House Extension Cost Calculator UK
The average extension cost in the UK is £1,800 to £3,000 per m² for a single-storey build in 2026, so a 20 m² rear extension is around £38,000 to £55,000 before VAT and fees. Double-storey extensions cost less per m² (about £1,600 to £2,800). London and the South East can be 20 to 40% higher. Use this extension cost estimator to price your own project.
- Single storey, per m²£1,800 – £3,000
- 20 m² rear extension£38,000 – £55,000
- Double storey, per m²£1,600 – £2,800
Prices checked November 2026 · UK figures include labour and materials unless stated
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House extension cost per m² (2026)
| Finish | Single storey per m² | Double storey per m² |
|---|---|---|
| Basic | £1,800 – £2,200 | £1,550 – £1,900 |
| Standard | £2,200 – £2,700 | £1,900 – £2,300 |
| High-end | £2,700 – £3,500+ | £2,300 – £3,000 |
Figures are for the build itself, excluding VAT and professional fees. London typically adds 20 to 40%.
House extension prices: examples
| Project | Size | Typical build cost (exc. VAT) |
|---|---|---|
| Small single-storey rear | 3 m × 4 m (12 m²) | £25,000 – £35,000 |
| Standard single-storey rear | 4 m × 5 m (20 m²) | £38,000 – £55,000 |
| Large kitchen extension | 30 m² | £60,000 – £84,000 |
| Double-storey rear | 2 × 18 m² (36 m²) | £70,000 – £100,000 |
Single storey vs double storey extension cost
A double storey extension costs more in total but less per square metre, because the foundations and roof serve two floors. For a standard finish, a single storey extension costs about £2,200 to £2,700 per m², and a double storey extension about £1,900 to £2,300 per m². Choose “Double storey” in the calculator and enter the total floor area across both floors.
Hidden costs to budget for
- Professional fees: architect or designer, structural engineer and building control, usually 10 to 15% of the build.
- VAT: most builders must charge 20% VAT on top.
- Party wall agreements if you build on or near a shared boundary.
- Kitchen or bathroom fit-out, which can add £8,000 to £25,000.
- Contingency: keep at least 10% aside for surprises such as poor ground or drains.
Do you need planning permission?
Many single-storey rear extensions fall under permitted development, within limits on depth, height and how much of the garden is covered. Larger homes can use the prior approval route for deeper extensions. Two-storey extensions, side extensions on some homes, and anything in a conservation area or on a listed building usually need full planning permission. Building regulations approval is always required.
How this extension cost calculator works
We multiply your floor area by a 2026 build rate per m² for your chosen finish, take 15% off for double-storey builds, and adjust for your region. Professional fees (10 to 15%), a 10% contingency and VAT are then added if you tick them.
Frequently asked questions
How much does an extension cost per m² in the UK?
Single-storey extensions typically cost £1,800 to £3,000 per m² in 2026 for the build. Double-storey extensions are cheaper per m², at around £1,600 to £2,800.
How much does a 4m x 5m extension cost?
A 20 m² single-storey extension usually costs about £38,000 to £55,000 for the build, before VAT and professional fees.
Is a double storey extension cheaper per m²?
Yes. The foundations and roof are shared across two floors, so the cost per m² is lower, although the total cost is higher.
How long does a house extension take to build?
A single-storey extension usually takes 10 to 16 weeks on site, and a double-storey one 16 to 24 weeks. Design and approvals can add several months beforehand.
Do I need an architect for an extension?
Not legally, but you need drawings for planning and building regulations. An architect or architectural technician can also help you get better value and avoid costly changes later.
Does an extension add value to a house?
A well-designed extension that adds a bedroom or a large kitchen-diner usually adds value, but it may not add as much as it costs, so check prices in your street first.
How much would a house extension cost?
Most UK extensions cost £1,800 to £3,000 per m² to build in 2026. Add 10 to 15% for professional fees, a 10% contingency and VAT at 20% for a realistic total budget.
Where our prices come from
We compare published 2026 UK cost guides and trade price data, then use the middle of the range. Figures are for budgeting only and are not a quote. See how we price.