Last Updated on September 17, 2026
Most UK single-storey extensions cost £1,800 to £3,000 per square metre for the build alone, before VAT, professional fees and fit-out. London runs £2,300 to £3,400 per m². A standard 30m² extension typically lands between £60,000 and £84,000.
Checked on 11 September 2026.
Extension cost per square metre
All figures below were seen on the named source pages on 11 September 2026. They are build costs. They do not include VAT, professional fees, or fit-out unless stated.
| Job | Typical rate | Source |
|---|---|---|
| Single storey, standard finish | £1,800 to £3,000 per m² | Checkatrade and HomeOwners Alliance |
| Single storey, shell only (no fit-out) | £1,200 to £1,700 per m² | Checkatrade |
| Double storey | around £1,900 per m² | HomeOwners Alliance |
| Single storey in London | £2,300 to £3,400 per m² | HomeOwners Alliance |
| Full spread, budget shell to premium finish | £750 to £3,600 per m² | Resi |
Double storey looks cheap per square metre because the foundations and roof are shared across two floors. The total bill is still much bigger.
Worked totals
Checkatrade’s standard-quality examples, seen 11 September 2026:
| Size | Total range |
|---|---|
| 20m² single storey | £40,000 to £56,000 |
| 30m² single storey | £60,000 to £84,000 |
| 50m² single storey | £100,000 to £140,000 |
| 60m² two storey | £108,000 to £180,000 |
Resi’s 30m² single-storey rear extension examples split by finish, seen 11 September 2026: outside London, budget £37,125 to £61,875, average £44,550 to £74,250, premium £48,262 to £80,437. In London, budget £45,000 to £75,000 rising to £58,500 to £97,500 at premium. Resi states these exclude VAT at 20% and exclude kitchens, glazing, landscaping and bathrooms.
What the £ per m² rate usually leaves out
This is where most homeowners get caught. The per-metre rate is the structure, not the finished room. On top, expect:
- VAT at 20%. A VAT-registered builder must add 20% to the quote. Resi’s figures exclude it. Checkatrade’s exclude it unless stated. Always ask which basis a quote uses. A builder under the VAT threshold does not charge it, which partly explains why one quote can undercut another by a fifth.
- Professional fees. HomeOwners Alliance, seen 11 September 2026: architects charge 3% to 15% of construction cost, structural engineers £100 to £200 per hour, a planning application £258, building control £100 to submit full plans plus £200 to £400 for inspections.
- Fit-out. A kitchen in the new space adds £15,000 to £35,000 (Checkatrade and HomeOwners Alliance both publish this range). A bathroom adds £5,500 to £9,000 (Checkatrade).
- Contingency. HomeOwners Alliance recommends setting aside around 10% of the total cost for the unexpected. Ground conditions, drains and party wall surprises are the usual culprits.
What moves the price
- Storeys. Double storey costs less per m² but far more in total.
- Finish level. Resi’s spread runs from £750 per m² for a budget shell to £3,600 per m² premium. The same footprint can triple in cost on finish alone.
- Location. London and the South East sit well above the Midlands, the North, Wales and Scotland. HomeOwners Alliance puts London at £2,300 to £3,400 per m² against a national £1,800 to £3,000.
- What the room is. A kitchen or bathroom extension carries plumbing, electrics and fit-out that a plain living room does not.
- Ground and access. Poor soil, nearby trees, drains under the footprint, or no side access for machinery all add cost before a single block is laid.
Two-minute quote check
Before you accept any extension quote:
- Ask for the price per m² and the floor area. Divide one by the other. If the rate is under about £1,500 per m² for a finished single-storey room, ask what has been left out.
- Ask whether VAT is included. Get the answer in writing. A £60,000 quote plus VAT is a £72,000 bill.
- Ask what is excluded. Kitchen, bathroom, glazing, landscaping, professional fees and building control are commonly not in the builder’s number.
- Ask for a payment schedule tied to stages, not dates. Foundations, damp course, wall plate, watertight, completion is a normal shape.
- Get three quotes on the same written spec. Quotes on different assumptions cannot be compared.
If you are the builder: what to publish
A short, honest price page wins work from vaguer competitors.
- Publish your real starting rate per m² and state plainly whether it includes VAT.
- List what your rate includes and what it never includes (fees, fit-out, landscaping).
- Show one or two recent jobs with the real final price and the floor area.
- State your payment stages and your typical deposit.
- If you are VAT-registered, say so. It explains your price against non-registered rivals.
That is it. No need for a cost calculator or a lead magnet. Clear numbers do the work.
Red flags
- A large upfront deposit. A request for a third or half of the job before work starts is not normal for an established firm. Materials deposits for made-to-order items are reasonable; big cash up front is not.
- No written contract or spec. A one-line quote for a five-figure job is a dispute waiting to happen.
- One project price with no breakdown. You cannot compare it, and neither can you tell what happens when something changes.
- A quote far below the ranges above with no explanation. Under roughly £1,200 per m² someone is excluding a lot, or planning to find “extras” later.
- Silence on VAT. If a firm will not say whether the figure includes VAT, assume it does not.
What this page does not cover
- Loft conversions, garage conversions and basements, which price differently (Checkatrade lists basements at £100,000 to £160,000 for 40m²).
- Listed buildings and conservation areas, where costs and consents are their own subject.
- Party wall agreements and their surveyor costs.
- Scotland’s separate building warrant system.
- Whether your project needs planning permission at all. Check with your local authority.
Method and limits
Every figure above was read directly on the Checkatrade, HomeOwners Alliance and Resi cost pages on 11 September 2026. These are published guide prices, not quotes, and the three sources do not fully agree, which is why ranges are shown rather than averages. MyBuilder’s guide was also fetched but its per-m² figures conflicted internally, so it is not cited. Prices move; treat anything here older than a few months as indicative, verify at source.
Sources
- Checkatrade house extension cost guide. Single-storey and shell-only £ per m², worked totals by size, kitchen and bathroom add-on costs. Seen 11 September 2026.
- HomeOwners Alliance house extension costs. London rates, double-storey rate, professional fees, 10% contingency, VAT advice. Seen 11 September 2026.
- Resi house extension cost guide. Full £750 to £3,600 per m² spread, budget/average/premium project totals, London versus outside London, list of exclusions. Seen 11 September 2026.
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