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..83f4c45c0
--- /dev/null
+++ b/domain/epc/property_overrides/glazing_mix_guard.py
@@ -0,0 +1,24 @@
+from __future__ import annotations
+
+from typing import Optional
+
+from domain.epc.property_overrides.glazing_type import GlazingType
+
+
+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).
+    """
+    raise NotImplementedError
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/tests/domain/epc/test_glazing_mix_guard.py b/tests/domain/epc/test_glazing_mix_guard.py
new file mode 100644
index 000000000..973b96b6a
--- /dev/null
+++ b/tests/domain/epc/test_glazing_mix_guard.py
@@ -0,0 +1,16 @@
+from __future__ import annotations
+
+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