Home / Cost guides / House extension cost
How much does a house extension cost in the UK?
Most single-storey extensions cost £1,800–£3,200 per m² to build in 2026. Here is where that range comes from, where the money goes, and a quick calculator for your own budget.
Rough build cost
- Budget range
- –
- Rate per m²
- –
- Plus contingency (10–15%)
- –
Excludes VAT, kitchen and bathroom fit-out, professional fees and planning. A rough guide only. A measured estimate from your drawings will be far tighter.
Get a measured estimate from £150Extension cost per m² in 2026
These are typical build costs per m² of new internal floor area, before VAT, for a finished shell with standard finishes.
| Extension | Cost per m² | 20 m² example |
|---|---|---|
| Single-storey, basic finish | £1,800–£2,200 | £36,000–£44,000 |
| Single-storey, standard finish | £2,200–£2,900 | £44,000–£58,000 |
| Single-storey, high finish | £2,900–£3,500 | £58,000–£70,000 |
| Double-storey, standard finish | £1,800–£2,700 | Per m² of both floors |
Small extensions cost more per m² because things like scaffolding, foundations, steelwork and the connection to the house cost much the same whatever the size. London is typically 20–40% above these figures, and the North of England a little below.
Where the money goes: a real example
This is the elemental summary from our sample estimate for a 16.28 m² single-storey rear extension in Merseyside, priced at Q4 2026 rates. The total came to £46,451 before VAT, or £2,853 per m².
| Element | Cost | Share of build |
|---|---|---|
| Preliminaries | £6,210 | 15.4% |
| Demolitions and alterations | £3,888 | 9.6% |
| Substructure | £4,545 | 11.3% |
| Ground floor | £1,743 | 4.3% |
| External walls | £6,370 | 15.8% |
| Roof | £6,347 | 15.7% |
| Windows and external doors | £5,820 | 14.4% |
| Internal finishes | £1,781 | 4.4% |
| Services | £2,200 | 5.4% |
| Drainage | £1,038 | 2.6% |
| External works | £450 | 1.1% |
| Construction cost | £40,393 | 100% |
The walls, roof and glazing make up nearly half the build cost. Preliminaries, which cover site set-up, scaffolding, skips and supervision, are the next biggest item on a small job and are often where quick quotes are short.
What pushes the price up
- Ground conditions. Clay, nearby trees or made ground can mean deeper footings or piles.
- Opening up the house. Knocking through the back wall needs steel beams, padstones and temporary support.
- Glazing. Large bifolds, sliders and roof lanterns can cost more than the walls they replace.
- Drains. Building over or near a public sewer needs water company approval and sometimes a diversion.
- Access. No side access means everything goes through the house by hand, which adds labour.
Costs outside the build
- Architectural drawings and a structural engineer's calculations.
- Building control fees, and a planning application if the extension isn't permitted development.
- Party wall agreements: roughly £700–£3,000 per neighbour where they are needed.
- Kitchen, bathroom, flooring and decorating, if not in the builder's price.
- VAT at 20% from a VAT-registered builder.
Single-storey rear extensions can often be built under permitted development up to 3m deep on a terraced or semi-detached house, or 4m on a detached house, and no more than 4m high. Check the current rules with your council before relying on this.
Common questions
How much does a single-storey extension cost per m² in 2026?
Most UK single-storey extensions come in at roughly £1,800 to £3,200 per m² of new floor area, with a standard specification usually around £2,200 to £2,900 per m². London and the South East run higher.
Is a double-storey extension cheaper per m²?
Yes. The foundations and roof are shared across two floors, so the rate per m² is usually lower, often 10 to 20% below a single-storey rate. The total cost is still higher because you are building more floor area.
Does the cost per m² include a kitchen?
Usually not. Most per m² figures cover the shell and basic finishes. A new kitchen, bathroom fittings and flooring are normally priced separately.
How much contingency should I allow?
Allow 10 to 15% of the build cost for a straightforward extension, and closer to 20% on older houses or where the ground and drainage are unknown.
Are the figures with or without VAT?
The figures on this page are before VAT. A VAT-registered builder will add 20%.
Pricing an extension?
Send us the drawings and we measure every item, price it at current rates for your area and send back an estimate like the sample above. From £150, in 3–5 working days.