House extension cost in the UK (2026 prices)

House extension prices depend on size, ground conditions, structure, glazing and finish far more than on a single "going rate". The honest answer is that a per-square-metre figure is a starting point for a conversation with a builder, never a quote. This guide sets out the published benchmarks we could verify, says where the figures come from, and shows what else goes on the bill.

Many popular price pages quote tidy ranges with no source. We have left those out. Where a published figure is paywalled or unclear, we say so instead of guessing.

How much does a house extension cost per square metre?

There is no single official UK price per square metre. Builders, quantity surveyors and councils use BCIS (the Building Cost Information Service, run by RICS) rates as a reference, and each adjusts for location and specification.

Which? published its 2026 extension cost guide on 15 May 2026, using BCIS figures, but its price tables are for members only, so we cannot reproduce them here (Which?, read October 2026). What is visible is useful: it budgets for architect design fees of up to 10% of construction cost, suggests setting aside around 10-15% for contingencies, and warns that fit-out for a kitchen, utility room, sliding doors or underfloor heating is additional.

A public benchmark we can quote is the City of Edinburgh Council's cost-of-works guide, which it uses to value work for building warrant fees. It is "based on BCIS rates from quarter 2 2025 to quarter 1 2026" and lists a single-storey extension at £2,190 per m2 and a multi-storey extension at £2,963 per m2 (City of Edinburgh Council, read October 2026). Treat it as one Scottish council's reference value. It is not a UK average, and the page does not say whether it includes VAT or fees.

Simple multiplication shows the scale. A 20 m2 single-storey extension at £2,190 per m2 gives £43,800, and 20 m2 of multi-storey space at £2,963 gives £59,260. That is arithmetic on a benchmark, not a prediction for your house, and it leaves out professional fees, VAT treatment, fit-out and contingency.

Single storey or double storey: which costs more?

A double-storey extension costs more in total but can cost less per square metre. Which? says a two-storey extension "isn't simply double the cost" of a single-storey one, and that a double-storey extension "will actually work out cheaper per square metre" because much of the cost is in the foundations (Which?, read October 2026).

The Edinburgh benchmark above points the other way, with the multi-storey rate higher than the single-storey one. Different methods give different answers, which is a good reason to ask for an itemised quote rather than lean on one rate.

In practice the choice often comes down to the plot and the planning position. A single-storey rear extension is the common permitted development route. Going up means more planning scrutiny, scaffolding, structural work where the new floor meets the old house, and more disruption for neighbours. See our planning permission guide for the rules.

House extension prices by type

If a website quotes a neat price band for each row with no named source, ask where it comes from.

What moves the price of an extension?

Ground and foundations come first. Poor soil, nearby trees or a drain running through the footprint can mean deeper or wider foundations. Your builder or structural engineer will only know once the ground is checked.

Structure is next. Knocking through to the existing house usually needs a steel beam sized by a structural engineer. Large openings for bi-fold or sliding doors add steelwork and glazing cost.

Glazing and rooflights are among the biggest swing items. A modest window and a wall of glass are very different budgets.

Kitchen and fit-out can outweigh the shell. Which? treats a kitchen, utility room and underfloor heating as additional to the build price, so check whether a quote includes them.

Access and site matter too. A terrace with no side access means materials are carried through the house, which slows work. Region affects labour rates and supply, so a figure from another part of the country, or from Scotland as above, will not transfer neatly to your town.

Specification covers everything you can see: bricks or render, flat roof or pitched, ceiling heights, finishes.

What fees sit on top of the build price?

Professional fees are separate from the builder's price. Budget for these.

• Architect or designer. Which? suggests allowing up to 10% of construction cost for architect design fees (read October 2026). Some designers charge a fixed fee instead.
• Structural engineer. Needed for beams and foundations, and usually requested by building control.
• Planning application, if needed. As at October 2026 the householder fee is £548 in England and £585 in Wales (Planning Portal, read October 2026). Prior approval for a larger home extension has its own fee, so use the Planning Portal fee calculator.
• Building control. Councils set their own charges. Bath & North East Somerset Council lists £876 including VAT for a 40 to 60 m2 extension and £984 for 60 to 80 m2 (Bath & NES Council, read October 2026). Other councils and private approved inspectors differ. Our building regulations guide explains what is inspected.
• Party wall. If you build on or near a boundary, the Party Wall etc Act 1996 requires you to notify neighbouring owners of certain work (GOV.UK, read October 2026). The government's explanatory booklet says the building owner usually pays the costs of drawing up a surveyor's award where the work is solely for their benefit, so allow for that. It also sets out the notice periods: at least two months for work to a party wall and one month for nearby excavation.

Scotland uses building warrants rather than building regulations approval, so Scottish readers should check their council's building standards page.

Do you need planning permission, and what does it cost?

Often not. In England the Planning Portal says a single-storey rear extension can extend up to 4 metres on a detached house or 3 metres on any other house under permitted development, and up to 8 metres or 6 metres with prior approval under the larger home extension scheme (Planning Portal, read October 2026). Prior approval is not available on Article 2(3) designated land, which includes conservation areas, and an Article 4 direction can remove the rights altogether.

Flats and maisonettes have no householder permitted development rights. Scotland and Wales have their own rules and fees. If you are unsure, a Lawful Development Certificate gives written confirmation from the council. Start with your own council's planning pages (the GOV.UK council finder will take you there), and see our planning permission guide.

Is VAT charged on a house extension?

Normally yes. HMRC's Notice 708 says "the construction of a new building and work to an existing building is normally standard-rated". Zero rating applies only where the extension itself contains an additional dwelling designed as such, for example a new flat (HMRC, GOV.UK, updated 26 August 2026, read October 2026). A builder who is not registered for VAT will not add it, so ask. Always check whether a figure is quoted with or without VAT, as Which? also advises.

How long does an extension take?

We have not found a reliable published timescale to quote, so treat any exact week count with caution. The order of events is dependable. Design and drawings come first, then planning or prior approval if needed, then building control and any party wall notices. Only after that does the build start, with foundations, structure, weathering, then first fix, finishes and fit-out. Councils have fixed periods for deciding applications, and a builder's start date may be weeks or months after you accept a quote. Ask each firm for a programme in writing.

How do you compare extension quotes?

Get at least three, each from a visit to the property, and ask for the same scope from each.

1. Check the drawings and specification are the same. A cheaper quote may use fewer items.
2. Ask for the price in sections: groundworks, structure, roof, windows and doors, electrics, plumbing, plastering, finishes.
3. Find out which items are provisional sums or prime cost sums, where the real price is not yet known. Kitchens and tiles are common examples.
4. Note exclusions: skip hire, scaffolding, building control fees, architect and engineer fees, making good the garden.
5. Check VAT and your contingency. Which? suggests 10-15%.
6. Confirm insurance, a written contract and a staged payment plan before you pay anything. Our guide on how to hire a builder covers the checks.

The lowest figure is not always the best value, and a quote that is far lower than the rest deserves a question about what is missing.

Find a checked builder for your extension

MyLocalBuilders.uk is a directory of checked local builders. The ranked list is ordered by verified reviews from the last 12 months, then by distance, and nobody pays to move up it. A labelled Featured slot sits above the list, and an enquiry goes to one builder only.

You can search for a builder near you or browse extension builders. We are filling the directory town by town, so some areas have fewer firms than others. Local pages cover London, Manchester, Birmingham, Leeds, Bristol, Glasgow, Cardiff and Newcastle.