Retrofitting solar to an existing home averaged £2,022 per kW in 2025/26 (DESNZ, MCS data), about £8,090 for a 4 kW system, against £1,599 per kW for new-build installs: retrofit costs roughly a quarter more. 0% VAT applies until 31 March 2027. Retrofit makes sense for existing homes; new-build phase install is cheaper and cleaner when the choice is open.
Retrofit vs new-build cost compared
| Array | New-build phase (~£1,200/kWp) | Retrofit (DESNZ average, £2,022/kW) | Difference |
|---|---|---|---|
| 3.74 kWp (reference array, 42.5 m² ground floor) | £4,490 | £7,560 | 41% lower at build stage |
| 4 kWp | £4,800 | £8,090 | 41% lower at build stage |
Rates: about £1,200 per kWp for roof-integrated PV fitted during construction (Spirit Energy) and £2,022 per kW for domestic retrofits, the 2025/26 mean in the DESNZ solar PV cost data (per kW of declared net capacity, so it leans high per kWp). On DESNZ's own like-for-like means the gap is 21%; the Spirit Energy build-stage rate is lower again. The gap comes mostly from access and labour: retrofit needs its own scaffold and works around a finished roof, while new-build installation shares the scaffold with the main build and goes in alongside the roofing. This site does not publish a line-by-line split because no source breaks the two rates down that way.
When retrofit is the right choice
Despite the premium, retrofit is the correct decision in three scenarios:
- Buying an existing non-PV home: If you have purchased a recently-built (2024-vintage) dwelling without solar, retrofit is the only route to add generation.
- Pre-FHS plots under the transitional period: A developer completing a pre-2027 plot to Part L 2021 spec may retrofit later if and when buyer demand justifies it.
- Fuel-poverty intervention: A housing association retrofitting solar to pre-FHS stock to cut tenant bills accelerates the outcome without waiting for stock turnover.
0% VAT on retrofit solar
Retrofit solar PV on existing dwellings attracts 0% VAT under the energy-saving materials relief, the same as new-build installations. The relief covers panels, mounting, inverter, battery and labour, and runs until at least March 2027. On a £5,550 retrofit that is a £1,110 saving versus the 20% rate. For the full finance picture — including green mortgages and the SEG export tariff — see our finance guide. The VAT on solar panels guide sets out when the 0% rate applies and what changes after 31 March 2027, and the guide to solar panels and building regulations covers the roof-strength and electrical checks a retrofit needs.
For the detailed integration and EPC argument, read new build vs retrofit solar. To ask about a particular home, send us the details and we reply by email.