How much does a house extension cost in the UK?
Extensions are usually discussed as a cost per square metre of new floor area, but the figure only means something once you know what it includes — and what it does not.
Per-m² figures typically cover the shell and basic finishes. Kitchens, bathrooms, flooring, decoration, landscaping and professional fees frequently sit outside them.
Professional fees, building control and structural design are real costs on almost every extension. If a quote does not mention them, they are probably yours to pay.
Indicative UK cost ranges
QuoteCheck does not hold a live UK pricing feed. The figures below are indicative planning ranges, assembled from commonly published UK ranges and typical job structures, and they are deliberately wide. Treat them as a starting point for questions, not as a valuation. Real prices depend on specification, access, ground conditions, region, season and how each firm prices its own labour and overheads.
| Item | Unit | Indicative range |
|---|---|---|
| Single-storey extension, standard finishHigher in London and the South East. | per m² | £2,000 – £2,800 |
| Single-storey, higher specificationLarge glazing, premium finishes. | per m² | £2,800 – £4,000 |
| Two-storey extensionOften lower per m² as foundations and roof are shared. | per m² | £1,800 – £3,000 |
| Architect / designer feesVaries by service level. | % of build | 5% – 12% |
| Structural engineerFor beam and foundation design. | per job | £500 – £1,500 |
| Building controlLocal authority or approved inspector. | per job | £500 – £1,200 |
| Party wall surveyorWhere a party wall award is required. | per job | £700 – £2,000 |
What moves the price
- Floor area
- The base of every per-m² calculation.
- Glazing
- Large sliding or bifold doors and rooflights are a major cost line.
- Groundworks
- Foundation depth, trees, drains and services under the footprint.
- Structural openings
- Steels to open up into the existing house.
- Services
- Heating, electrics and any relocated kitchen or bathroom.
- Access
- Terraced properties where everything passes through the house.
- Finishes
- Kitchen, flooring and decoration are often separate.
Worked example — 20m² single-storey rear extension (illustrative only)
- Shell at £2,400/m²
- £48,000
- Structural steels
- £5,500
- Large sliding doors
- £6,500
- Fees: design, engineer, building control
- £5,000
- Excluded: kitchen, flooring, decoration
- Budget separately
- Indicative total before VAT
- £65,000
Fictional figures. The purpose is to show how much of an extension budget sits outside the headline per-m² rate.
What your quote should show
- Floor area and per-m² equivalent
- Foundation type and assumed depth
- Steels and structural work
- Glazing specification and supplier
- Which services are included
- Fees: design, engineer, building control, party wall
- Exclusions in writing
- Programme, payment stages and variation process
Before you agree to the quote
- What is usually excluded
- Kitchens, bathrooms, flooring, decoration, blinds, landscaping and making good to the existing house are frequently outside a build quote. So are professional fees, party wall costs and any temporary kitchen. Ask for exclusions in writing before comparing totals.
- VAT
- Most extension work by a VAT-registered builder attracts 20% VAT. Some conversions of a property that has been empty for a long time, or work relating to a listed or newly built dwelling, can be treated differently — confirm the treatment in the quote rather than assuming the headline figure includes it.
- Staged payments
- Extensions are normally paid in stages tied to completed work — for example groundworks, damp proof course, roof, plaster and completion — rather than to dates. Avoid large payments ahead of work, and keep a final retention until snagging is done.
- How long a quote should stand
- Many builders state a validity period, commonly 28 to 90 days, because material prices move. If no period is stated, ask for one in writing along with what happens if the start date slips.
- Unexpected costs
- The common ones are foundation depth greater than assumed, drains or services under the footprint, tree roots, asbestos in an older property, and structural surprises once the wall is opened up. Ask how each would be priced if found.
- Comparing quotes
- Convert each quote to a cost per m² of new floor area, then add back the value of each one's exclusions. A quote that includes fees, glazing and finishes is not comparable to a shell-only price.
- Questions to ask before accepting
- Who supervises the site day to day? What is the programme and what happens if it overruns? How are variations priced? What insurance and guarantee are in place? Who deals with building control?
Frequently asked questions
How much does a house extension cost per m² in the UK?
Indicatively £2,000–£4,000 per m² for a single-storey extension, with London and the South East at the upper end. Confirm what the rate includes before comparing quotes.
Do I need planning permission for an extension?
Many single-storey rear extensions fall under permitted development, but limits apply and listed or conservation properties differ. Confirm with your local planning authority.
Why do extension quotes vary so much?
Usually because they cover different scopes. One may include fees, glazing and finishes while another prices the shell only.
Should I pay a deposit for an extension?
A modest deposit to secure a start date or order long-lead items such as glazing is common. It should be proportionate to what is actually being ordered, and the items it covers should be named in the quote.
Is VAT charged on extension work?
Normally yes, at 20%, where the builder is VAT registered. Certain conversions and work to long-empty or listed properties can be treated differently, so ask the builder to state the treatment in writing.
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