When you complete new extension, your house become bigger and instantly have more square foot. So extension is an investment.
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1. Design & Planning
Before anyone picks up a shovel, you’ll need proper plans in place — and that usually means hiring an architect or designer. For a fairly straightforward extension, expect to pay somewhere between £1,500 and £5,000, depending on who you hire and how complex the job is.
If planning permission is needed (which it often is in London), that’s another chunk — not just the council fee, but drawings, possible tweaks, maybe a structural engineer’s input too. Oh, and if your property shares a wall with a neighbour, you might need a party wall agreement — those don’t come free either. Think around £700 and up, depending on the situation.
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2. Getting Approval (or Not)
Some extensions fall under Permitted Development, but plenty don’t — especially in London. If you need full planning permission, expect a bit of paperwork and a wait. Council fees are around £200–£500, depending on your borough.
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3. The Build Itself
Now we’re getting to the bit that really hits the wallet — the actual build.
Single or a double-storey? Funny enough, the price per metre might be a bit higher, but you’re getting more bang for your buck in terms of added space. It’s not always double the cost for double the size — and that’s something worth thinking about when weighing your options.
This usually includes:
- Foundations and structural work
- Brick/blockwork
- Roof
- Windows and doors
- First fix electrics and plumbing
- Insulation and plastering
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4. Fixtures, Fittings & Finishes
Want a bespoke kitchen? Fancy brass taps? These aren’t in the base build price. You can find furniture in many stores like Ikea or more bespoke shops, all depending on your budget
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5. How to avoid Unexpected Extras
You might find dodgy drains, need a steel beam you didn’t expect, or have to redo part of an old wall. Always keep 10–15% of your budget aside for the “just in case” moments.
Speak to XMX LONDON about your project and share you budget to create a fix proposal
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6. VAT
It hurts, but it’s part of the picture. Most residential extension work is subject to 20% VAT, unless it’s part of a qualifying renovation.
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7. Professional Fees
You will require:
– Architect
– Structural Engineer
– Party wall surveyor fees (unless your house is fully detached)
– Building Control
XMX LONDON provides all the professionals you need, just go to xmxlondon.co.uk, email to [email protected] or contact the office on 020 7183 9333
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So What’s the Total?
For a typical 20m² single-storey extension, expect to pay somewhere between £70,000 and £100,000 in London in 2025 — all in. That includes design, build, finishes, VAT, and some contingency.
It’s not pocket change, but done well, it adds huge value to your home and your everyday life.
Want a more accurate idea based on your house and plans? We’re happy to talk it through. No pressure, just straight-up advice from a builder who’s seen it all before.