diff --git a/domain/epc/property_overrides/glazing_mix_guard.py b/domain/epc/property_overrides/glazing_mix_guard.py
index 83f4c45c0..36d8fce58 100644
--- a/domain/epc/property_overrides/glazing_mix_guard.py
+++ b/domain/epc/property_overrides/glazing_mix_guard.py
@@ -1,9 +1,17 @@
 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``.
@@ -21,4 +29,12 @@ def glazing_mix_guard(description: str) -> Optional[GlazingType]:
     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
+    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