How much does a house extension cost in 2026?
A typical single-storey rear extension costs about £1,750 to £3,400 per m² including VAT in 2026, so a standard 4 m × 5 m room is around £41,000 to £52,000 before design and council fees. Two-storey extensions cost less per square metre, about £1,550 to £3,000 per m², and London prices run roughly 30% above the national guide.
What does the price per square metre include?
Extension costs are usually compared per square metre of new internal floor area because it is the simplest way to compare jobs of different sizes. Our guide figures cover the build itself: foundations, drainage, walls, roof, steelwork, the opening into the house, windows and doors, insulation, plastering, electrics and a basic floor finish, all including VAT at 20%.
They do not include design fees, structural calculations, council planning and building control fees, party wall surveyors, the kitchen, bathroom fittings or decoration. On a typical project, fees and fittings add 10–20% to the build cost, and a mid-range kitchen adds £9,000 to £20,000 on top.
Guide prices by extension type
These are national guide ranges at standard specification, before any regional adjustment. Our price guide shows all three specification tiers and the regional factors we use.
| Extension type | Per m² (all tiers) | Example at standard spec |
|---|---|---|
| Single-storey rear | £1,750 to £3,400 per m² | 4 m × 5 m: £41,000 to £52,000 |
| Double-storey | £1,550 to £3,000 per m² | 32 m² over two floors: £57,500 to £75,500 |
| Side return | £2,100 to £3,800 per m² | 11 m²: £26,000 to £33,000 |
| Wraparound | £2,000 to £3,700 per m² | 25 m²: £57,500 to £72,500 |
| Orangery | £2,400 to £4,500 per m² | 16 m²: £44,500 to £56,000 |
| Glass box | £3,000 to £5,800 per m² | 12 m²: £42,000 to £53,000 |
| Over a garage | £1,800 to £3,400 per m² | 12 m²: £25,000 to £32,500 |
Why do two-storey extensions cost less per square metre?
The most expensive parts of any extension are the foundations, the groundworks and the roof. A two-storey extension builds those once but gets two floors of space from them, so the cost per square metre of floor area falls. The total project is still larger, and planning is stricter, but the value per square metre is usually better.
How much more does an extension cost in London and the South East?
Labour, access, parking and waste removal all cost more in London, and terraced sites often need party wall work and smaller machinery. We apply a factor of about 1.30 for London and 1.12 for the South East, and lower factors in the North East, Wales and Northern Ireland. Your written quote replaces the guide figure once we have surveyed the house.
What pushes the cost up?
- Ground conditions. Clay, trees nearby or made ground can mean deeper or piled foundations.
- Drains. A sewer or manhole in the way may need diverting and a build-over agreement.
- Glazing. Large sliding doors, roof lanterns and structural glass are among the biggest single costs.
- Steelwork. Wide openings and corner removals need bigger beams and sometimes posts.
- Access. Terraced houses with no side access mean materials and spoil go through the house.
- Finishes. Flooring, kitchens, lighting and joinery can double the cost of the shell.
Do I pay VAT on an extension?
Yes. Building work to extend an existing home is standard-rated for VAT at 20%. Our guide prices include it. Some conversions and new dwellings are charged at reduced or zero rates, but an ordinary house extension is not one of them.
How do I get an accurate price?
Start with our guide-price tool for a range, then book a site visit. After the survey we confirm the planning route and give you a written price. Where drawings are needed, the design stage is priced in writing first and the build price is fixed once the design and specification are agreed.
This guide is general information, not legal or professional advice for your property. Rules can change; we check the current position for your address at the survey.