House extension cost UK 2026
Indicative ranges by type, the decisions that move them, and the costs people forget until the invoice arrives.
| Extension type | Typical UK range, built and finished |
|---|---|
| Single-storey extension | £24,900 – £50,200 |
| Side extension | £16,300 – £37,700 |
| Two-storey extension | £50,200 – £90,300 |
Source: BestBuilders’ 2026 regional cost model — published national cost guides adjusted by a regional labour index, averaged across 519 UK towns. Indicative, not quotes.
What moves the price more than size does
- Ground conditions. Trees, a high water table, shallow existing foundations or a sewer crossing the footprint add depth — and cost — before anything visible is built.
- Steel. Opening the back wall means a beam, a structural engineer, padstones and often bigger foundations. Removing the whole rear wall costs materially more than leaving a pier.
- Glazing. Bifolds, sliders and rooflights are where budgets quietly double. A large sliding door can cost more than the roof above it.
- Drainage. Building over or near a public sewer needs a build-over agreement, sometimes a diversion.
- Finish level. The shell is fairly predictable; kitchens, flooring, underfloor heating and lighting are where two identical shells end up £20,000 apart.
Costs that are easy to forget
Drawings and a structural engineer, the planning or prior approval fee, Building Control, a party wall surveyor if a neighbour dissents, skips and welfare, and VAT. Together these are a meaningful percentage of the build rather than a rounding error — budget for them at the start.
How to use a range like this
As a sanity check, not a budget. Get three itemised quotes from builders who have seen the property and compare them line by line — particularly what each assumed about foundations, steel and glazing, because that is usually where two quotes that look different are simply pricing different jobs.
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