House extension cost in London: a simple price guide
Here's the short answer. A single storey extension cost in London is usually £2,200–£3,000 per m². A double storey extension cost is usually £2,800–£3,800 per m². So if you build a normal 20m² single-storey extension at the back of your house, expect to pay somewhere between £44,000 and £60,000 in total. That includes materials, the builders' work, and basic fittings.
These numbers change depending on things like glass doors, steel beams, and how easy it is to get materials to your house. We explain all of that below, using real numbers from jobs we've actually done in London and nearby areas.
Most people aren't scared of the extension itself. They're scared of getting a cheap quote, saying yes, and then being told "extra costs" a few weeks later. This guide is here to stop that from happening to you.
House extension cost by type
| Extension type | Cost per m² | Total cost | How long it takes |
| Single storey rear extension | £2,200–£3,000 | £35,000–£65,000 | 10–14 weeks |
| Side return extension | £2,300–£3,100 | £30,000–£55,000 | 8–12 weeks |
| Double storey extension | £2,800–£3,800 | £70,000–£140,000 | 14–20 weeks |
| Kitchen extension cost (with glass doors) | £2,600–£3,500 | £45,000–£75,000 | 10–16 weeks |
| Wraparound extension | £2,900–£3,900 | £75,000–£150,000 | 16–22 weeks |
These prices are for building only, in London and nearby areas like Surrey, Kent, Essex, and Berkshire. Places outside inner London are usually 8–12% cheaper. You can check our Estimated Project Costs page for updated prices on everything we do.
Simple rule to remember: a single storey extension costs less per m² than a double storey one, but a double storey often gives you more value for your money. Why? Because you're only paying for one foundation and one roof, not two.
What actually makes the price go up
Most quotes don't go wrong because someone is lying. They go wrong because these five things get missed or guessed at too early:
Big glass doors and windows. Bifold doors and glass roof panels look amazing in the pictures, but they cost a lot more than a normal wall with a window. This alone can add £8,000 to £20,000 to a small extension.
Steel beams. If you're knocking down a wall to make an open-plan kitchen, you'll need steel beams. These need to be checked and approved by an engineer and signed off by building control. Cheap quotes often forget this cost completely.
Getting materials to your house. A terraced house with no side gap costs more to build, because workers have to carry everything by hand or use a crane. A house with a driveway is much easier and cheaper.
Ground and drainage work. Moving drains, dealing with hard clay soil, or building near a tree that's protected by law — all of this adds cost that only shows up once digging actually starts. That's why we do a proper survey before quoting, not after.
Waiting for planning permission. Every extra month you wait for approval is a month where material and labour prices can go up. It's not a building cost, but it still adds to your final bill.
A good, honest quote lists all five of these clearly from the start. If your quote doesn't mention steel, glass type, or how easy your house is to access, ask why. Those costs are probably going to show up later as "extras."
This is why a kitchen extension cost is usually higher than a plain box extension — most people want the big glass doors and open-plan feel, which brings in the glazing and steel costs above. A side return extension tends to sit a little lower, since you're building into an existing gap rather than starting from scratch, but access can still be tight, so it's worth getting a proper survey either way.
Extension planning permission vs permitted development
Most single storey rear extensions in London don't need full planning permission. They fall under something called permitted development, as long as you stay within these rules:
- No more than 4 metres out from the back wall (detached house), or 3 metres (semi-detached or terraced house)
- No taller than 4 metres
- Doesn't cover more than half your garden
If you go bigger than this — a wraparound, a double storey, or building close to your boundary — you'll need full planning permission. This usually takes 8 to 13 weeks in most London boroughs.
Two things trip people up: conservation areas and something called an Article 4 direction. Both of these remove your automatic right to build, even for a small extension. We check for this at your free survey, before you design anything, so you never end up designing something that isn't allowed.
Note that building regulations are different from planning permission. You need building regulations approval no matter how big or small your extension is. This covers things like safety, fire protection, insulation, and drainage, and it's checked in stages as the build happens, not just at the end.
A real project: rear extension in Clapham
We recently finished a 24m² single storey rear extension for a Victorian house in Clapham. It had a full glass roof, steel beams for an open-plan kitchen, and new drainage. The total cost was £58,000, finished in 12 weeks, with no surprise costs added — because we priced in everything above right from the start.
You can see the full before and after photos and cost breakdown on our recent projects page.
Extension or loft conversion — which one is right for you?
If your garden is small and you don't want to lose outdoor space, a loft conversion can add the same amount of extra room without touching your garden at all. We've written a full comparison in our loft conversion cost guide, including which option usually adds more value when you come to sell.
Get a real, accurate extension quote
The only way to get a number you can trust is a proper site visit, not a guess over the phone. Our quotes always list steel, glass, ground work, and site access as separate items, so nothing shows up as a surprise once the work has started.
Get Your Free Quote →
Check out our full Extensions service page to see how we work, or visit Estimated Project Costs for current prices across all our services.
Frequently asked questions
1.How much does a house extension cost in London?
A single storey extension usually costs £2,200–£3,000 per m², and a double storey extension usually costs £2,800–£3,800 per m². The exact price depends on glass, steel work, and how easy it is to access your house.
2.How long does a house extension take to build?
A single storey extension usually takes 10–14 weeks from start to finish. A double storey extension usually takes 14–20 weeks.
3.Do I need planning permission for a house extension?
Many single storey rear extensions don't need full planning permission — they fall under permitted development, as long as they stay within certain size limits. Conservation areas and Article 4 rules can remove this right, so it's worth checking first.
4.Is Zonex Group a fully accredited construction company?
Yes. Zonex Group is fully insured and accredited, and we work across London, Surrey, Essex, Kent, Berkshire, and Manchester. We also hold a 5.0 rating on Google from our clients.
5.What does a commercial fit out include?
Visit our Commercial Fit Outs page for the full details and typical costs.