House extension cost calculator (UK)

Single-storey extension cost calculator: 2026 UK prices

Most people using this page are pricing a single-storey rear extension, so that is what the calculator starts on. For a good-quality finish, builders are working to £1,900–£2,700 per m² for the works at UK-average rates. That covers the shell and a standard fit-out. It does not yet include the glazing, the rooflights, fees or contingency, and those are what turn a per-m² figure into a real budget.

With a 3-panel bi-fold, two standard rooflights, 9–14% fees, a 10% contingency and 20% VAT, the calculator gives these typical all-in ranges (no kitchen, no steel, normal side access):

“UK average” is the calculator’s baseline rate. Its Midlands setting is 3% below it, the North and Wales 8% below, and the East and South West 3% above.

London and the South East: the calculator puts London at about 1.25× the UK average and the South East at about 1.12×. Most of that gap is labour and demand, not materials. Terraces where everything comes through the house, which is common in London, add roughly another 12%.

What is in the price, and what usually is not

• Included: foundations, drainage connections, walls, roof, windows and doors, plastering, electrics and heating, decoration and a standard floor finish.
• Often extra: a steel beam to open up the old back wall (£2,200–£6,500), building over or diverting a drain (£900–£4,000), deeper foundations near trees or on clay (£200–£450 per m²), party wall surveyors (£1,200–£3,500) and putting the garden back (£1,500–£6,000). Tick the ones that apply in the calculator.
• Not included anywhere: furniture, blinds and anything you buy yourself.

VAT, fees, building regs and planning

The figures include 20% VAT, because a new extension to an existing home is standard-rated. A builder who is not VAT-registered will not charge it, and that alone explains some of the spread between quotes. The 9–14% fees cover drawings, structural calculations, building control (typically £600–£1,200 on its own) and, if you need one, the planning application. In England a householder application currently costs £548, with a rise to £575 planned from 8 December 2026 (Planning Portal). In England, many single-storey rear extensions on houses fall under permitted development: 3m deep on a terrace or semi and 4m on a detached house, or 6m and 8m with prior approval (not available in conservation areas or on other designated land). Flats and maisonettes have no permitted development rights for extensions. Building regulations approval is needed either way. Check your own case with the planning permission checker and the building regulations checker.

Run your own size in the calculator ↑

Kitchen extension cost: what a kitchen extension costs in the UK

A kitchen extension is usually a single-storey rear extension with two extra lines: the kitchen itself, and a steel beam so the new room opens into the old one. The calculator starts with both ticked, so its first figure is a kitchen extension.

• Kitchen: £9,000–£22,000 for mid-range units, worktops and appliances, fitted.
• Steel and opening up: £2,200–£6,500 for the beam, padstones, temporary props and the engineer’s calculations.

On a 20m² extension with a 3-panel bi-fold and two rooflights, that gives £64,000–£113,000 all-in at UK-average rates, £72,000–£127,000 in the South East and £80,000–£142,000 in London. A 25m² kitchen-diner with a roof lantern comes to roughly £77,000–£135,000 at UK-average rates and £96,000–£168,000 in London. Fees and contingency are added to the kitchen line too, because the design, electrics and plumbing for it go through the same job.

Why is the kitchen line lower than our kitchen cost calculator, which puts a mid-range kitchen at £18,000–£32,000? That figure is for replacing a kitchen in an existing room, so it includes the electrics, plastering and flooring. In an extension those are already in the build rate above, so the kitchen line only needs to cover units, worktops, appliances and fitting.

Where kitchen extension budgets usually slip

• Moving the sink or hob a long way from the existing drain or gas run.
• A bigger heating load: underfloor heating (£75–£120 per m²) or a boiler that can no longer cope (£2,400–£4,200).
• Glazing creep: a slim-frame slider instead of a bi-fold can add £5,000–£12,000. A lot of glazing can also mean extra energy calculations for building control.
• The old kitchen: it becomes a utility, a WC or a hallway, and that work is rarely in the first quote.

Change the kitchen and glazing options in the calculator ↑

2-storey extension cost: double-storey prices in the UK

A two-storey extension costs more in total but less per m², because you pay for the foundations and the roof once and get two floors out of them. At UK-average rates the works run at £1,650–£2,350 per m² of floor area, roughly 10–15% below the single-storey rate.

Using the calculator for a 2-storey extension: choose Double storey and enter the total floor area of both floors. A 4m × 5m footprint is about 40m², not 20m².

Typical all-in ranges (French doors downstairs, no rooflights, no steel or fit-out):

What a 2-storey budget has to cover that a single storey does not

• Upstairs rooms: an en-suite or bathroom is not a calculator option. Price it separately with the bathroom cost calculator.
• Breaking through upstairs: new openings in the existing first-floor wall, and often a steel beam downstairs too (£2,200–£6,500).
• Matching the house: brick, roof tiles and window style matter far more on a two-storey extension, because it is visible from further away.
• Scaffold, access and neighbours: a two-storey extension on or near the boundary almost always means party wall notices. See the party wall checker.

Planning permission for a two-storey extension

Two-storey extensions are much less likely to fall under permitted development than single-storey ones. In England a rear extension of more than one storey is limited to 3m deep and has to be at least 7m from the rear boundary. On designated land, such as conservation areas, National Parks and AONBs, two-storey extensions are taken out of permitted development altogether. Plenty of two-storey extensions need a householder planning application, so allow for the fee and the time it takes before work can start.

Switch the calculator to Double storey ↑ · Check planning for your case