How much does a house extension cost in the UK?

A finished 20 square metre single-storey rear extension costs £53,000 to £79,000 including VAT across most of the UK, or £72,000 to £120,000 in London. Per square metre that is £2,640 to £3,960 nationally and £3,600 to £6,000 in London. Professional and statutory fees sit on top of those figures.

Almost every extension cost you will read online is quoted before VAT, and most do not say so. That single omission accounts for a fifth of your budget, which is why the numbers on this page are shown both ways.

What does an extension cost per square metre?

Per square metre is how the trade prices extensions, and it is the only figure that lets you compare one quote with another. What it includes varies enormously though, so the basis matters more than the rate.

A shell is foundations, walls, roof, and openings made weathertight. Finished means plastered, wired, plumbed, floored and decorated, ready to use. The gap between the two is roughly the same again, which is why a shell-only quote can look like a bargain and then double.

Those are market rates: the shell figure from Checkatrade, the national finished range from Homebuilding & Renovating in February 2026, and the London range from Architecture for London in January 2026. All three publish ex VAT, so the inc-VAT column is calculated at 20%.

What does an extension cost by type?

Size drives the number more than type does, but type determines how much structural work sits underneath it.

Those are the London per-square-metre range applied to typical footprints, not separate estimates, so treat them as arithmetic and not as observed project prices. Outside London, expect roughly a quarter to a third less.

A wraparound is the most expensive of the four because it is the largest, and it combines a rear extension with a side return in one structure. If you have read that a wraparound starts cheaper than a rear extension, the two figures were measuring different sizes.

If your extension is really about creating a bigger kitchen, which most rear and side extensions are, our guide to what a kitchen extension costs covers the fit-out as well as the build.

What does a side return extension cost?

A side return extension costs £43,000 to £120,000 finished and including VAT for a typical 12 to 20 square metre footprint in London, and roughly a quarter to a third less elsewhere. It fills the narrow alley running alongside a Victorian or Edwardian terrace, which is why it is the most common extension in inner London: most terraces have one going spare.

Per square metre it usually costs more than a rear extension of the same area. That catches people out, because a side return is the smaller job. Three things drive it.

Access. A side return is reached through the house or along a passage too narrow for a barrow, so materials come in by hand and spoil goes out the same way. Labour hours climb for work that would be straightforward in an open garden.

Party walls on both sides. You are building along a boundary with a neighbour on one flank and frequently up to a boundary on the other. That means notices in both directions and, where either neighbour dissents, two sets of surveyor fees rather than one.

More structure for the area. Taking out the side wall of the original house needs steel, and the new roof has to tie into two existing rooflines instead of one.

The economics improve if you combine it with a rear extension to form a wraparound. Foundations, scaffolding and the roof tie-in are largely shared, so the second half of the footprint costs less per square metre than the first. Anyone weighing up both should price them together rather than doing one now and one later.

The fees nobody puts in the headline figure

Build cost is not project cost. These sit on top, and on a modest extension they add up to more than most people budget.

Two of those deserve a word.

Planning. The £548 householder fee applies from 1 April 2026, a 3.8% rise indexed to September 2025 inflation. A draft instrument published in July 2026 would raise it to £575 from 8 December 2026, so check the current figure if you are applying late in the year. If your extension falls within permitted development and you want written confirmation, a Lawful Development Certificate for a proposed use costs £274, half the full application fee. A certificate for an existing use costs the full £548.

Party wall. There is no statutory fee. The Party Wall etc. Act 1996 says only that "the reasonable costs incurred" shall be paid as the surveyors determine, and the government's own guidance is blunter still: surveyors' fees are a matter for individual surveyors, and there are no set charges. Where a neighbour consents in writing, you may pay nothing at all. Where both sides appoint separately, budget at the top of the range per neighbour.

Building control is the one most guides leave out entirely, and the spread is wider than anyone admits. Councils set their own charges under the Building (Local Authority Charges) Regulations 2010 on a cost-recovery basis, so no national figure exists. Published schedules put a domestic extension up to 40 square metres at £700 to £1,300 including VAT across most of England, and inner London sits well above that: Tower Hamlets £1,551.60, Islington £1,584, Camden £1,875.55. Westminster does not publish an area band at all and prices by the value of the work instead. Full plans and building notice cost the same in most authorities, so choosing between them is a question of risk and not of price.

Taken together, fees on a £70,000 build run from about 5% to as much as 25% of the build cost. That is an uncomfortably wide spread, and the architect appointment is what moves it. A drawings-only package with a neighbour who consents in writing lands near the bottom. A full-service architect appointment can be 15% on its own, and once you add two party wall surveyors and a structural engineer on an older property you are at the top. Our guide to the renovation costs that catch homeowners off guard covers the ones that surface most often.

Do you pay VAT on an extension?

Yes, at 20%, on labour and materials both. Extending an existing home is standard-rated and there is no relief for it.

Two adjacent cases do carry relief, and they are worth knowing because people confuse them with extensions.

A property empty for two years or more. Renovation work qualifies for a reduced 5% rate. An extension to that property does not automatically qualify, so the split between renovation and new build matters and your builder needs to apportion it.

A conversion that changes the number of dwellings. Splitting a house into flats, or combining flats back into a house, attracts 5% on the qualifying work.

Neither of those covers a straightforward extension to an occupied home. If a quote for one looks suspiciously low, check for a missing VAT line rather than assuming a relief applies.

What drives extension costs up

Four things account for most of the spread between two extensions of identical size.

Groundworks you cannot see until you dig. Trees, drains, a shallow existing foundation or made-up ground all change the foundation design. This is the single largest source of unwelcome surprises.

Glazing. Bifolds, sliders and roof lanterns are where specification runs away fastest, and the difference between adequate and impressive is tens of thousands on a large opening.

Structural openings. Removing the rear wall to open the extension into the existing house means steel, and the wider the opening the heavier and more expensive the steel and its supports.

Where you are. London and the South East run well above the national average, and almost all of the difference is labour rather than materials.

Hold 10% to 15% back as contingency, and more on a property built before 1940. Extensions uncover more than any other type of domestic work because they involve the ground, the drains and the existing structure all at once.

Does an extension add value?

You will read that an extension adds 10% to 20% to a property's value. That figure has no source behind it, so here is one that does.

Nationwide's house price research, based on hedonic regression across its own mortgage lending, found that a 10% increase in floor space adds about 5% to price. Adding an extra double bedroom to a two-bedroom house adds around 13%. An extension delivering both a bedroom and a bathroom adds up to 24%.

That is a more useful way to think about it than a flat percentage, because it says what actually drives the uplift: usable floor area, and specifically the kind that changes how the house is described. A larger kitchen improves your life more than it improves your valuation. A fourth bedroom changes which buyers can consider the house at all.

Two caveats. The uplift is capped by the ceiling price on your street, and over-specifying rarely returns the money. And you will see a claim that new floor area is worth £4,000 per square metre in London. No lender, surveyor or statistical body publishes that figure, so treat it as marketing.

How long does an extension take?

Ten to sixteen weeks on site for a single-storey rear extension, and longer where the ground throws up surprises or the glazing has a long lead time.

Start to finish is the number that matters, and it is much longer. Design, planning, party wall agreements and building control all sit in front of the build. A planning application alone runs eight weeks from validation, and party wall agreements can take a further two months where a neighbour dissents. Our guide to how long a house extension takes sets out the stages.

Budget six to nine months from first drawing to finished room, and be pleasantly surprised rather than caught out.

How Beams helps

Extensions have more moving parts than any other domestic project. Design, planning, party wall, building control, structure and fit-out all interact, and a decision made in the second month shows up in the cost in the sixth.

So we define the scope first, then set a realistic cost expectation, match you with builders suited to the job, and line up as many as three site visits. Every builder quotes from the same information, which is the only way three numbers become comparable.

That is what a managed house extension with Beams looks like: one agreed price, payments released against milestones, and a 12-month workmanship warranty behind it.

Common questions about extension costs

Is it cheaper to extend or to move?
Extending usually wins, and by more than people expect once stamp duty, agents' fees and moving costs are counted. Moving makes sense where the problem is the location or the plot rather than the amount of space.

Do I need planning permission for an extension?
Often not. Many single-storey rear extensions fall within permitted development, subject to depth, height and boundary limits, and to your property not being listed or in a conservation area. Where you want certainty without a full application, a Lawful Development Certificate for a proposed use costs £274.

Does an extension need building regulations approval even under permitted development?
Yes, always. Planning permission and building regulations are separate consents, and permitted development removes only the first. Building control signs off structure, insulation, drainage, ventilation and fire safety, and you will want the completion certificate when you sell.

What is the cheapest type of extension?
A single-storey rear with a flat roof and a modest specification, kept within permitted development so there is no planning application, and with services left where they are. Depth is cheaper than complexity: a slightly larger simple box usually costs less than a small clever one.

Where these figures come from

The figures on this page are what the market currently charges for an extension, compiled from published industry cost data rather than from any single builder's price list. Most published extension costs are quoted before VAT without saying so, which is the main reason they appear to disagree so violently, so every source below is listed with its date and its VAT basis.

Per-square-metre ranges: Architecture for London (London £3,000 to £5,000 per m², construction only, ex VAT, January 2026), Homebuilding & Renovating (£2,200 to £3,300 per m² finished, ex VAT, February 2026) and Checkatrade (shell only £1,200 to £1,700 per m², ex VAT). Inc-VAT equivalents are calculated at 20%.

Planning and certificate fees: Planning Portal, A guide to fees for Planning Applications in England, applying from 1 April 2026

Building control: published council fee schedules for Camden, Islington, Tower Hamlets, Southwark, Greenwich, Redbridge, Sutton, Hounslow, Ealing, Havering, Wandsworth, Hillingdon, Bromley, Lambeth, Harrow, Hackney, Cumberland, Bristol and Sheffield, set under the Building (Local Authority Charges) Regulations 2010

Party wall: Party Wall etc. Act 1996, section 10(13), and Ministry of Housing, Communities and Local Government explanatory guidance

Value uplift: Nationwide house price research, October 2025

Architect, structural engineer and party wall surveyor fee ranges: HomeOwners Alliance, January and March 2026. RIBA publishes no fee percentage, and the Party Wall Act sets no statutory fee

Every project prices differently once the ground, the structure and the specification are known. These figures are a starting point for a conversation rather than a quote.