How much does a house extension cost in the UK?

If you're weighing up whether to extend rather than move, cost is usually the first question - and often the hardest to pin down. Extension prices vary enormously depending on size, location and specification, but with a few reliable benchmarks, you can build a realistic budget before you even approach a builder. Here's what to expect per square metre, what's typically included in a quote, and the hidden costs that catch people out.

Key takeaways

• Single-storey extensions typically cost £1,800-£3,000 per m² nationally; London and the South East can run 20-40% higher.
• Double-storey extensions cost less per m² because foundations and roofing are shared, but total project costs are usually higher due to the extra floor area.
• Per-m² quotes usually cover construction and basic finishes only - VAT, professional fees and kitchen or bathroom fit-out are typically extra.
• Many single-storey rear extensions fall under Permitted Development and don't need a full planning application.
• A contingency of 10-15% is standard practice, and should be treated as untouchable unless something goes wrong.
  
  

What does an extension cost per square metre?

According to the Federation of Master Builders, the average cost of a single-storey extension in the UK sits between £1,800 and £3,000 per square metre, depending on specification and location. Properties in London and the South East typically fall at the higher end of this range, or above it, reflecting higher labour and material costs in the region.

To put that into context: a modest 20m² single-storey extension - enough for a larger kitchen-diner or family room - could cost anywhere from around £36,000 to £60,000 before VAT, professional fees or interior fit-out, depending on where you live and the finish you choose.

These figures are a useful starting point for budgeting, but every site is different. Ground conditions, access, and the shape of the extension can all move the final number, so it's worth getting at least three like-for-like quotes from builders once you have a clear idea of what you want.

Single storey vs double storey: which offers better value?

It might seem counterintuitive, but double-storey extensions typically cost less per square metre than single-storey ones. That's because the foundations and roof (two of the most expensive elements of any build) are shared across two floors rather than paid for once to serve a single room.

That doesn't necessarily make a double-storey extension the cheaper option overall. Because you're building more floor area, the total project cost is usually higher, even if the price per square metre is lower. A double-storey extension also tends to involve more complex structural work, scaffolding, and in the case of semi-detached or terraced properties, a party wall agreement with your neighbour.

Which route makes sense depends on what you actually need from the space. If you're looking for a kitchen-diner or a bit more living space, a single-storey extension is usually simpler and more likely to fall within Permitted Development. If you need an extra bedroom or bathroom as well, building up can deliver more for your money in the long run.

If you're still deciding between extension types - side return, wraparound, dormer and so on - our guide on which type of house extension to choose breaks down the pros and cons of each.

What's included - and what's not - in a per-m² quote

Per-square-metre figures are a helpful shorthand, but it's important to know what they do and don't cover. Typically, they include the shell of the extension: foundations, brickwork, roofing, windows and doors, basic electrics and plumbing, and standard plastering and decoration.

What's usually excluded:

• VAT at 20% - most builder quotes are given excluding VAT, so it's worth adding this to any headline figure before you compare quotes.
• Professional fees - architect and structural engineer fees typically add 7–15% of the build cost, plus a separate fee if you need a party wall surveyor.
• Kitchen or bathroom fit-out - units, worktops and appliances are priced separately and can add anywhere from a few thousand pounds to tens of thousands, depending on specification.
• Planning application fees - payable in full where a full application is required (see planning section below).

Once these are added, many homeowners find their all-in cost comes out 20–30% higher than the headline per-m² figure alone suggests - which is exactly why it pays to ask a builder or quantity surveyor what's included before you commit to a number.

Hidden costs people forget to budget for

Beyond VAT and professional fees, there's a shorter list of costs that regularly catch people out partway through a project:

• Party wall agreements - required if you share a wall with a neighbour, and can take weeks to arrange, so it's worth starting early.
• Scaffolding - often quoted separately from the main build cost.
• Rewiring or fuse box upgrades - sometimes needed to bring the rest of the house up to standard once an extension is added.
• Contingency - a genuine 10–15% of your total budget, set aside and left untouched unless something goes wrong. This is different from a "nice to have" fund for upgrades, it should only be used to cover the unexpected, such as poor ground conditions discovered once digging begins.
  
  

Do you need planning permission?

Many single-storey rear extensions fall under Permitted Development rights, meaning they don't require a full planning application - provided they meet certain conditions. In England, this generally means the extension doesn't extend more than 3 metres beyond the original rear wall for an attached house (4 metres for a detached house), isn't higher than the existing roofline, and doesn't cover more than half the garden.

Even if your extension qualifies, it's worth applying for a Lawful Development Certificate from your local authority. It isn't a legal requirement, but it provides a formal record that the work was compliant - something buyers' solicitors often ask for when you come to sell.

If your extension exceeds Permitted Development limits, sits in a conservation area, or affects a listed building, you'll need a full planning application. Current fees are listed on the Planning Portal, so it's worth checking there directly since these are revised periodically.

Extend or move? A quick comparison

Extending is often, though not always, the cheaper option compared with moving - particularly once stamp duty, estate agent fees, and conveyancing costs are factored in on a new purchase. A well-planned extension can also add meaningful value to your home, although how much depends heavily on your local market and the quality of the finish.

That said, an extension is disruptive to live through, and the value it adds isn't guaranteed to match what you spend, particularly if the work is over-specified for the area. It's worth speaking to a local estate agent about ceiling prices for your street before committing to a large-scale extension purely as an investment.

Thinking about financing your extension? A home improvement loan could help spread the cost of your project, whether you're planning a small single-storey addition or a larger renovation.

Explore home improvement loans