Extension cost calculator
Set the size, spec and region and get the range a real builder would work to — including the fees, the steel and the drain diversion that never make it into the headline number.
Last updated July 2026 · built by working UK builders
Assumes a VAT-registered contractor. New-build extensions to an existing home are standard-rated at 20% VAT — that is included above.
Excludes furniture, blinds, and anything you buy yourself.
The rates behind this calculator
All figures are 2026 UK prices for a competent, insured, VAT-registered contractor. Costs are shown as ranges because building work is delivered in ranges. A “×” value multiplies the whole build cost.
How many storeys?
| Option | Cost | Effect on total |
|---|---|---|
| Single storey £1,900–£2,700 per m² | £1,900–£2,700 per m² | Added |
| Double storey Cheaper per m² — same roof and foundations | £1,650–£2,350 per m² | Added |
| Wrap-around More external wall and roof per m² of floor | £2,100–£3,000 per m² | Added |
Finish level
This is the single biggest swing factor after size. Same shell, very different bill.
| Option | Cost | Effect on total |
|---|---|---|
| Builder’s standard Plastered, painted, standard doors and radiators, laminate or vinyl floor | — | ×0.88 |
| Good quality Underfloor heating, engineered wood, decent glazing, tiled utility | — | ×1 |
| High specification Slim-frame aluminium, structural glazing, bespoke joinery, stone floors | — | ×1.28 |
Glazing to the new opening
| Option | Cost | Effect on total |
|---|---|---|
| Standard windows and a back door | £0 | Added |
| French doors | £1,400–£2,600 | Added |
| 2-panel sliding doors (approx. 3m) | £3,200–£6,000 | Added |
| 3-panel bi-fold (approx. 3m) | £3,400–£6,200 | Added |
| 5-panel bi-fold (approx. 5m) | £5,500–£10,500 | Added |
| Slim-frame aluminium slider (approx. 4m) | £8,000–£18,000 | Added |
Rooflights
| Option | Cost | Effect on total |
|---|---|---|
| None | £0 | Added |
| 2 standard rooflights | £1,400–£2,800 | Added |
| 3 standard rooflights | £2,100–£4,200 | Added |
| Roof lantern | £2,600–£6,500 | Added |
| Flush structural glass (walk-on or fixed) | £4,500–£11,000 | Added |
What else is in the job?
Tick everything the builder will be doing while they are there.
| Option | Cost | Effect on total |
|---|---|---|
| Removing a load-bearing wall / structural steel Beam, padstones, temporary props, engineer’s calcs | £2,200–£6,500 | Added |
| New kitchen fitted in the extension Mid-range units, worktops and appliances | £9,000–£22,000 | Added |
| New downstairs WC | £2,200–£4,500 | Added |
| Wet underfloor heating Approx. £75–£120 per m² | £75–£120 per m² | Added |
| Boiler upgrade to cope with the extra load | £2,400–£4,200 | Added |
| Building over or diverting a drain Build-over agreement with the water authority | £900–£4,000 | Added |
| Deep foundations (trees, clay, or made ground) Adds roughly £200–£450 per m² of footprint | £200–£450 per m² | Added |
| Party wall surveyors needed | £1,200–£3,500 | Added |
| Making good the garden afterwards | £1,500–£6,000 | Added |
Site access
Everything has to get in and the muck has to get out.
| Option | Cost | Effect on total |
|---|---|---|
| Driveway / wide side access | — | ×0.96 |
| Normal side passage | — | ×1 |
| Through the house only | — | ×1.12 |
Where is the property?
Labour rates and builder demand vary more than materials do.
| Option | Cost | Effect on total |
|---|---|---|
| London | — | ×1.25 |
| South East | — | ×1.12 |
| South West | — | ×1.03 |
| East of England | — | ×1.03 |
| Midlands | — | ×0.97 |
| North of England | — | ×0.92 |
| Wales | — | ×0.92 |
| Scotland | — | ×0.95 |
| Northern Ireland | — | ×0.85 |
Applied to every total
| Item | Amount |
|---|---|
| Design, engineer & building control | 9–14% of build cost |
| Contingency | 10% of build cost |
| VAT | Included at 20% unless stated |
What the number actually includes
The build cost above covers the whole shell and fit-out: foundations, drainage connections, brickwork or render, roof, doors and windows, plastering, first and second fix electrics and plumbing, decoration and a standard floor finish. Professional fees cover drawings, structural calculations, building control and — if you need it — the planning application. The contingency is money you should genuinely expect to spend.
The five things that move the price most
1. Ground conditions
Nobody knows what the foundations will cost until the trench is open. Clay soil, a mature tree within influencing distance, made ground or an old cellar can turn a £4,000 foundation into a £20,000 one. Any builder who prices foundations to a fixed sum without a trial hole is either very confident or about to hand you a variation.
2. Drains
If a public sewer runs under or within three metres of the extension you will need a build-over agreement from the water authority, and possibly a diversion or a new manhole. It is rarely more than £4,000 but it is almost never in the first quote.
3. Glazing
A three-panel bi-fold and a slim-frame aluminium slider look similar in a drawing and differ by £12,000 in reality. Glazing is the easiest place to save money without the room feeling cheaper — and the easiest place to lose control of a budget.
4. What the extension connects to
Extensions rarely stop at the new walls. Removing the old rear wall needs a steel beam and padstones. Adding 20m² of underfloor heating may need a bigger boiler. The old kitchen becomes a utility. Budget for the works either side of the join.
5. Access
A terraced house where every barrow of muck and every pack of blocks goes through the hallway costs meaningfully more than a detached with a driveway. It is labour, not materials — but it is real, and it is why the same drawing prices differently street to street.
Where extension budgets usually go wrong
- Comparing quotes that cover different work. Put every quote onto the same line-by-line schedule before you compare totals. It is the single most valuable hour you will spend.
- Forgetting the finishes. Flooring, blinds, lighting, garden reinstatement and decorating the rooms you disturbed will add £5,000–£15,000 that no builder quoted.
- Paying too much up front. Stage payments should follow completed work, not the calendar. A large deposit before anything arrives on site is a risk you do not need to take.
- Not costing the disruption. Six months without a kitchen has a price, even if it is not on an invoice.
Will you get the money back when you sell?
Sometimes. A well-built extension that adds a bedroom or opens up the ground floor typically returns most of its cost in a rising market, and adding floor area is one of the few improvements valuers reliably reward. But every street has a ceiling price, and spending £90,000 on a house four doors from the top of the road rarely comes back. If resale is your main reason for building, get a local agent's view on the ceiling before you commit.
Frequently asked questions
How much does a 20m² single-storey extension cost in 2026?
Most 20m² rear extensions land between £50,000 and £80,000 all-in outside London, and £65,000 to £105,000 inside the M25. That covers the shell, glazing, plastering, electrics, heating, fees and contingency — but not a kitchen. Add £9,000–£22,000 if a kitchen is going in.
Why do builders quote such different prices for the same extension?
Three reasons, in order of size: what is actually included (many quotes exclude the kitchen, flooring, decorating or the drain diversion), how busy the builder is, and how they have priced the unknowns underground. A quote 30% below the others is usually missing something rather than being generous.
Is a double-storey extension cheaper per square metre?
Yes. You pay for the foundations and the roof once and get two floors out of them, so the rate per m² drops by roughly 10–15%. The total is still higher because you are building twice the floor area.
Do I need planning permission for an extension?
Many single-storey rear extensions fall under permitted development, but the limits on depth, height, materials and how close you build to the boundary are strict, and they do not apply at all to flats, listed buildings or most conservation areas. Use our planning permission checker for your specific case.
What should I hold back as contingency?
10% on a new-build extension on a straightforward site, 15% if the property is pre-war or the ground is unknown. The money goes on foundations that have to go deeper than expected, drains in the wrong place, and decisions you change halfway through.