Retrofitting solar costs £1,565-£1,686 per kWp on the MCS 2026 average (£6,300-£6,700 for a 4 kWp system) — roughly 20-30% more than the same capacity at new-build phase (~£1,200/kWp in-roof). 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
| System (4 kWp) | New-build phase | Retrofit | Difference |
|---|---|---|---|
| Panels + inverter | £2,940 | £3,200 | +£260 |
| Mounting | £480 (in-roof) | £300 (on-roof rails) | −£180 |
| Scaffold + access | £0 (shared with build) | £650 | +£650 |
| Labour | £530 | £1,400 | +£870 |
| Total | £3,950 | £5,550 | +£1,600 (41%) |
The cost gap is almost entirely labour and access: retrofit needs its own scaffold mobilisation and roughly 3× the on-site install time, because the roof is already complete and the crew works around the finished structure. New-build phase installation shares the scaffold with the main build and installs the array concurrently with roofing.
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 get a retrofit quote, request a consultation.