How Much Does an Extension Cost in the UK? Every Type Priced for 2026

A standard 20m² single storey rear extension costs £54,000 to £62,000 and a 40m² double storey rear extension costs £86,500 to £99,500 at West Midlands rates, including VAT, drawings, a structural engineer, Building Regulations and the planning fee. Inner London is the most expensive part of the UK (£71,000 to £88,500 for the single storey job) and Northern Ireland the cheapest (£47,500 to £56,500). This guide prices every type of extension, shows how size and spec move the number, and lists what you still need to budget for.

How much does each type of extension cost?

Six types cover almost every home extension in the UK. Here they are at standard spec, each at a typical size, priced by the same engine that runs the calculator.

A single storey rear extension is the most common job: it usually opens up the kitchen and dining space and can often go ahead under permitted development. Side extensions are smaller but often sit on or near the boundary, so a party wall agreement is more likely. A wraparound combines both and is priced here as two separate structures added together. For a full trade-by-trade breakdown of the rear extension, read our single storey extension cost guide.

Why does a double storey extension cost less per square metre?

A double storey rear extension on a 5m by 4m footprint gives you 40m² of new floor for £86,500 to £99,500, about £2,310 per m². The single storey version gives you 20m² for £54,000 to £62,000, about £2,870 per m². The foundations, the roof, the drainage and the professional fees are paid for once and shared by both floors. What you add is a second floor structure, taller walls, stairs and more finishing.

The catch is planning. Double storey extensions usually need a full householder planning application and are judged on how they affect neighbours’ light and privacy, so allow time for the decision.

How much does size change the price?

Bigger extensions cost more in total but less per square metre, because parts of the job (drainage connections, scaffold set-up, fees) barely change with size. All of these are single storey rear extensions at standard spec:

Inc VAT and professional fees, West Midlands rates.

How much does the spec level add?

The calculator has three spec levels. Standard uses blockwork walls and a flat GRP roof. Mid moves to cavity walls and higher professional fees for more detailed drawings. High uses face brick and a pitched roof with slate. For the same 20m² single storey rear extension:

Spec changes the shell less than most people think. The bigger jumps come from what goes inside: a new kitchen, bifold doors, a roof lantern or a bathroom. Our kitchen extension guide prices those in detail.

How much does an extension cost in my region?

Our base prices come from completed jobs in the West Midlands. Every other region is priced with a cost index calibrated from published regional tender price data, so the same extension costs more in London and the South East and less in the North East, Wales and Northern Ireland.

Prices also move within a region. Commuter towns tend to sit above their regional average and big northern cities below. Our South East and North West pages show the local postcode areas, and the calculator prices down to your own postcode.

What fees do you pay on top of the build?

Every extension price in this guide already includes these. On the standard 20m² rear extension:

Ex VAT. Professional fees scale with your region; planning fees are set nationally. Wales charges £585, Scotland £312 and Northern Ireland £327 for a householder application.

Do you need planning permission for an extension?

Not always. Many single storey rear extensions in England fall under permitted development, which usually means no full planning application if the extension stays within set depth and height limits and doesn’t face the road. Larger rear extensions may be possible through the prior approval process. Double storey and most side extensions usually need full planning, and homes in conservation areas, listed buildings and flats have tighter rules. Rules differ in Wales, Scotland and Northern Ireland, so check with your council before you commit to a design. If you are relying on permitted development, a lawful development certificate gives you proof for when you sell.

Building Regulations approval is needed for almost every extension, whether or not you need planning. If you build on or near a shared boundary, the Party Wall etc. Act 1996 usually requires you to serve notice on your neighbours in England and Wales.

What isn’t included in an extension estimate?

• Hidden ground problems: soft clay, tree roots or a public sewer under the footprint.
• Knocking through: steel beams and making good where the extension meets the house.
• Moving services: a boiler, gas meter or soil stack in the way.
• Fit-out: kitchens, bathrooms and big glazing, unless you add them in the calculator.
• Finishing: flooring, decorating, blinds and the garden after the build.

Keep a contingency for these. A builder can only price them properly after a site visit.

How do I budget for an extension?

1. Run your size, rooms and postcode through the free calculator to get a range.
2. Brief your architect with that budget so the drawings match what you can spend.
3. Get at least three written quotes from builders working from the same drawings.
4. Compare like for like: VAT, fees, glazing, kitchen and what happens if the ground is poor.

Thinking about going up instead of out? Our loft conversion cost guide prices every loft type the same way. And if you want to know exactly how these numbers are built, read how Quoteio prices a project.

Common questions