The Short Answer
On our estimates, a single-storey rear extension of 3m x 6m (18m²) costs about £47,300 to £69,400 all in, including VAT: £43,200 to £57,600 for the build at £2,400 to £3,200 per m², and £4,100 to £11,800 for drawings, a structural engineer and building control. Expect about three to six months on site.
The biggest swings come from glazing and the roof. Flat-roofed extensions tend to cost less; a lantern and a wall of bifolds can add £7,600 to £11,100 on our estimates. On a semi or terrace, 3m deep is the most you can build without any application to the council.
Extension cost calculator
The build priced per square metre, plus the fees most quotes leave out: design, engineer, building control, planning and party wall.
Estimated total
£57,310 to £83,380
| Single-storey extension, 20m² of floor, standard | £48,000 to £64,000 |
| Design: drawings for planning and building regulations | £3,000 to £9,000 |
| Structural engineer | £300 to £1,200 |
| Building control | £800 to £1,600 |
| Contingency (10%) | £5,210 to £7,580 |
Single storey extension prices by size
| Size | Floor area | Build (standard) | With fees |
|---|---|---|---|
| 3m deep x 4m wide (semi or terrace, no prior approval) | 12m² | £28,800 to £38,400 | £32,900 to £50,200 |
| 3m x 6m | 18m² | £43,200 to £57,600 | £47,300 to £69,400 |
| 4m x 5m (detached, or with prior approval) | 20m² | £48,000 to £64,000 | £52,100 to £75,800 |
| 4m x 7.5m | 30m² | £72,000 to £96,000 | £76,100 to £107,800 |
Finish and roof
| Finish | What you get | Per m² |
|---|---|---|
| Basic | Simple box, standard doors and windows | £2,000 to £2,600 |
| Standard | Good windows and doors, a rooflight or two | £2,400 to £3,200 |
| Premium | Big glazing, roof lanterns, high-end finishes | £3,000 to £4,200 |
- Flat roof: flat-roofed extensions tend to cost less. A flat rooflight adds £900 to £1,800; a lantern £2,200 to £4,500.
- Pitched or lean-to roof: a lean-to is a single slope against the house wall; a pitched roof sheds water and fits in with the look of the house.
- Near a boundary: within 2m of it, permitted development limits the eaves to 3m, whatever the roof.
- Glazing: aluminium bifolds up to 3m are £3,600 to £5,400 fitted. Windows, doors and rooflights over 25% of the floor area (plus what the extension covers up) need an energy calculation.
Planning permission
A single-storey rear extension on a house in England is permitted development up to 3m deep on a semi or terrace and 4m on a detached house, no more than 4m high. Through the larger home extension prior approval you can go to 6m and 8m: the fee is £249, neighbours get 21 days to comment, and the council has 42 days to decide.
Beyond that, or in a conservation area, it's a householder application at £548, and most are decided within 8 weeks. The full list of limits, and a checker, are on our house extension page.
What a quote should include
Citizens Advice's checklist for any quote: a fixed total rather than a day rate, a breakdown of work and materials, whether VAT is included, how long it's valid, and when the price can go up. The contract should say who clears the rubbish and pays for skips. For an extension I'd also want to see:
- Foundations, with what happens if the ground is poor or there's a drain or tree in the way (often a provisional sum).
- Knocking through into the house, with the steel, and making good inside.
- Electrics, heating, plastering and decorating, or a clear line where the builder stops.
- Scaffolding, and who pays building control.
Get single-storey extension quotes
Single-storey extensions are priced per square metre, then by glazing and finish.
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Frequently asked questions
How much does a single storey extension cost?
On our estimates, about £2,400 to £3,200 per m² to a standard finish, including VAT. A 3m x 6m rear extension is £47,300 to £69,400 with design, engineer and building control.
How much does a 3m by 4m extension cost?
About £28,800 to £38,400 to build to a standard finish, plus £4,100 to £11,800 in fees, including VAT, on our estimates.
Is 30k enough for an extension?
Only for a very small one at a standard finish: on our estimates, fees and contingency leave £30,000 buying roughly 4 to 9m². See the budget guide on our house extension page.
Is 50k enough for an extension?
For a modest one: roughly 10 to 17m² at a standard finish after fees and contingency, on our estimates.
Can I build a single storey extension without planning permission?
Often. On a house in England, up to 3m deep (4m detached) and 4m high is permitted development, subject to other limits. Flats and designated areas are different.
Is a flat or pitched roof cheaper on an extension?
Flat-roofed extensions tend to cost less. A pitched roof sheds water and can suit the look of the house better.
How long does a single storey extension take?
Around three months to six months on site, after drawings and approvals.
How we priced thisView sources (7, 6 official)
Rules come from the legislation and official guidance, read at source. Prices are our estimates: we check them against trade price data, retailer shelf prices and specialists' price lists, then build them up from their parts so the totals add up. Below are the sources behind the page; the trade price guides we only compare against aren’t listed. Where sources disagreed, we've said so. More on our method.
- OfficialThe Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2 Part 1, legislation.gov.uk. Checked 16 Oct 2026.
- OfficialA guide to fees for planning applications in England (from 1 April 2026), Planning Portal. Checked 15 Oct 2026.
- OfficialWhen will I get a decision?, Planning Portal. Checked 15 Oct 2026.
- OfficialApproved Document L: Conservation of fuel and power, Volume 1 Dwellings (2021 edition incorporating 2023 amendments), GOV.UK. Checked 16 Oct 2026.
- OfficialBefore you get work done on your home (England), Citizens Advice. Checked 15 Oct 2026.
- IndustrySingle-storey extension costs: a practical guide for homeowners (4 March 2026), Federation of Master Builders. Checked 15 Oct 2026.
- OfficialBuilding control charges: domestic alterations 2026 to 2027, Royal Borough of Windsor and Maidenhead. Checked 15 Oct 2026.
