Working file C2
Deep retrofit versus heat pump only (UK 2026)
Side-by-side capex, net-after-grants and payback for an ASHP-only project versus a phased PAS 2035 deep retrofit. Q3 2026 Ofgem cap baked in.
Live calculator
Live calculator
Q3 2026 cap baked inTotal capex
£36,085 to £63,912
Grant offset
£7,500
Net after grants
£28,585 to £56,412
Payback (yrs)
408 yr to 1880 yr
Live math. Coefficients on /sources/. ASHP SCOP assumed 3.2; saving = current gas cost minus electricity cost of same kWh demand at the Q3 2026 cap.
The two quotes
Quote A is an ASHP and a radiator upsize. Quote B is a phased fabric-first programme with ventilation, heating, renewables and controls drawn out across a five to ten year medium-term improvement plan. The £-delta is real and worth resolving in writing before either quote is signed.
When heat-pump-only is defensible
- Post-2000 build with U-values already in the 0.18 to 0.28 W/m²K band.
- Cavity-filled 1930s semi where loft is already at 300mm and floors are uncertain.
- Owner-occupier with no realistic five-year horizon to phase a deeper programme.
When deep retrofit pays back
- Pre-1919 solid wall where IWI plus glazing changes the delivered SCOP of the heat pump.
- System-built or no-fines walls where fabric work is a precondition for an MCS-compliant heat-pump design.
- Households on enhanced PTC or LA Flex routes where ECO4 or GBIS strip a real layer off the net cost.
Payback ribbonYears to break even, at Q3 2026 Ofgem cap.
ECO4 cliff
BUS review (~2028)
0 yr5 yr10 yr15 yr20 yr
6 to 15 years to break even at the Q3 2026 Ofgem cap.
Surveyed 22 June 2026. Ofgem Q3 2026 cap applied (gas 7.33p, elec 26.11p, gas standing 29.04p). Next scheduled review 26 September 2026 (Q4 cap publication). [ref C2]
By Oliver Wakefield-Smith, Founder, Digital Signet. Corrections to oliver at digitalsignet dot com.