Is an extension cheaper than moving house in London?

Usually, yes. Moving a £900,000 London home costs around £60,000–£80,000 in stamp duty, agent fees, legal fees and removals before you have gained a single square metre. A £70,000 extension adds the space and typically adds 10–15% to the property value.

Does the price include the kitchen?

Build quotes usually exclude kitchen units, worktops and appliances because they vary from £8,000 to £60,000 depending on the brand. We include the plumbing, electrics and first-fix for the kitchen in every quote.

Yes. ICV Buildings works to a staged payment schedule agreed in the contract: a deposit to secure the start date, then payments at foundations, watertight shell, first fix and completion.

How much does a house extension cost in London in 2026?

In 2026 a single-storey rear extension in London costs £2,500–£3,500 per m² to build, so a typical 20 m² kitchen extension is £55,000–£80,000 before the kitchen itself. Double-storey extensions cost £90,000–£160,000. Add 10–15% for professional fees, VAT at 20% and a contingency of 10%.

Cost per square metre in 2026

London build costs sit well above the national average because of labour rates, access, parking and the age of the housing stock. For a standard-specification single-storey rear extension, budget £2,500–£3,500 per m² for the shell, roof, glazing, first and second fix and basic finishes. A premium specification with large structural glazing, polished concrete, underfloor heating and bespoke joinery runs £3,500–£4,500 per m².

Double-storey extensions are cheaper per square metre because the foundations and roof are shared between two floors: expect £2,200–£3,000 per m² across both levels. The absolute cost is higher because you are adding twice the area and usually a bathroom upstairs.

Typical project budgets

The figures below are what ICV Buildings quotes for standard-specification projects in Greater London. They include design liaison, structural steels, Building Regulations compliance, electrics, plumbing and plastering, but exclude the kitchen units, appliances and VAT.

Side-return extension on a Victorian terrace (12–15 m²): £65,000–£95,000
Rear kitchen extension (20 m²): £55,000–£80,000
Wrap-around extension (30–35 m²): £95,000–£140,000
Double-storey rear extension (40–50 m²): £110,000–£160,000
Garden room or outbuilding (15–20 m²): £30,000–£50,000

What pushes the price up

Glazing is the biggest single variable. A set of aluminium bi-folds costs £6,000–£12,000, while a frameless sliding system or structural glass roof can add £20,000–£40,000. Steelwork is the second: opening up the whole rear wall and removing internal walls can mean three or four steel beams and a goalpost frame, adding £8,000–£15,000.

Ground conditions matter in London. Clay soil near trees often requires deeper foundations or piling, and Thames Water build-over agreements apply when you build within 3 m of a public sewer. Both can add £5,000–£20,000. Finally, conservation area or listed status increases design fees and may require more expensive matching materials.

Hidden costs to budget for

On top of the build cost, allow for an architect or designer (£3,000–£8,000), a structural engineer (£1,500–£3,000), a planning application fee (£258 for a householder application in 2026) or a lawful development certificate, Building Regulations fees (£800–£1,500) and a party wall surveyor if your neighbours appoint one (£1,000–£3,000 per neighbour). VAT at 20% applies to all extension work on an existing house.

We recommend holding a 10% contingency for the unexpected: hidden drains, asbestos in old floor tiles, or a decision to upgrade the specification halfway through.

How to keep the cost down

Keep the roof simple: a flat roof with two or three rooflights costs far less than a pitched or glazed roof. Keep structural openings to one wide span rather than several. Choose standard-size aluminium or uPVC glazing over bespoke systems, and use a fixed-price contractor so the number you agree is the number you pay.

Permitted development also saves time and fees. Most single-storey rear extensions up to 3 m deep on a terrace or semi, or 4 m on a detached house, do not need planning permission outside conservation areas.

Frequently asked questions

Is an extension cheaper than moving house in London?: Usually, yes. Moving a £900,000 London home costs around £60,000–£80,000 in stamp duty, agent fees, legal fees and removals before you have gained a single square metre. A £70,000 extension adds the space and typically adds 10–15% to the property value.
Does the price include the kitchen?: Build quotes usually exclude kitchen units, worktops and appliances because they vary from £8,000 to £60,000 depending on the brand. We include the plumbing, electrics and first-fix for the kitchen in every quote.
Can I pay in stages?: Yes. ICV Buildings works to a staged payment schedule agreed in the contract: a deposit to secure the start date, then payments at foundations, watertight shell, first fix and completion.