How much does a house extension cost in the UK?

A house extension in the UK typically costs between £2,000 and £3,500 per square metre, rising to around £2,500 to £5,000 per square metre in London. For a typical single-storey rear extension, that works out at roughly £40,000 to £90,000 across most of the UK, or £80,000 to £150,000 in London once VAT and professional fees are included.

The final figure depends on four things: the type of extension, its size, the specification, and where you live. This guide breaks down each type, what sits inside a quote, and the extra costs that catch people out.

If you're extending in London, our house extensions page explains how Beams can help.

Cost depends on the type of extension

The biggest single factor is which kind of extension you're building. All figures below are for construction, before VAT and professional fees, and London sits at the higher end of each range.

Single-storey rear extension
The most common extension and usually the most affordable per square metre. Around £2,000–£3,500 per sqm across the UK, or £2,500–£5,000 in London. A typical 20–30 sqm rear extension runs to roughly £80,000–£150,000 all-in in London.

Side return extension
Fills the narrow alley alongside a Victorian or Edwardian terrace. Similar rates to a rear extension, though the awkward, confined footprint can nudge the per-sqm cost up. In London, total project costs often land around £75,000–£120,000.

Wraparound extension
A rear and side return combined into an L-shape — the most ground-floor space, and a higher total cost to match, typically from around £60,000 well into six figures in London depending on size and finish.

Double-storey extension
Costs more in total than a single-storey, but often less per square metre — one set of foundations and one roof serve two floors, so you get more space for each pound. Expect higher total project costs, commonly £120,000–£200,000+ in London.

Kitchen extension
Most rear and side return extensions are really about creating a bigger kitchen. The build is costed as above; the kitchen itself is separate. You can see the options on our kitchen ranges.

What's included in an extension quote

A typical builder's quote covers the shell and the core finishes:

Groundworks and foundations, brickwork and blockwork, the roof structure and covering, windows and external doors, first-fix electrics and plumbing, plastering, and basic internal finishing. Better quotes itemise each so you can see where the money goes.

What a quote usually excludes is the kitchen units and appliances, bathroom fittings, flooring, decoration and any landscaping — these are specified and costed separately.

The extra costs to budget for

Several factors push costs above the typical ranges.

The per-square-metre rate is only part of your total. Build these in too:

• VAT — add 20% to all net figures. Most builder quotes are shown before VAT.
• Professional fees — typically 10–15% of the build cost across the whole team: architect or designer, structural engineer (£1,500–£4,000) and a party wall surveyor where one is needed.
• Planning application fee — a householder planning application in England costs £548 from April 2026. A Lawful Development Certificate, useful proof that your project is permitted development, is cheaper.
• Party Wall surveyor — if your extension affects a shared wall or sits close to a boundary, budget £1,000–£2,500 per affected neighbour.
• Contingency — 10–15% is sensible, more on older properties where foundations and existing structure can spring surprises.

What drives extension costs up

Several factors push a project above the typical ranges:

• Location — London and the South East run 20–40% above the national average, on labour rates alone.
• Specification — the gap between a basic flat-roof build and a high-spec extension with a roof lantern, full-width glazing, underfloor heating and a premium kitchen can be £70,000 or more on the same footprint.
• Glazing — large sliding or bi-fold doors and roof lanterns are some of the most expensive elements per square metre.
• Groundworks — difficult access, deep foundations near trees or drains, or underpinning all add cost before the walls go up.
• Structural work — steel beams to open up the back of the house, and any work to load-bearing walls, need engineer's calculations and add to both fees and build.

A house extension typically adds 10–20% to your home's value

In London, where space is at a premium, a well-designed extension can add as much as £4,000 per square metre of new floor area. The extensions that add most are those that create a larger, brighter kitchen-diner or an extra bedroom — the spaces buyers value most. For most homeowners, extending also works out cheaper than the stamp duty, fees and upheaval of moving to a larger home.

For how long the work takes, see our guide to how long a house extension takes. To get the project off on the right foot, read how to plan a house extension.