diff --git a/domain/sap10_calculator/docs/HANDOVER_CERT_9501_AND_HEATPUMPS.md b/domain/sap10_calculator/docs/HANDOVER_CERT_9501_AND_HEATPUMPS.md
index 3e05df3c..105aaa9c 100644
--- a/domain/sap10_calculator/docs/HANDOVER_CERT_9501_AND_HEATPUMPS.md
+++ b/domain/sap10_calculator/docs/HANDOVER_CERT_9501_AND_HEATPUMPS.md
@@ -52,39 +52,80 @@ PYTHONPATH=/workspaces/model python -m pytest \
 
 ## Outstanding workstreams (in priority order)
 
-### 1. Heat-pump workstream — 7 ASHP certs (DEFERRED until go-ahead)
+### 1. Heat-pump workstream — cert 0380 IN PROGRESS
 
-Cert refs (per the prior handover): 0350, 0380, 2225, 2636, 3800,
-9285, 9418. Predominantly PCDB index 104568 (one model 102421). The
-mapper has never been validated against a heat-pump cert.
+Cert refs: 0380, 0350, 2225, 2636, 3800, 9285, 9418. Predominantly
+PCDB index 104568 (one model 102421).
 
-Cert 0380 fixtures are already staged (commit `17646c8a`). Original
-probe showed:
+#### Cert 0380 state at session end (semi-detached bungalow, ASHP, age D)
 
-| Path | Cascade SAP | Δ vs worksheet 88.5104 |
-|---|---|---|
-| Summary mapper | 18.08 | **-70.43** (catastrophic — Summary identifies HP as 80% boiler) |
-| API mapper | 70.14 | **-18.37** |
+Worksheet target SAP: **88.5104**
 
-The Summary mapper is fundamentally broken on heat pumps; the API
-mapper is partially-broken. Likely 15-30 slice workstream. Sketch
-(from the prior handover, unchanged):
+| Path | Cascade SAP | Δ vs worksheet | Status |
+|---|---|---|---|
+| Summary mapper | 30.14 | **-58.37** | Still broken on HPs (Summary identifies HP wrong) |
+| API mapper | 91.43 | **+2.92** | HLC EXACT match worksheet; cost gap dominates |
 
-1. **API mapper**: surface `main_heating_index_number`, set
-   `main_heating_category` for HPs, `main_fuel_type=29` (electric
-   heat pump).
-2. **Cascade**: ensure `cert_to_inputs._main_heating_efficiency`
-   reads PCDB HP COP correctly. Investigate Table 4a/4b vs PCDB
-   precedence for HPs.
-3. **Fuel cost**: HW + space heating on electricity tariffs
-   (Table 12) — check if the cascade has electric-tariff fuel-cost
-   plumbing wired up.
-4. **Appendix N**: HP-specific efficiency adjustments (climate +
-   flow temperature). Likely the biggest cascade-side gap.
-5. **Summary mapper**: separate slice — needs to identify HPs from
-   the Summary PDF's heating section.
+API path closed gaps this session (Slices 101a-c):
 
-**Do NOT start HP slices without an explicit go-ahead from the user.**
+- **101a** API `glazing_type=14` → SAP 10.2 Table 24 post-2022 DG
+  (U=1.4, g=0.72). Closed windows HLC.
+- **101b** Cavity wall + filled cavity + external insulation:
+  added `WALL_INSULATION_CAVITY_PLUS_EXTERNAL=6` +
+  `WALL_INSULATION_CAVITY_PLUS_INTERNAL=7` constants + composite
+  U formula `1/(1/U_filled + R_ins)` with 2-d.p. rounding to match
+  the dr87 worksheet. Walls HLC 14.87 → 11.62 (worksheet exact).
+- **101b** SAP 10.2 Table 11 cat-4 (HP) secondary fraction = 0
+  (dict was missing the entry; HP certs fell through to DEFAULT
+  0.10). Removed £72 incorrect secondary heating cost.
+- **101c** SAP 10.2 Table 4f cat-4 (HP) pumps/fans = 0 (dict was
+  missing the entry; HP certs fell through to DEFAULT 130 kWh →
+  £17 incorrect pumps/fans cost). Worksheet line (249) shows 0 kWh.
+
+(37) total fabric heat loss EXACT match worksheet 96.0889.
+
+#### Remaining cert 0380 API gaps (HW cascade — dominant Δ +2.92 SAP):
+
+Component cost breakdown (cert 0380 API vs worksheet):
+
+| Component | Cascade £ | Worksheet £ | Δ £ |
+|---|---|---|---|
+| main heating | 313.86 | 316.36 | -2.50 |
+| secondary | 0.00 | 0.00 | ✓ |
+| **hot water** | **66.36** | **115.82** | **-49.46 (BIG)** |
+| pumps_fans | 0.00 | 0.00 | ✓ |
+| lighting | 23.17 | 23.75 | -0.58 |
+| electric shower | 88.93 | 88.93 | ✓ |
+| PV credit | -338.11 | -338.11 | ✓ |
+| **TOTAL** | **154.21** | **206.75** | **-52.54** |
+
+Hot water cascade for HP needs:
+
+1. **HP HW-specific COP** — worksheet uses 1.711 (PCDB-derived for
+   model 104568); cascade uses 2.3 (Table 4a generic HP COP). HW
+   needs higher water temp than space heating → lower COP. PCDB
+   likely has a separate `water_heating_efficiency` field.
+2. **Cylinder storage + primary losses** — worksheet (47) lodges
+   cylinder size 160 L with (56) storage loss ~444 kWh + (59)
+   primary loss ~503 kWh. Cascade computes HW heat demand of 1157
+   kWh vs worksheet 1502 kWh (Δ -345 kWh, likely the storage loss
+   not applied). Cert 0380's `cylinder_size: 3` is an integer
+   schema code (not litres) — needs lookup to actual volume.
+3. **Possibly two**: HW heat demand AND HP HW COP both need
+   fixing to land at worksheet's 878 kWh fuel.
+
+#### Summary path (cert 0380) — still catastrophic
+
+The Summary mapper produces SAP 30.14 vs worksheet 88.51 (Δ -58.37).
+The Elmhurst extractor likely identifies the HP as a different
+heating system. Significant work — defer until API path is closed.
+
+#### Remaining HP certs
+
+Cert 0380's HW + HP HW COP work will likely benefit ALL 7 ASHP
+certs (they share PCDB idx 104568 or 102421). Once cert 0380 closes
+to 1e-4 on the API path, the other 6 ASHPs should close in 1-3
+slices each (shape variations).
 
 ### 2. 8 cohort golden certs without worksheets