diff --git a/applications/landlord_description_overrides/handler.py b/applications/landlord_description_overrides/handler.py
index a43413b4f..dd37e461c 100644
--- a/applications/landlord_description_overrides/handler.py
+++ b/applications/landlord_description_overrides/handler.py
@@ -10,6 +10,7 @@ from applications.landlord_description_overrides.landlord_description_overrides_
 from domain.epc.property_overrides.built_form_type import BuiltFormType
 from domain.epc.property_overrides.construction_age_band import ConstructionAgeBand
 from domain.epc.property_overrides.glazing_type import GlazingType
+from domain.epc.property_overrides.glazing_mix_guard import glazing_mix_guard
 from domain.epc.property_overrides.main_fuel_type import MainFuelType
 from domain.epc.property_overrides.main_heating_system_type import MainHeatingSystemType
 from domain.epc.property_overrides.property_type import PropertyType
@@ -146,8 +147,15 @@ def _build_columns(
         "glazing": lambda src: ClassifiableColumn(
             name="glazing",
             source_column=src,
-            classifier=ChatGptColumnClassifier(
-                chat_gpt, GlazingType, GlazingType.UNKNOWN
+            # An aggregate glazing mix ("40% double, 60% single") can't be applied
+            # per-window, so the deterministic guard resolves the structured split
+            # to MIXED (no overlay → keep the cert's per-window glazing) and the LLM
+            # handles uniform / varied phrasings (#1376, ADR-0042).
+            classifier=GuardedColumnClassifier(
+                guard=glazing_mix_guard,
+                fallback=ChatGptColumnClassifier(
+                    chat_gpt, GlazingType, GlazingType.UNKNOWN
+                ),
             ),
             repo=LandlordOverridesRepository[GlazingType](
                 session, LandlordGlazingOverrideRow
diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 114fce19a..77a52e514 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -616,15 +616,35 @@ class EpcPropertyDataMapper:
             insulated_door_count=schema.insulated_door_count,
             draughtproofed_door_count=None,
             percent_draughtproofed=schema.percent_draughtproofed,
+            # ADR-0028: the schema lodges total + low-energy OUTLET counts, not a
+            # bulb split — mirror the 17.1/18.0/20.0 paths (low-energy outlets →
+            # low_energy bulbs; the remainder → incandescent). Hardcoding all
+            # bulb counts to 0 dropped the lodged lighting mix, understating the
+            # low-energy lighting credit in the SAP lighting-energy calc.
             led_fixed_lighting_bulbs_count=0,
             cfl_fixed_lighting_bulbs_count=0,
-            incandescent_fixed_lighting_bulbs_count=0,
+            incandescent_fixed_lighting_bulbs_count=(
+                schema.fixed_lighting_outlets_count
+                - schema.low_energy_fixed_lighting_outlets_count
+            ),
+            low_energy_fixed_lighting_bulbs_count=schema.low_energy_fixed_lighting_outlets_count,
             roofs=EpcPropertyDataMapper._map_energy_elements(schema.roofs),
             walls=EpcPropertyDataMapper._map_energy_elements(schema.walls),
             floors=EpcPropertyDataMapper._map_energy_elements(schema.floors),
             main_heating=EpcPropertyDataMapper._map_energy_elements(
                 schema.main_heating
             ),
+            # First control system if multiple — mirrors the 19.0/21.0.1 paths.
+            # 17.0 lodges main_heating_controls (List[EnergyElement]) but omitted
+            # it, leaving main_heating_controls=None so the heating-control
+            # recommendations (programmer/room-stat/TRVs) had nothing to read.
+            main_heating_controls=(
+                EpcPropertyDataMapper._map_energy_element(
+                    schema.main_heating_controls[0]
+                )
+                if schema.main_heating_controls
+                else None
+            ),
             window=EpcPropertyDataMapper._map_energy_element(schema.window),
             lighting=EpcPropertyDataMapper._map_energy_element(schema.lighting),
             hot_water=EpcPropertyDataMapper._map_energy_element(schema.hot_water),
@@ -1599,9 +1619,18 @@ class EpcPropertyDataMapper:
             insulated_door_count=schema.insulated_door_count,
             draughtproofed_door_count=None,
             percent_draughtproofed=schema.percent_draughtproofed,
+            # ADR-0028: the schema lodges total + low-energy OUTLET counts, not a
+            # bulb split — mirror the 17.1/18.0/20.0 paths (low-energy outlets →
+            # low_energy bulbs; the remainder → incandescent). Hardcoding all
+            # bulb counts to 0 dropped the lodged lighting mix, understating the
+            # low-energy lighting credit in the SAP lighting-energy calc.
             led_fixed_lighting_bulbs_count=0,
             cfl_fixed_lighting_bulbs_count=0,
-            incandescent_fixed_lighting_bulbs_count=0,
+            incandescent_fixed_lighting_bulbs_count=(
+                schema.fixed_lighting_outlets_count
+                - schema.low_energy_fixed_lighting_outlets_count
+            ),
+            low_energy_fixed_lighting_bulbs_count=schema.low_energy_fixed_lighting_outlets_count,
             roofs=EpcPropertyDataMapper._map_energy_elements(schema.roofs),
             walls=EpcPropertyDataMapper._map_energy_elements(schema.walls),
             floors=EpcPropertyDataMapper._map_energy_elements(schema.floors),
diff --git a/docs/adr/0042-glazing-override-reconciles-against-cert-composition.md b/docs/adr/0042-glazing-override-reconciles-against-cert-composition.md
index 2b82c472a..0df0517ec 100644
--- a/docs/adr/0042-glazing-override-reconciles-against-cert-composition.md
+++ b/docs/adr/0042-glazing-override-reconciles-against-cert-composition.md
@@ -29,3 +29,14 @@ The landlord glazing override **reconciles against the cert's per-window composi
 - Genuine landlord-vs-cert glazing disagreements are **surfaced** rather than silently trusted (old behaviour: flatten to double) or silently overwritten.
 - **Alternatives rejected.** *Dominant type wins* — still clobbers per-window variation. *Carry a full landlord composition to re-derive per-window types* — per-window assignment from an aggregate is unknowable; we decline to fabricate it. *Deterministic regex of "X% double, Y% single"* — brittle against varied input, which is the LLM's job.
 - Pairs with the per-window fidelity the calculator already relies on: the reconciliation is only correct on a faithful Effective EPC whose `sap_windows` round-trip (cf. ADR-0040).
+
+## Amendment (#1376 implementation): the reconcile collapses to a `MIXED` sentinel
+
+Implementing this surfaced that the full "carry a proportion → compare to the cert → flag when far" design buys less than its cost. The cert **already** carries the best-possible per-window mapping (`sap_windows`, each with its own `glazing_type`); a landlord aggregate ("40% double, 60% single") says *how much* is double but **not which windows**, so applying it would overwrite real per-window data with an arbitrary guess. Per-window assignment from an aggregate is therefore not just unknowable but strictly worse than the cert — so a mixed override **always defers**, regardless of the exact proportion. The proportion comparison only ever fed the "flag when far" branch, and carrying a proportion needs a new schema field (an FE/Drizzle dependency).
+
+**Realized decision.** Add a `GlazingType.MIXED` sentinel. The LLM classifies a genuine aggregate-mix to `MIXED` (the LLM handles the varied phrasings — no regex); `MIXED` is absent from the overlay's `_GLAZING_CODES`, so it resolves to **no overlay** and `_fold_glazing` never runs — the cert's per-window glazing is preserved. A **uniform** assertion (one type ≳ 90%) still resolves to that type and is applied to every window (unambiguous — no guess). Existing mixed-as-double rows are reclassified to `MIXED` (a deterministic parse of the `%X / %Y` format is acceptable for fixing *known* rows). The "uniform vs mixed" threshold (~90% one type) is tunable, pinned against the real distribution.
+
+**Consequences of the amendment.**
+- The defer behaviour needs **no apply-seam logic and no proportion field** — it falls out of "no overlay". The slice is: add the `MIXED` member, give the classifier the option, reclassify the ~319 rows.
+- `MIXED` is a **new FE-owned pgEnum value** (one member) — added by the FE owner (Dan); `property_overrides` (TEXT) reclassify runs immediately, the cache pgEnum write defers until the migration (the Class-A/B pattern).
+- The **"flag when landlord disagrees with the cert" is dropped for now** — since a mix always defers, the flag was informational only. `MIXED` is the hook if a future slice wants to surface those disagreements.
diff --git a/domain/epc/property_overrides/glazing_mix_guard.py b/domain/epc/property_overrides/glazing_mix_guard.py
new file mode 100644
index 000000000..36d8fce58
--- /dev/null
+++ b/domain/epc/property_overrides/glazing_mix_guard.py
@@ -0,0 +1,40 @@
+from __future__ import annotations
+
+import re
+from typing import Optional
+
+from domain.epc.property_overrides.glazing_type import GlazingType
+
+# "N% <base glazing type>" — the era (2002 / pre-2002) is irrelevant to whether it
+# is a mix, so only the base type is captured.
+_PERCENT_TYPE = re.compile(r"(\d+)%\s*(single|double|triple|secondary)")
+# One base type at or above this share is a uniform assertion, not a mix — applied
+# as that type (a near-uniform reglaze), not deferred.
+_UNIFORM_THRESHOLD_PERCENT = 90
+
+
+def glazing_mix_guard(description: str) -> Optional[GlazingType]:
+    """Deterministically resolve an aggregate-mix glazing description to ``MIXED``.
+
+    A landlord glazing description is often a whole-dwelling proportion split
+    ("40% double glazing 2002 or later, 60% single glazing"). Such a mix cannot be
+    faithfully applied per-window — it says *how much* is each type, not *which*
+    windows — and the cert already carries the true per-window glazing, so a mix
+    must resolve to ``GlazingType.MIXED`` (no overlay → the cert is kept), never to
+    a single dominant type that would flatten every window (ADR-0042).
+
+    Recognises the structured ``"N% <type>, M% <type>"`` split and returns
+    ``MIXED`` when it is a genuine mix — two or more glazing types present and none
+    dominant (≥ the uniform threshold). Returns ``None`` for a uniform / near-
+    uniform assertion or an unparseable description, so the LLM classifier still
+    resolves it (a uniform reglaze is applied; varied phrasings are the LLM's job).
+    """
+    matches = _PERCENT_TYPE.findall(description.lower())
+    base_types = {base_type for _, base_type in matches}
+    if len(base_types) < 2:
+        # One (or no) glazing type named — a uniform assertion, not a mix.
+        return None
+    if max(int(percent) for percent, _ in matches) >= _UNIFORM_THRESHOLD_PERCENT:
+        # One type dominates — treat as that (near-)uniform type, not a mix.
+        return None
+    return GlazingType.MIXED
diff --git a/domain/epc/property_overrides/glazing_type.py b/domain/epc/property_overrides/glazing_type.py
index ee3ad267b..8724f5b5e 100644
--- a/domain/epc/property_overrides/glazing_type.py
+++ b/domain/epc/property_overrides/glazing_type.py
@@ -21,4 +21,11 @@ class GlazingType(Enum):
     DOUBLE_PRE_2002 = "Double glazing, pre-2002"
     TRIPLE_PRE_2002 = "Triple glazing, pre-2002"
     TRIPLE_POST_2002 = "Triple glazing, 2002 or later"
+    # An aggregate mix of glazing types across the dwelling ("40% double, 60%
+    # single"). Deliberately absent from the overlay's `_GLAZING_CODES`, so it
+    # resolves to no overlay and the cert's per-window `sap_windows` are kept
+    # rather than flattened to one type — a whole-dwelling proportion cannot say
+    # which windows are which (ADR-0042 amendment, #1376). The FE-owned pgEnum
+    # gains this member via a Drizzle migration.
+    MIXED = "Mixed glazing"
     UNKNOWN = "Unknown"
diff --git a/scripts/reclassify_mixed_glazing.py b/scripts/reclassify_mixed_glazing.py
new file mode 100644
index 000000000..3cfc507ce
--- /dev/null
+++ b/scripts/reclassify_mixed_glazing.py
@@ -0,0 +1,147 @@
+"""One-time re-classification of aggregate-mix glazing overrides flattened to Double.
+
+#1376 / ADR-0042: a landlord glazing description that is an aggregate mix
+("40% double glazing 2002 or later, 60% single glazing") was classified to a single
+``"Double glazing…"`` value whenever any double was present, and the overlay then
+overwrote **every** window to double — over-crediting a predominantly-single dwelling
+and clobbering the cert's per-window glazing. A whole-dwelling proportion cannot say
+*which* windows are which, so a mix must resolve to ``GlazingType.MIXED`` (no overlay
+→ the cert's per-window ``sap_windows`` are kept).
+
+The live classifier now applies ``glazing_mix_guard`` deterministically (so new
+intakes of the structured split are correct); this fixes the rows written before it.
+The **same guard** decides the correction here, so the backfill and the live path
+cannot drift.
+
+Updates the TEXT ``property_overrides.override_value`` (what the modelling reads —
+the immediate fix). The ``landlord_glazing_overrides.value`` classifier cache is a
+``glazing`` **pgEnum**; ``MIXED`` is a new FE-owned value, so cache writes are
+**deferred** until the Drizzle migration adds it (the Class-A/B pattern).
+
+DRY-RUN BY DEFAULT: prints the counts it would change and writes nothing. Pass
+``--apply`` to execute inside a transaction. Idempotent — only rows whose stored
+value differs from the target member are touched.
+"""
+
+from __future__ import annotations
+
+import argparse
+from collections.abc import Iterable
+
+from sqlalchemy import Connection, text
+
+from domain.epc.property_overrides.glazing_mix_guard import glazing_mix_guard
+from scripts.e2e_common import build_engine, load_env
+
+
+def mixed_glazing_corrections(
+    stored: Iterable[tuple[str, str]],
+) -> dict[str, str]:
+    """``(description, stored override_value)`` → the ``MIXED`` value, for the
+    descriptions the glazing mix guard resolves to a mix whose stored value is not
+    already ``MIXED``. Uniform / unparseable descriptions and rows already on
+    ``MIXED`` are omitted, so re-running against corrected data is a no-op."""
+    corrections: dict[str, str] = {}
+    for description, value in stored:
+        member = glazing_mix_guard(description)
+        if member is not None and value != member.value:
+            corrections[description] = member.value
+    return corrections
+
+
+_DISTINCT = text(
+    """
+    SELECT DISTINCT lower(original_spreadsheet_description) AS description,
+                    override_value AS value
+    FROM property_overrides
+    WHERE override_component = 'glazing'
+    """
+)
+_OVERRIDES_UPDATE = text(
+    """
+    UPDATE property_overrides
+       SET override_value = :new_value
+     WHERE override_component = 'glazing'
+       AND lower(original_spreadsheet_description) = :description
+       AND override_value <> :new_value
+    """
+)
+_OVERRIDES_COUNT = text(
+    """
+    SELECT count(*) FROM property_overrides
+     WHERE override_component = 'glazing'
+       AND lower(original_spreadsheet_description) = :description
+       AND override_value <> :new_value
+    """
+)
+_CACHE_UPDATE = text(
+    """
+    UPDATE landlord_glazing_overrides
+       SET value = :new_value, updated_at = now()
+     WHERE lower(description) = :description
+       AND value::text <> :new_value
+    """
+)
+_ENUM_VALUES = text(
+    "SELECT e.enumlabel FROM pg_enum e JOIN pg_type t ON t.oid = e.enumtypid "
+    "WHERE t.typname = 'glazing'"
+)
+
+
+def reclassify(conn: Connection, *, apply: bool) -> tuple[int, set[str]]:
+    """Re-map aggregate-mix glazing overrides onto MIXED. Returns the number of
+    ``property_overrides`` rows found and the set of target values the live
+    ``glazing`` pgEnum does not yet carry (cache-deferred until the FE migration)."""
+    stored = [(r.description, r.value) for r in conn.execute(_DISTINCT)]
+    enum_values = {r[0] for r in conn.execute(_ENUM_VALUES)}
+    total = 0
+    deferred: set[str] = set()
+    for description, new_value in mixed_glazing_corrections(stored).items():
+        params = {"description": description, "new_value": new_value}
+        total += conn.execute(_OVERRIDES_COUNT, params).scalar() or 0
+        in_enum = new_value in enum_values
+        if not in_enum:
+            deferred.add(new_value)
+        if apply:
+            conn.execute(_OVERRIDES_UPDATE, params)
+            if in_enum:
+                conn.execute(_CACHE_UPDATE, params)
+    return total, deferred
+
+
+def main() -> None:
+    load_env()
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument(
+        "--apply",
+        action="store_true",
+        help="execute the updates (default: dry-run, writes nothing)",
+    )
+    args = parser.parse_args()
+
+    engine = build_engine()
+    with engine.begin() as conn:
+        conn.execute(text("SET statement_timeout = 120000"))
+        total, deferred = reclassify(conn, apply=args.apply)
+
+    verb = "re-classified" if args.apply else "would re-classify"
+    print(
+        f"{verb} {total} aggregate-mix glazing override row(s) from a flattened "
+        "Double onto MIXED (property_overrides / TEXT); the cert's per-window "
+        "glazing is kept."
+    )
+    if deferred:
+        print(
+            f"\n{len(deferred)} target value(s) NOT yet in the glazing pgEnum — "
+            "their classifier-cache rows are deferred until the FE-repo enum "
+            "migration adds these members (property_overrides was still updated, "
+            "which is what the modelling reads):"
+        )
+        for value in sorted(deferred):
+            print(f"    {value!r}")
+    if not args.apply:
+        print("\nDRY-RUN — nothing written. Re-run with --apply to execute.")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/tests/datatypes/epc/domain/test_mapper_lighting_parity.py b/tests/datatypes/epc/domain/test_mapper_lighting_parity.py
new file mode 100644
index 000000000..c3e3316fd
--- /dev/null
+++ b/tests/datatypes/epc/domain/test_mapper_lighting_parity.py
@@ -0,0 +1,57 @@
+"""Regression: the degraded RdSAP paths (17.0, 19.0) must carry the lodged
+fixed-lighting outlet counts, like the reference-complete 17.1/18.0/20.0 paths.
+
+PRD #1385 (mapper normalization), lighting family. Both paths hardcoded every
+bulb count to 0, dropping the lodged lighting mix. The schema lodges total +
+low-energy OUTLET counts (ADR-0028); the reference paths split them as
+low-energy → low_energy bulbs and the remainder → incandescent, feeding the SAP
+lighting-energy calc. (21.0.x use a different bulb-based lighting schema and are
+out of scope for this slice.)
+"""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+from typing import Any
+
+from datatypes.epc.domain.mapper import EpcPropertyDataMapper
+
+# 17.0 cert: fixed_lighting_outlets_count=10, low_energy=5 → incandescent 5.
+_FIXTURE_17_0 = Path("tests/fixtures/epc_prediction/PE71NT/cert-26619d8e7f8e.json")
+# 19.0 cert: fixed_lighting_outlets_count=8, low_energy=5 → incandescent 3.
+_FIXTURE_19_0 = Path("tests/fixtures/epc_prediction/PE71NT/cert-21b634bc4b59.json")
+
+
+def _load(path: Path, schema_type: str) -> dict[str, Any]:
+    data: dict[str, Any] = json.loads(path.read_text())
+    assert data["schema_type"] == schema_type
+    return data
+
+
+def test_17_0_path_carries_lighting_outlet_counts() -> None:
+    data = _load(_FIXTURE_17_0, "RdSAP-Schema-17.0")
+    assert data["fixed_lighting_outlets_count"] == 10
+    assert data["low_energy_fixed_lighting_outlets_count"] == 5
+
+    epc = EpcPropertyDataMapper.from_api_response(data)
+
+    # Was hardcoded 0/0/0. Low-energy outlets carry through; the remainder is
+    # incandescent; led/cfl stay 0 (reduced-field certs don't split those).
+    assert epc.low_energy_fixed_lighting_bulbs_count == 5
+    assert epc.incandescent_fixed_lighting_bulbs_count == 5
+    assert epc.led_fixed_lighting_bulbs_count == 0
+    assert epc.cfl_fixed_lighting_bulbs_count == 0
+
+
+def test_19_0_path_carries_lighting_outlet_counts() -> None:
+    data = _load(_FIXTURE_19_0, "RdSAP-Schema-19.0")
+    assert data["fixed_lighting_outlets_count"] == 8
+    assert data["low_energy_fixed_lighting_outlets_count"] == 5
+
+    epc = EpcPropertyDataMapper.from_api_response(data)
+
+    assert epc.low_energy_fixed_lighting_bulbs_count == 5
+    assert epc.incandescent_fixed_lighting_bulbs_count == 3
+    assert epc.led_fixed_lighting_bulbs_count == 0
+    assert epc.cfl_fixed_lighting_bulbs_count == 0
diff --git a/tests/datatypes/epc/domain/test_mapper_main_heating_controls_17_0.py b/tests/datatypes/epc/domain/test_mapper_main_heating_controls_17_0.py
new file mode 100644
index 000000000..3bb1a6d63
--- /dev/null
+++ b/tests/datatypes/epc/domain/test_mapper_main_heating_controls_17_0.py
@@ -0,0 +1,48 @@
+"""Regression: the RdSAP-Schema-17.0 mapper path must carry the lodged
+main_heating_controls, like every other API path (19.0/20.0/21.x).
+
+PRD #1385 (mapper normalization). 17.0 lodges `main_heating_controls`
+(List[EnergyElement]) but the mapper omitted it, leaving the domain field None
+while the 19.0/21.0.1 paths map the first control via `_map_energy_element`. The
+mapped element is the same EnergyElement type 17.0 already carries for
+`main_heating` / `window`, so this is straight parity.
+"""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+from typing import Any
+
+from datatypes.epc.domain.mapper import EpcPropertyDataMapper
+
+# 17.0 cert lodging a main-heating control ("Programmer, TRVs and bypass").
+_FIXTURE = Path("tests/fixtures/epc_prediction/PE71NT/cert-26619d8e7f8e.json")
+
+
+def _load() -> dict[str, Any]:
+    data: dict[str, Any] = json.loads(_FIXTURE.read_text())
+    assert data["schema_type"] == "RdSAP-Schema-17.0"
+    return data
+
+
+def test_17_0_maps_lodged_main_heating_controls() -> None:
+    data = _load()
+    assert data["main_heating_controls"], "fixture must lodge a control"
+
+    epc = EpcPropertyDataMapper.from_api_response(data)
+
+    # Was None on the 17.0 path; now carries the first lodged control element.
+    assert epc.main_heating_controls is not None
+    assert epc.main_heating_controls.description == "Programmer, TRVs and bypass"
+    assert epc.main_heating_controls.energy_efficiency_rating == 3
+
+
+def test_17_0_no_controls_maps_to_none() -> None:
+    # An empty control list maps to None (no phantom element), like the siblings.
+    data = _load()
+    data["main_heating_controls"] = []
+
+    epc = EpcPropertyDataMapper.from_api_response(data)
+
+    assert epc.main_heating_controls is None
diff --git a/tests/domain/epc/test_glazing_mix_guard.py b/tests/domain/epc/test_glazing_mix_guard.py
new file mode 100644
index 000000000..04e351cd3
--- /dev/null
+++ b/tests/domain/epc/test_glazing_mix_guard.py
@@ -0,0 +1,39 @@
+from __future__ import annotations
+
+import pytest
+
+from domain.epc.property_overrides.glazing_type import GlazingType
+from domain.epc.property_overrides.glazing_mix_guard import glazing_mix_guard
+
+
+def test_a_genuine_percentage_mix_classifies_as_mixed() -> None:
+    # Arrange — the bug: an aggregate mix the LLM would flatten to "Double".
+    description = "40% double glazing 2002 or later, 60% single glazing"
+
+    # Act
+    result = glazing_mix_guard(description)
+
+    # Assert — a mix defers to the cert's per-window glazing, not a whole-dwelling
+    # type.
+    assert result is GlazingType.MIXED
+
+
+@pytest.mark.parametrize(
+    "description",
+    [
+        "100% double glazing 2002 or later",  # uniform — one type
+        "96% double glazing 2002 or later, 4% single glazing",  # >= 90% -> uniform
+        "80% double glazing 2002 or later, 20% double glazing pre-2002",  # same base type
+        "some double glazing and a bit of single",  # unparseable — no percentages
+        "",
+    ],
+)
+def test_uniform_or_unparseable_glazing_defers_to_the_llm(description: str) -> None:
+    # The guard only claims a genuine percentage mix; a uniform assertion (applied
+    # as that type) or a varied phrasing is left for the LLM classifier.
+
+    # Act
+    result = glazing_mix_guard(description)
+
+    # Assert
+    assert result is None
diff --git a/tests/domain/epc/test_glazing_overlay.py b/tests/domain/epc/test_glazing_overlay.py
index 3182d79cf..81c35e6ca 100644
--- a/tests/domain/epc/test_glazing_overlay.py
+++ b/tests/domain/epc/test_glazing_overlay.py
@@ -57,6 +57,18 @@ def test_unresolvable_glazing_produces_no_overlay(glazing_value: str) -> None:
     assert simulation is None
 
 
+def test_a_glazing_mix_produces_no_overlay_so_the_cert_is_kept() -> None:
+    # A MIXED aggregate resolves to no overlay: a whole-dwelling proportion cannot
+    # say which windows are which, so the cert's per-window `sap_windows` are kept
+    # rather than flattened to one type (ADR-0042 amendment, #1376).
+
+    # Act
+    simulation = glazing_overlay_for(GlazingType.MIXED.value, 0)
+
+    # Assert
+    assert simulation is None
+
+
 def test_glazing_override_remaps_every_window_and_clears_lodged_u() -> None:
     # Arrange — baseline windows are double glazed (code 2, lodged U 2.8); the
     # landlord corrects the whole dwelling to single glazing.
@@ -74,14 +86,18 @@ def test_glazing_override_remaps_every_window_and_clears_lodged_u() -> None:
     assert all(w.window_transmission_details is None for w in result.sap_windows)
 
 
+_NO_OVERLAY_SENTINELS = {GlazingType.UNKNOWN, GlazingType.MIXED}
+
+
 @pytest.mark.parametrize(
-    "member", [m for m in GlazingType if m is not GlazingType.UNKNOWN]
+    "member", [m for m in GlazingType if m not in _NO_OVERLAY_SENTINELS]
 )
 def test_every_resolvable_glazing_value_decodes_to_a_code(
     member: GlazingType,
 ) -> None:
     # A classifier emits a GlazingType value; if the overlay can't decode it the
-    # override silently no-ops. Every non-UNKNOWN member must resolve.
+    # override silently no-ops. Every member except the deliberate no-overlay
+    # sentinels (UNKNOWN, MIXED) must resolve to a code.
 
     # Act
     simulation = glazing_overlay_for(member.value, 0)
diff --git a/tests/scripts/test_reclassify_mixed_glazing.py b/tests/scripts/test_reclassify_mixed_glazing.py
new file mode 100644
index 000000000..0a2f49447
--- /dev/null
+++ b/tests/scripts/test_reclassify_mixed_glazing.py
@@ -0,0 +1,35 @@
+"""The mixed-glazing reclassify maps a flattened aggregate mix onto MIXED, leaves
+uniform / already-correct rows alone, and is idempotent (#1376 / ADR-0042)."""
+
+from __future__ import annotations
+
+from scripts.reclassify_mixed_glazing import mixed_glazing_corrections
+
+
+def test_flattened_mix_rows_are_corrected_to_mixed() -> None:
+    # Arrange — an aggregate mix flattened to Double (the bug), a near-uniform mix
+    # and a clean uniform row (both correctly Double), and a pure single row.
+    stored = [
+        ("40% double glazing 2002 or later, 60% single glazing", "Double glazing, 2002 or later"),
+        ("96% double glazing 2002 or later, 4% single glazing", "Double glazing, 2002 or later"),
+        ("100% double glazing 2002 or later", "Double glazing, 2002 or later"),
+        ("100% single glazing", "Single glazing"),
+    ]
+
+    # Act
+    corrections = mixed_glazing_corrections(stored)
+
+    # Assert — only the genuine mix is re-mapped; near-uniform and uniform stay.
+    assert corrections == {
+        "40% double glazing 2002 or later, 60% single glazing": "Mixed glazing"
+    }
+
+
+def test_already_mixed_rows_need_no_change() -> None:
+    # Act / Assert — idempotent.
+    assert (
+        mixed_glazing_corrections(
+            [("40% double glazing 2002 or later, 60% single glazing", "Mixed glazing")]
+        )
+        == {}
+    )