diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index f9e8f0d1..e500c32e 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -1867,6 +1867,12 @@ _ELMHURST_PARTY_WALL_CODE_TO_SAP10: Dict[str, int] = {
     "CU": 4,  # Cavity masonry unfilled — same U=0.5 cascade; Elmhurst
               # encodes party-wall cavity type with the masonry sub-code
               # (CU vs CF filled) — observed first on cert 001479 Main.
+    # "U Unable to determine" — the cohort's modal lodgement. The cohort
+    # hand-built convention uses 0 as the explicit "unknown" sentinel
+    # (rather than None) so cross-mapper field parity is preserved; the
+    # cascade resolves 0 to the same `u_party_wall` default (0.25) as
+    # None, so cascade output is unchanged for cohort-style certs.
+    "U": 0,
 }
 
 
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py
index 6e953323..8b2d8dbd 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py
@@ -221,6 +221,11 @@ def build_epc() -> EpcPropertyData:
     epc.sap_heating.shower_outlets = ShowerOutlets(
         shower_outlet=ShowerOutlet(shower_outlet_type="Non-electric shower"),
     )
+    # Summary §14 "Heat pump age: Unknown" — surfaced by the Elmhurst
+    # mapper as the str dual-encoding that internal_gains.py reads.
+    # `make_main_heating_detail` doesn't expose the str kwarg, so set
+    # here post-construction for cross-mapper field parity.
+    epc.sap_heating.main_heating_details[0].central_heating_pump_age_str = "Unknown"
     return epc