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Extension calculator

What an extension or a loft costs, and what the finished house has to be worth

Choose the job, the size and where it is, then set each professional fee and the contingency to whatever you have been quoted. We publish no percentage for value added, deliberately: every figure in circulation traces back to an agent's press release rather than to what houses actually sold for. So the tool asks what you think it would be worth finished and tells you what it has to reach to pay for itself, at both ends of the cost range.

For: More space on a building you already own

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The job
What are you building?
Up to 4m on a detached house or 3m on a semi or terrace is usually permitted development, and 8m or 6m with prior approval.
m²
A single storey rear extension is usually 15 to 30 square metres, and a garage about 15.
What the property is
Permitted development on a semi-detached allows 3m, or 6m with prior approval to the rear and 50 cubic metres in the roof, and none of it applies in a conservation area, a National Park or on a listed building.
Region
Level of finish
What a private buyer expects in a family house. The baseline every rate here is quoted at.
Is it permitted development?
No application fee. A lawful development certificate is £274 and is still worth having: without one you are relying on your own reading of the rules when you come to sell.
Fees on top of the build
Defaults are national figures. Put your own quotes in.
£
£
£1,500 to £2,500 where steel or a new opening is involved. A basement is several times that.
£
£1,500 to £3,000 per neighbour, and a mid terrace has two. You pay for their surveyor as well as your own, and the notice period is two months before you can start.
£
£700 to £1,300 on an extension up to 40 square metres, more in inner London. It is not a statutory fee, so it is worth getting two.
%
An architect is 3 to 7% of construction cost. A technician drawing a simple extension is less, and it shows.
%
Ten per cent contingency plus five for inflation is the usual allowance. On anything that involves opening up an old building, more.
What it is worth
We publish no percentage for value added, on purpose. Every figure in circulation traces back to an agent's press release rather than to what houses actually sold for.
£
£
Your number. Get it from an agent who sells on your street, and ask them what a house of that size and condition actually achieved rather than what it was listed at.
£
The ceiling, and no amount of work goes through it. This is the single most useful number on the page and the one people most want to argue with.
It is line ball-£6,015 to +£18,135It works at the cheap end of the build and not at the dear end, so on this one the tender is the whole story. At the top of the range the finished house has to reach £396,015.
£51,865 to £76,015All in
£40,000 to £60,000The building work
£2,593 to £3,801Per m² all in
£396,015Value it must reach
The building work78%Fees9%Contingency13%
What it costs
The building work£40,000 to £60,000
Up to 4m on a detached house or 3m on a semi or terrace is usually permitted development, and 8m or 6m with prior approval.
Fees on top£5,100 to £6,100
Planning £0, engineer £2,000, building control £1,100, and design at 5% of the build.
Contingency and inflation£6,765 to £9,915
15%. On anything that involves opening up an old building this is the line most likely to be spent in full.
All in£51,865 to £76,015
What to have available before you start, not what to hope for. Include VAT: an extension or a loft on a home somebody lives in is standard rated, with no relief at any scale.
Per square metre, all in£2,593 to £3,801
Published guides put a single storey extension at £2,000 to £3,000 a square metre including VAT, and two storey at £1,600 to £2,500. That is the range to check this against.
Whether it pays
What you expect it to add£70,000
From £320,000 to £390,000.
What it has to reach to break even£396,015
At the dear end of the build. At the cheap end it only has to reach £371,865, which is the difference a properly tendered job makes.
Headroom to the street ceiling£110,000
The most that can sensibly be spent here is about £110,000, because that is the gap between where the house is now and where the street tops out.
Return on the money spent109%
Over one hundred per cent means the work adds more than it costs. Under it does not, which is a perfectly reasonable decision if you are extending to live in the space rather than to sell it.
Consents
Permitted developmentYou are relying on it
On a semi-detached: 3m, or 6m with prior approval to the rear, 50 cubic metres in the roof. Removed entirely in conservation areas, National Parks and on listed buildings, and often removed by an Article 4 direction as well.
Party wallNone allowed for
Required where you cut into a shared wall, build against it, or dig within three metres of a neighbour's foundations. Two months' notice before work starts, which is a programme item and not just a cost.
Where these figures come from

Build costs. Published UK guides at 2026 prices, including VAT and labour: single storey extension £2,000 to £3,000 a square metre, two storey £1,600 to £2,500, garage conversion £900 to £1,500. Loft conversions: rooflight £20,000 to £40,000, dormer £40,000 to £70,000, mansard £55,000 to £95,000. Basements £2,500 to £4,200 a square metre.

Region and specification. Rates are quoted at West Midlands prices. London runs 30% to 45% higher and the South East 20% to 25%. Level of finish moves the price further than size does.

Fees. Householder planning application £548 from 1 April 2026, plus a £91 Planning Portal charge. A lawful development certificate is £274. Building control £700 to £1,300 on an extension up to 40 square metres. Structural engineer £1,500 to £2,500. Party wall £1,500 to £3,000 per neighbour. Architect 3% to 7% of construction cost.

Value added. Deliberately not published here. We could find no figure in circulation that traced back to transaction evidence rather than to a press release, so the finished value is yours to set and ours to sense-check on the phone.

Published UK build costs as at September 2026, including VAT and labour, and every figure adjustable. An extension, a loft or a refurbishment of a home somebody lives in is standard rated for VAT with no relief at any scale; the reliefs apply to new dwellings and to conversions that change the number of dwellings, which is a separate calculator. Permitted development allowances differ by house type and are removed entirely in conservation areas, in National Parks, on listed buildings, under an Article 4 direction, and on flats. Party wall notices need two months before work can start. An estimate, not advice, and not a quotation: nobody here puts a firm figure on a building before they have stood in it.

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