diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 7e3add21..06771a7e 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -3319,14 +3319,28 @@ def _synthesise_reduced_field_windows(
     ]
 
 
+# ADR-0028: multiple_glazing_type "ND" (Not Defined) → the DG-modal
+# default (cascade code 2 → daylight g_L 0.80), as for 18.0/19.0/17.x.
+_RDSAP20_ND_GLAZING_TYPE: int = 2
+
+
 def _synthesise_20_0_0_sap_windows(schema: RdSapSchema20_0_0) -> List[SapWindow]:
     """ADR-0027/0028 seam: 20.0.0 glazed_type codes 1-8+ND are identical to
     21.0.1's, so route multiple_glazing_type through the verified cascade (fixes
-    code 1 "DG pre-2002" read as single), then call the shared core."""
+    code 1 "DG pre-2002" read as single), then call the shared core. The "ND"
+    string (no cascade mapping) falls back to the DG-modal default, mirroring
+    `_synthesise_18_0_sap_windows` — without this the cascade raised
+    UnmappedApiCode on every ND-glazed 20.0.0 cert."""
+    mgt = schema.multiple_glazing_type
+    glazing_type = (
+        _api_cascade_glazing_type(mgt)
+        if isinstance(mgt, int)
+        else _RDSAP20_ND_GLAZING_TYPE
+    )
     return _synthesise_reduced_field_windows(
         schema.glazed_area,
         schema.total_floor_area,
-        _api_cascade_glazing_type(schema.multiple_glazing_type),
+        glazing_type,
     )
 
 
diff --git a/datatypes/epc/schema/rdsap_schema_20_0_0.py b/datatypes/epc/schema/rdsap_schema_20_0_0.py
index dbd5feef..9a4b78f2 100644
--- a/datatypes/epc/schema/rdsap_schema_20_0_0.py
+++ b/datatypes/epc/schema/rdsap_schema_20_0_0.py
@@ -267,7 +267,9 @@ class RdSapSchema20_0_0:
     energy_rating_current: int
     lighting_cost_current: float
     main_heating_controls: List[EnergyElement]
-    multiple_glazing_type: int
+    # "ND" (Not Defined) appears in the corpus alongside the 1-8 integer
+    # codes — type as Union to match the data (mirrors RdSAP-Schema-18.0).
+    multiple_glazing_type: Union[int, str]
     open_fireplaces_count: int
     heating_cost_potential: float
     hot_water_cost_current: float