Extension cost calculator
A house extension costs £2,000 to £3,000 per m², so 20m² works out at £40,000 to £60,000. Enter your own size below.
Work out what a house extension costs. Enter the footprint, say whether it is single or two storey, and tick a kitchen fit-out if the new space needs one. Gives you a price range for your region, built from our own guide prices rather than quotes.
Extension cost calculator
Set it up to match your job. Everything updates as you change it, and nothing is sent anywhere.
Built from the same figures as our house extension guide: £2,000 to £3,000 per square metre for a single storey build, and £2,000 to £2,700 per square metre for two storey, which is slightly less per square metre because the second floor sits on foundations and under a roof you are paying for anyway. A kitchen fit-out adds £8,000 to £20,000. Two-storey floor area is the footprint counted twice, so a 5m by 4m two-storey extension is 40m² of new space, not 20m². Regional figures use the same multipliers as every other page here. What the rate covers is the shell and a basic finish: foundations, structure, roof, windows, doors, basic electrics, plastering and building control. What sits outside it, and what you should budget for separately, is design and drawings at roughly 7 to 15 per cent of the build, a structural engineer for any beams, and a party wall surveyor at around £700 to £1,500 per neighbour if you build near a boundary. Bi-fold or sliding doors are another £4,000 to £12,000 for a 3 to 4 metre span. VAT applies to most extension work.
Extension cost by region
| Size | UK average | London | North / Scotland / Wales |
|---|---|---|---|
| 12 m² | £24,000 to £36,000 | £31,000 to £47,000 | £22,000 to £33,000 |
| 20 m² | £40,000 to £60,000 | £52,000 to £78,000 | £37,000 to £55,000 |
| 30 m² | £60,000 to £90,000 | £78,000 to £117,000 | £55,000 to £83,000 |
| 40 m² | £80,000 to £120,000 | £104,000 to £156,000 | £73,500 to £110,500 |
Worked totals at £2,000 to £3,000 per m², before any extras. UK guide prices reviewed June 2026, not quotes. Set the calculator above to your own job to add the extras.
How this estimate is put together
Extensions are priced by floor area, and almost every builder will quote you a rate per square metre before anything else. That is why this calculator asks for the footprint rather than a room count: multiply length by depth, multiply again by the storeys, and you have the number the rate applies to.
The range is wide on purpose. £2,000 per square metre is a straightforward rectangular extension on good ground with a simple roof and a standard finish. £3,000 is the same floor area with deeper foundations, a more complicated roof, wider structural openings into the existing house, or a specification above builder's basic. Most projects land somewhere in the middle, which is why the guide's typical figure is £2,500.
Why two storey costs less per square metre
It reads like a mistake and it is not. A two-storey extension costs less per square metre than a single storey one, roughly £2,000 to £2,700 against £2,000 to £3,000, because the two most expensive parts of the job do not double. You dig one set of foundations and you build one roof, then you get two floors underneath it instead of one.
The total bill is still much higher, because you are buying twice the floor area. On a 5m by 4m footprint, single storey gives you 20m² and two storey gives you 40m². At the mid rate that is roughly £50,000 against £94,000. The per-square-metre figure is what improves, not the cheque.
The costs that sit outside the rate
The per square metre rate is for the building work. It is not the project. The gap between the two is where extension budgets usually go wrong, and it is rarely the builder's fault: the rate you were quoted was honest, it just never included the things nobody quotes for at the start.
Design and drawings run about 7 to 15 per cent of the build cost. A structural engineer is needed for any beam. Building control fees are separate. If you build within three metres of a boundary you will likely need a party wall agreement, at roughly £700 to £1,500 per neighbour. Then the things that go inside: a fitted kitchen, bi-fold doors at £4,000 to £12,000 for a typical span, flooring, and decorating. A useful habit is to take the calculator's figure, add the professional fees, add the fit-out, and treat that as the real number.
Planning permission and what it costs you in time
Many single storey rear extensions fall under permitted development if they stay within the size limits, which means no planning application. Two storey work, anything in a conservation area, and larger single storey extensions usually need permission. A householder application in England is a fixed fee and the council has eight weeks to decide, so the real cost is the delay rather than the fee.
Permitted development is not the same as no paperwork. You still need building control, and a lawful development certificate is worth having even when you do not need planning, because it is what a buyer's solicitor will ask for years later.
Common questions
How much does a house extension cost in the UK?
About £2,000 to £3,000 per square metre for a single storey extension, so a 20m² extension is roughly £40,000 to £60,000 for the building work, typically around £50,000. A two storey extension is £2,000 to £2,700 per square metre but covers far more floor area. London runs about 30% above the UK average and the North, Scotland and Wales about 8% below.
How much does a 20m2 extension cost?
Roughly £40,000 to £60,000 for the shell and a basic finish, at the £2,000 to £3,000 per square metre rate, with £50,000 the typical figure at the mid rate. A kitchen fit-out in the new space adds £8,000 to £20,000 on top, and bi-fold doors another £4,000 to £12,000, so a finished 20m² kitchen extension commonly lands well above the build figure alone.
Is a two storey extension cheaper than two single storey ones?
Considerably, yes. Building up rather than out means one set of foundations and one roof serving two floors, which is why the rate drops to about £2,000 to £2,700 per square metre. Two separate single storey extensions of the same total area would each need their own foundations and roof. Two storey work is more likely to need planning permission, though, and it is more disruptive to live through.
What is not included in the per square metre price?
Design and drawings at roughly 7 to 15 per cent of the build, a structural engineer for beams, building control fees, and a party wall surveyor at about £700 to £1,500 per neighbour if you build near a boundary. Everything that goes inside the room is also separate: a kitchen, bi-fold doors at £4,000 to £12,000, flooring and decorating. The rate buys you a finished shell, not a finished room.
Why do extension quotes vary so much for the same size?
Because floor area is only the starting point. Ground conditions decide how deep the foundations go and you cannot see that until you dig. A wide structural opening into the existing house needs a bigger steel. A hipped or pitched roof costs more than a flat one. Access matters too, since a builder who can get a digger to the back of the house works faster than one barrowing everything through your hallway. Get three written quotes and compare what each one includes rather than the headline figure.
These are independent guide prices based on typical UK jobs in 2026. Your actual cost depends on your property, spec, access and where you live. Always get at least three written quotes before committing. These are independent Costwise guide prices, reviewed June 2026, not quotes and not a survey of real quotes. A real price depends on access, condition and what the job actually involves, so use this to sanity-check what you are told and always get three written quotes.