Working file C1

Retrofit cost calculator (UK 2026)

A surveyor's working calculator for UK house retrofit cost in 2026. Compares heat-pump-only against phased deep retrofit, applies the BUS, ECO4, GBIS and 0% VAT grants, and reports the running-cost change at the Q4 2026 Ofgem cap.

The calculator

Live calculator

Q4 2026 cap baked in

ArchetypeInternal floor area (m²)Current EPC bandTarget EPC bandCurrent annual gas spend (£) , Q4 2026 cap is 7.97p / kWh plus 29.68p / day standingDelivered heat pump SPF , trial median 2.78Boiler it replaces, seasonal efficiency (%)

Grant eligibility

BUS (£7,500) ECO4 (~£8,000) GBIS (~£1,500) 0% VAT route

Total capex

£36,085 to £63,912

Grant offset

£7,500

Net after grants

£28,585 to £56,412

Annual running-cost change

£101

Heat pump leg, line by line

Gas standing charge dropped: £108 / yr
Heat delivered by the boiler: 8,443 kWh
Electricity a heat pump needs: 3,037 kWh
Worst case (boiler 92%, SPF 2.55): minus £43 / yr
Best case (boiler 76%, SPF 3.05): £249 / yr
Heat pump leg net of the BUS grant: £2,300 to £11,000
Years for running-cost savings to repay that: 66 yr

Live math, coefficients on /sources/. Gas standing charge comes off the bill first, the rest buys gas at 7.97p, the boiler turns that into heat at the efficiency you set, and the heat pump buys the same heat as electricity at 26.32p divided by the SPF you set. At this cap gas at 85% costs 9.38p per kWh of heat and a heat pump at SPF 2.78 costs 9.47p, which is why the standing charge does most of the work. Payback is quoted on the heat-pump leg alone: the fabric, glazing and solar legs all cut demand too, and this file does not model that, so it publishes no whole-project payback.

What this calculator does

It takes archetype, internal floor area, current and target EPC band, current annual gas spend, and grant eligibility, and returns four numbers: total capex (low to high), grant offset, net cost after grants, and the annual running-cost change from the gas-to-electricity fuel switch at the Q4 2026 Ofgem cap.

It does not pretend to replace a Retrofit Assessor (£400 to £800) or Coordinator (£1,500 to £3,500). It frames the conversation before those fees are spent.

Preset routes

Coefficients

Every coefficient is exposed on the sources manifest as a JSON object with the publishing URL beside it. The math is not proprietary; it is a transparent stack of published unit costs.

Surveyed 1 October 2026. Ofgem cap for 1 October to 31 December 2026 applied (gas 7.97p, elec 26.32p, gas standing 29.68p, elec standing 54.83p). Next scheduled review 25 November 2026 (January cap announcement). [ref C1]

By Oliver Wakefield-Smith, Founder, Digital Signet. Corrections to oliver at digitalsignet dot com.