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/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")]
+        )
+        == {}
+    )