diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index a9e9fbf7..b30b8177 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -541,7 +541,28 @@ class EpcPropertyDataMapper:
                 secondary_heating_type=None,
                 cylinder_insulation_thickness_mm=None,
             ),
-            sap_windows=[],
+            # ADR-0028: 990/1000 omit sap_windows -> synthesised from the
+            # glazed_area band + TFA via the shared reduced-field core. The 10
+            # rich certs keep their lodged per-window geometry (used directly:
+            # window_width = area, height = 1.0).
+            sap_windows=[
+                SapWindow(
+                    frame_material=None,
+                    glazing_gap=0,
+                    orientation=w.orientation,
+                    window_type=w.window_type,
+                    glazing_type=w.glazing_type,
+                    window_width=_measurement_value(w.window_area),
+                    window_height=1.0,
+                    draught_proofed=False,
+                    window_location=w.window_location,
+                    window_wall_type=0,
+                    permanent_shutters_present=False,
+                )
+                for w in schema.sap_windows
+            ]
+            if schema.sap_windows
+            else _synthesise_17_0_sap_windows(schema),
             sap_energy_source=SapEnergySource(
                 mains_gas=es.mains_gas == "Y",
                 meter_type=str(es.meter_type),
@@ -3210,6 +3231,28 @@ def _synthesise_19_0_sap_windows(schema: RdSapSchema19_0) -> List[SapWindow]:
     )
 
 
+# ADR-0028: multiple_glazing_type "ND" (Not Defined, 54/1000 17.0 certs) → the
+# DG-modal default, as for 18.0/17.1.
+_RDSAP17_0_ND_GLAZING_TYPE: int = 2
+
+
+def _synthesise_17_0_sap_windows(schema: RdSapSchema17_0) -> List[SapWindow]:
+    """ADR-0028 seam: reuses the inherited 20.0.0 coefficients via the shared
+    core (968/1000 band-1 with no measured band-1 windows; only 10 rich certs).
+    17.0 glazed_type codes (observed 1-4, 7) are a subset of 20.0.0's verified
+    1-8 space: route integer codes through the verified cascade; the "ND" string
+    falls back to the DG-modal default. Own seam so 17.0 can diverge."""
+    mgt = schema.multiple_glazing_type
+    glazing_type = (
+        _api_cascade_glazing_type(mgt)
+        if isinstance(mgt, int)
+        else _RDSAP17_0_ND_GLAZING_TYPE
+    )
+    return _synthesise_reduced_field_windows(
+        schema.glazed_area, schema.total_floor_area, glazing_type
+    )
+
+
 # ADR-0028: multiple_glazing_type "ND" (Not Defined, 56/1000 17.1 certs) → the
 # DG-modal default, as for 18.0.
 _RDSAP17_1_ND_GLAZING_TYPE: int = 2