A single storey extension of 20 to 40 square metres costs around £2,700 per square metre to build, with most projects landing somewhere between £2,300 and £3,350. On a typical 35 square metre rear extension that is about £94,500 of building work, and roughly £145,000 by the time you have added contingency, fees, statutory costs and VAT.
Those two numbers being so far apart is the reason house extension costs confuse people. Almost every figure you will read online is the first one. Almost every figure you actually pay is the second.
I had a quantity surveyor price the whole thing properly in September 2026, so the rates below are current and they are his rather than mine. They are at Midlands and Wales prices, exclude VAT and professional fees, and include the builder’s preliminaries and margin. That last point matters more than it sounds, and I will come back to it.
WHAT AN EXTENSION COSTS PER SQUARE METRE
Here are the rates, per square metre of new floor area. Each one has a cautious figure, a most likely figure, and a figure for when it goes against you.
- Single storey, under 20 m²: £2,850 to £4,200, most likely £3,350
- Single storey, 20 to 40 m²: £2,300 to £3,350, most likely £2,700
- Single storey, over 40 m²: £2,050 to £3,050, most likely £2,450
- Two storey, per m² across both floors: £2,000 to £2,950, most likely £2,300
- Over an existing structure or garage: £2,300 to £4,500, most likely £3,100
Read the first line again, because it is the one that surprises people. The smallest extensions have the worst rate per square metre, and it is not close.
Those are baseline figures. Where you are in the country, how easily a lorry can get to the back of the house, and whether you intend to live there through the works all move them, sometimes by a lot. Our Extension Cost Calculator runs the same rates against your own project if you want a number rather than a range.
WHY SMALL EXTENSIONS COST MORE PER SQUARE METRE
Every extension needs foundations, a roof, a connection into the existing house, drainage moved, scaffolding, a skip, and a builder who turns up for however long it takes. Those costs barely change between a 15 square metre infill and a 45 square metre wraparound. What changes is the number of square metres you divide them by.
So a small extension carries the same fixed costs spread across less floor. You pay roughly 37% more per square metre on a 15 square metre job than on a 50 square metre one, for work that is in most respects identical.
This is why the first thing I ask when someone tells me they are extending is how much they are extending by. The self-builders and renovators I work with often arrive having decided on three metres because that is what they think they are allowed, when four and a half would have cost barely more per month on the borrowing and solved the problem properly.
The conversation I have most often starts with someone trying to keep the floor area down, on the understanding that a smaller extension is better value. Shrinking the footprint spreads the same fixed costs across fewer square metres, so every metre taken out makes the ones left behind dearer. Going bigger is usually the more efficient project of the two.
Thinking about an extension?
An hour on your own project, before the design is fixed and the quotes are in.
SINGLE STOREY, TWO STOREY, OR OVER THE GARAGE
The two storey rate looks wrong when you first see it. £2,300 per square metre across both floors, against £2,700 for a single storey of the same specification? Going up really is cheaper per square metre than going out, for a reason that is easy to miss: the expensive parts of an extension are underneath it and on top of it, and a two storey spreads one set of foundations and one roof across twice the floor.
That does not make it the right answer. It usually needs planning permission where a modest single storey might not, and it changes the look of the house from the street. But if you need the space and the planning position allows it, the arithmetic is firmly on its side.
Building over an existing single storey or a garage is the one to be careful with. The rate runs from £2,300 to £4,500, and that spread is measuring the structural unknown underneath. What is down there was not designed to carry another floor, and until a structural engineer has opened up the foundations and looked, nobody can tell you which end of that range you are at. I would not sign anything on a garage conversion-and-build-over until that check is done.
WHAT THE PER SQUARE METRE FIGURE DOES NOT INCLUDE
A rate per square metre describes building work. It is not a project cost, and treating it as one is how people end up £30,000 short.
Here is what sits outside it.
Everything you choose. Kitchens, bathrooms, flooring, lighting, built-in furniture. The rates assume sensible middle-of-the-road choices. A £40,000 kitchen is a £40,000 kitchen whatever the rate per square metre says, and on a lot of extensions the kitchen is the single biggest line in the whole job.
Upgraded glazing sits outside it too. Bifolds, sliding doors and roof lanterns above a standard allowance. If the entire point of your extension is a wall of glass onto the garden, say so when you are pricing it, because the difference is easily £10,000 to £20,000.
Professional fees, which are bigger than most people expect. More on those below.
The abnormals. Asbestos where you did not expect it, a drain running exactly where the foundations need to go, a neighbour’s wall that turns out to have no foundation at all, made ground, a tree with a root protection area. Nobody can see any of that from a floor area, and any one of them can add five figures.
And VAT, which on an extension is 20% and is not optional.
That is also why published figures vary so wildly. You will see £1,200 per square metre on one site and £2,800 on another, and both are telling the truth about different things. The cheaper numbers are usually a build-line rate: shell, standard finish, nothing else. Ours includes the builder’s preliminaries and margin, which is what he charges you rather than what the work costs him. When you compare two rates, the useful question is what each one has inside it.
THE COSTS NOBODY BUDGETS FOR
These are the ones I see missed most often, and on a modest extension they add up to more than the kitchen.
- Measured survey of the existing house, because you almost certainly have no drawings of what you already own: £650 to £1,800
- Structural engineer, close to certain on anything with an opening or a steel: £750 to £2,500
- Householder planning application in England, from 1 April 2026: £548
- Building control, whether local authority or an approved inspector: £600 to £2,200
- Party wall surveyor, per neighbour, if you are within three metres of a boundary: £900 to £4,000
- Drawings for planning and building regulations: around 7% of construction cost
- A full architectural service through to completion: around 12%
Two notes on that list. The planning fee nearly doubled on 1 April 2026, so if you are working from a figure you read last year, it is wrong. And if your extension falls under permitted development you avoid the £548, but I would still spend £274 on a lawful development certificate. It is half the householder fee, it takes the argument away before it starts, and your solicitor will ask for it when you sell.
One thing you can stop worrying about: biodiversity net gain does not apply to householder applications. Home extensions, conservatories and loft conversions are all specifically exempt. It can catch a self-build plot, but not a householder extension.
If you are building in Scotland, the system differs in a way that catches people out, and it is worth getting straight before you start rather than after. You need a building warrant rather than building regulations approval, and it has to be granted before any work begins. Starting without one is a criminal offence, not a paperwork problem you can tidy up later. Most extensions need a warrant, although some very small works are exempt. Warrant fees are set by each council rather than nationally, so the £548 above is an England figure and tells you nothing about what yours will be.
VAT, AND WHY EXTENSIONS GET THE WORST OF IT
An extension to a house you live in is standard rated at 20%. There is no way round it, and it applies to the builder’s labour and materials as well as to your professional fees.
This is worth knowing because renovation work is not always 20%. A house that has stood empty for two years or more qualifies for 5% on the building work. So does converting a non-residential building into a home, and so does changing the number of dwellings in a building. Those reduced rates are worth thousands and are routinely missed, but none of them rescue an ordinary extension. Our guide to house renovation costs sets out the three routes in detail.
If any of those situations might apply to you, get it confirmed in writing before the job starts. The contractor has to charge the right rate at the time; it cannot be fixed afterwards. HMRC Notice 708 is the authority, and I am an architect rather than an accountant.
WHERE THE MONEY ACTUALLY GOES
A 35 square metre single storey rear extension, at Midlands and Wales prices, with drawings for planning and building regulations only and the usual consultants:
- Building work, 35 m² at £2,700: £94,500
- Contingency at 15%: £14,175
- Construction cost: £108,675
- Drawings for planning and building regulations, 7%: £7,607
- Measured survey: £1,200
- Structural engineer: £1,600
- Planning application: £548
- Building control: £1,100
- Total excluding VAT: £120,730
- VAT on the building work at 20%: £21,735
- VAT on professional fees at 20%: £2,411
Total: £144,876
That is £3,449 per square metre excluding VAT, and £4,139 including it, against a headline build rate of £2,700. The gap is not padding. It is the part of the project that exists whether or not anyone wrote it down.
On the contingency: 15% is not pessimism. An extension is priced before anyone has opened the ground or taken the render off the wall it is joining. The contingency is the part of the job nobody can see yet, and the projects that go badly wrong are almost always the ones that started without one.
HOW TO SANITY-CHECK A BUILDER’S QUOTE
A few things I would look for before signing anything.
- Ask what the rate includes: preliminaries, scaffolding, welfare, skips, and the builder’s margin. If a quote comes in well under the ranges above, something is outside it, and you want to know what before you find out on site.
- Check the quote names a contingency, or at least confirm who carries the risk if the ground is not what everyone assumed.
- Look at how the kitchen and the glazing are handled. Provisional sums are fine, but a provisional sum set low is how a quote wins a job it should not have.
- Get the structural engineer in before you price rather than after. A steel that turns out to be twice the size changes the foundation, and that is an expensive conversation to have with a builder who has already started.
- Within three metres of a boundary, assume the Party Wall Act applies and budget for it.
One more thing, because it is the pattern behind most of the extension jobs I see go wrong. People get excited by the lowest quote and either do not ask the right questions, or ask them too late. A price that wins on being cheapest tends to get back to a sensible margin through changes and claims once the work is underway, and it frequently ends up costing more than the next quote up would have.
WORK OUT YOUR OWN NUMBER
The ranges above will get you to the right order of magnitude. What they cannot do is account for your region, your access, whether you are living in the house while the work happens, or which VAT position you are in. Those make a difference of tens of thousands on a job this size.
Our Extension Cost Calculator runs the same quantity surveyor’s rates against your own project and gives you a cautious, likely and against-you figure, with the breakdown showing where each pound goes. If you are also refurbishing the existing house, the Renovation Cost Calculator covers both. Both are free and neither asks for anything to use.
All the rates here were priced in September 2026 at Midlands and Wales levels, and the statutory fees are the England figures from 1 April 2026. We review them annually, and the planning fee in particular moves most Aprils.
COMMON QUESTIONS
How much does a 4m x 4m extension cost? Sixteen square metres puts you in the worst rate band, at around £3,350 per square metre, so roughly £53,600 of building work. Add contingency, fees and VAT and you are realistically looking at £84,000 or more. Extending to 5m x 5m costs about 24% more in total but gives you 56% more floor, at a far better rate per square metre.
Is a two storey extension cheaper than two single storey ones? Considerably. One set of foundations and one roof spread across twice the floor area brings the rate down to around £2,300 per square metre across both floors. Whether you can get permission for it is a separate question.
How long does an extension take to build? A straightforward single storey extension is typically twelve to twenty weeks on site once you have started, with design and planning taking three to six months before that. The planning determination alone is eight weeks for a householder application if nobody objects.
Do I need planning permission for an extension? Often not. Many single storey rear extensions fall under permitted development, subject to size, height and how much of the garden you are covering. The rules are detailed enough to deserve their own article, and the cost difference is the £548 application fee against £274 for a lawful development certificate.
Should I move instead? Sometimes, and it is worth doing the sum honestly. Against £145,000 for the extension above, set stamp duty, agents’ fees, legal fees and removals on a larger house, plus the difference in price. In parts of the country extending wins comfortably. In others it does not, and nobody tells you that until you ask.
GETTING IT RIGHT BEFORE YOU COMMIT
Most people come to me once they have a builder’s price in front of them and a decision to make about it. By then the design is fixed, the quote is what it is, and the useful conversations have already happened without anyone in the room who was on your side.
A Self Build Strategy Session is an hour on your own project. The design, the budget, the quote you have been given, the planning position, or a build that has already started going wrong. You get the recording, a transcript, and a written summary setting out your biggest risks and what to do about them in what order.
If you are early, before the drawings and before the builder, that is the cheapest hour you will ever spend on the project.
Thinking about an extension?
An hour on your own project, before the design is fixed and the quotes are in.
