House extension costs: a 2026 guide
Extension costs vary enormously with size, design and specification. The figures below are an indicative 2026 guide, the only reliable way to get a real price is to prepare drawings and a proper tender.
Indicative 2026 costs
All figures below are build-cost guides for 2026 and exclude VAT, fees and finishes. London and the South East run higher because of labour and site costs.
| Project | London & South East | Rest of UK |
|---|---|---|
| Single-storey extension (per m²) | £3,000–£4,500 | £2,000–£3,000 |
| Two-storey extension | Around the single-storey cost plus ~50% | |
| Loft conversion with roof lights (approx. 4×5m) | £30,000–£38,000 | £27,000–£33,000 |
| Loft conversion with dormer (approx. 4×5m) | £45,000–£55,000 | £40,000–£48,000 |
| Hip-to-gable loft conversion | £55,000–£70,000 | £45,000–£65,000 |
Treat these as starting points, not quotes. Roughly 40% of a build cost is labour, and the specification you choose can move the figure significantly in either direction.
The four things that drive the cost
1. Size. Extensions are usually costed per square metre, so floor area is the biggest single driver of the headline figure.
2. Design. The form, structure and complexity all affect price. Getting the design fixed before you build matters: once a builder is on site, changes are charged as extras.
3. Specification. There are hundreds of choices, bricks and cladding, windows and doors, roof coverings, flooring and tiling, heating, lighting, kitchens and bathrooms, drainage, foundations and landscaping, and each one moves the cost. A detailed specification removes ambiguity and protects you from surprises.
4. Project management. Good coordination keeps the build on programme and on budget, and avoids costly mistakes.
Costs beyond the build
As well as construction, budget for planning application and building regulation fees, and for design, drawing and structural design fees. Investing in a thorough design and a clear specification is the best way to keep the build cost under control, and a well-designed extension adds real value to your home.
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