerEfficiencyBand.D, (80.0, 68.0)),  # code 126
+        (127, BoilerEfficiencyBand.E, (71.0, 59.0)),  # code 125
+        (127, BoilerEfficiencyBand.G, (66.0, 54.0)),  # code 124
+        # Oil combi (base 130) — D/E to code 129, F/G to code 128 (⚠ ADR-0068).
+        (130, BoilerEfficiencyBand.C, (82.0, 73.0)),  # code 130
+        (130, BoilerEfficiencyBand.D, (77.0, 68.0)),  # code 129
+        (130, BoilerEfficiencyBand.F, (71.0, 62.0)),  # code 128
+    ],
+)
+def test_c_and_below_bands_resolve_to_the_table_4b_pair(
+    base_code: int, band: BoilerEfficiencyBand, expected: tuple[float, float]
+) -> None:
+    assert band_seasonal_efficiency_pct(base_code, band) == expected
+
+
+@pytest.mark.parametrize("band", [BoilerEfficiencyBand.A, BoilerEfficiencyBand.B])
+@pytest.mark.parametrize("base_code", [102, 104, 127, 130])
+def test_a_and_b_bands_are_pending_elmhurst_validation(
+    base_code: int, band: BoilerEfficiencyBand
+) -> None:
+    # A/B exceed Table 4b's 84% ceiling — they are judgement values that MUST be
+    # pinned against an accredited Elmhurst build before they are enabled
+    # (ADR-0068 gated pre-req). Until then: no slot, so an A/B boiler keeps its
+    # condensing code default (no invented numbers ship).
+    assert band_seasonal_efficiency_pct(base_code, band) is None
+
+
+@pytest.mark.parametrize(
+    "base_code",
+    [
+        151,  # Solid fuel boiler — Table 4a 75%, intrinsic (SEDBUK doesn't rate it)
+        191,  # Electric boiler — Table 4a 100%, intrinsic
+        192,  # Electric CPSU — intrinsic electric
+        120,  # Gas CPSU — non-condensing, not SEDBUK-banded
+    ],
+)
+@pytest.mark.parametrize("band", list(BoilerEfficiencyBand))
+def test_non_banded_boilers_never_take_a_slot(
+    base_code: int, band: BoilerEfficiencyBand
+) -> None:
+    assert band_seasonal_efficiency_pct(base_code, band) is None
+
+
+@pytest.mark.parametrize(
+    ("archetype", "band", "expected"),
+    [
+        ("Gas boiler, regular", BoilerEfficiencyBand.D, (80.0, 70.0)),
+        ("Gas boiler, combi", BoilerEfficiencyBand.G, (66.0, 57.0)),
+        ("Oil boiler, regular", BoilerEfficiencyBand.E, (71.0, 59.0)),
+        ("Oil boiler, combi", BoilerEfficiencyBand.D, (77.0, 68.0)),
+    ],
+)
+def test_overlay_sets_the_band_efficiency_slot_on_gas_and_oil_boilers(
+    archetype: str, band: BoilerEfficiencyBand, expected: tuple[float, float]
+) -> None:
+    simulation = main_heating_overlay_for(archetype, 0, band)
+
+    assert simulation is not None
+    assert simulation.heating is not None
+    # The base code is unchanged (it still drives combi/regular HW behaviour);
+    # only the efficiency slot is set from the band.
+    assert simulation.heating.seasonal_efficiency_override_pct == expected
+
+
+@pytest.mark.parametrize(
+    ("archetype", "band"),
+    [
+        # No band → no slot (today's behaviour: keep the condensing code default).
+        ("Gas boiler, combi", None),
+        # A/B pending Elmhurst → no slot yet.
+        ("Gas boiler, combi", BoilerEfficiencyBand.A),
+        # C on gas regular equals the code default — still set, but a no-op value.
+        # Non-banded boilers ignore the band letter entirely.
+        ("Electric boiler", BoilerEfficiencyBand.D),
+        ("Solid fuel boiler", BoilerEfficiencyBand.G),
+        ("Gas CPSU", BoilerEfficiencyBand.D),
+    ],
+)
+def test_overlay_sets_no_slot_when_band_absent_or_not_applicable(
+    archetype: str, band: Optional[BoilerEfficiencyBand]
+) -> None:
+    simulation = main_heating_overlay_for(archetype, 0, band)
+
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.seasonal_efficiency_override_pct is None
+
+
+def test_overlay_defaults_to_no_band_when_not_supplied() -> None:
+    # Back-compat: the band arg is optional; existing callers pass none.
+    simulation = main_heating_overlay_for("Gas boiler, combi", 0)
+
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.seasonal_efficiency_override_pct is None