How much does a house extension cost in the UK?
Most UK house extensions are priced by five main factors: the floor area in square metres, the specification of the build, the structural work involved, how easy the site is to access, and the region you live in. Two homes on the same street can pay very different prices for the same size extension once these factors are taken into account.
This extension cost calculator gives you a rough budgeting estimate for a UK extension in 2026. It is not a fixed quote and should not be treated as one — real builder quotes will vary based on your site, drawings, specification and market conditions. Use the range as a starting point for conversations with architects and builders.
Extension cost per square metre UK
As a rough guide, here are typical UK extension costs per m² in 2026 for build only, before VAT, professional fees and fit-out extras like kitchens and bathrooms.
| Extension type | Standard £/m² | Mid-range £/m² | Premium £/m² |
|---|---|---|---|
| Single storey rear | £1,900–£2,400 | £2,300–£2,900 | £2,800–£3,500 |
| Double storey | £1,700–£2,200 | £2,100–£2,700 | £2,600–£3,300 |
| Kitchen extension | £2,300–£3,000 | £2,900–£3,800 | £3,700–£5,000 |
| Side return | £2,300–£3,000 | £2,900–£3,800 | £3,700–£4,800 |
| Wraparound | £2,500–£3,300 | £3,200–£4,200 | £4,000–£5,200 |
London and the South East typically add around 20% on top of these figures. The Midlands, the North and parts of Scotland and Wales tend to be 5–10% below the UK average.
Single storey extension costs
A single storey extension is the most common project for UK homeowners. The cost per m² covers foundations, drainage connections, external walls, roof structure and covering, insulation, internal plastering, first and second fix electrics, heating extension, and standard doors and windows.
Smaller extensions (under about 15m²) often work out at a higher rate per m² because the fixed costs of setting up the site, foundations and roof are spread over a smaller area.
Double storey extension costs
Double storey extensions can be better value per m² than single storey because you get two floors of space for roughly one set of foundations and one roof. The total cost is higher, but the price per square metre is usually 10–20% lower than an equivalent single storey build.
They do come with more structural and design considerations — larger steels, more planning sensitivity, party wall issues and upstairs layout changes such as new bedrooms or bathrooms.
Kitchen extension costs
Kitchen extensions often cost more per m² than a plain rear extension because the fit-out is included: kitchen units, worktops, splashbacks, plumbing, extra electrics and sockets, tiled or engineered flooring, task and feature lighting, integrated appliances and higher-spec finishes.
A mid-range kitchen typically adds £15,000–£25,000 on top of the build. A premium kitchen with stone worktops and branded appliances can easily reach £40,000 or more.
Side return and wraparound extension costs
Side return and wraparound extensions are popular on Victorian and Edwardian terraces, but they tend to be more complex per m². You often need steelwork to open up the existing structure, drainage moves, a bespoke roof design (flat with a lantern, or a pitched glazed roof), and careful detailing at the boundary with neighbours.
Restricted access — no side gate, or all materials carried through the house — can add 5–10% to the build cost on its own.
What affects the cost of an extension?
- Ground conditions and soil type
- Foundation depth and type
- Drainage runs and manhole moves
- Site access and material handling
- Steelwork and knock-throughs
- Roof type (flat, pitched, glazed)
- Bifolds or sliding doors
- Kitchens and bathrooms
- Finish level and specification
- Regional labour rates
- Skip hire and waste removal
- VAT status of the builder
- Planning permission requirements
- Building control inspections
- Structural engineer calculations
- Party wall agreements
- Contingency for unknowns
What this calculator includes and excludes
This calculator gives a rough build-cost estimate for a UK extension. It includes the base build (foundations, structure, roof, walls, standard finishes) plus any optional extras you tick, an optional VAT line, and a contingency percentage.
It does not automatically include:
- Architect, designer and structural engineer fees
- Planning application and building control fees
- Party wall surveyor fees
- Kitchens, bathrooms, wardrobes and joinery (unless selected)
- Landscaping, patios, fencing and garden reinstatement
- Decorating, curtains, blinds and furniture
- Utility upgrades (new consumer unit, boiler, gas meter)
- Unexpected ground works, asbestos or structural surprises
Treat the number it gives you as a homeowner budgeting range, not a fixed quote.
Example extension costs
These example ranges are indicative UK 2026 figures at a mid-range specification, before VAT, for a typical site outside London.
| Project | Indicative range |
|---|---|
| Small 12m² rear extension | £28,000–£38,000 |
| 20m² single storey extension | £46,000–£62,000 |
| 30m² kitchen extension (incl. fit-out) | £100,000–£140,000 |
| 40m² wraparound extension | £130,000–£175,000 |
| 50m² double storey extension | £110,000–£150,000 |
Add VAT (20%) if your builder is VAT registered, plus a 10–15% contingency and professional fees on top.
How to use this estimate when comparing builder quotes
Once you have a rough budget from this calculator, the next step is comparing builder quotes properly. Don't just look at the headline number — check what each quote includes and excludes.
Ask every builder for a written, itemised quote covering preliminaries, groundworks, structure, roof, external works, internal finishes, kitchens, bathrooms, electrics, plumbing, heating, decoration, VAT, and any provisional sums. If one quote is much cheaper, it usually means something is missing.
Why builder quotes can vary so much
It's normal for extension quotes to vary by 20–40% between builders on the same job. Common reasons include:
- Different specification assumptions (bricks, insulation, windows)
- Missing items — some builders exclude decoration, flooring or scaffolding
- Access assumptions and skip locations
- Structural work priced from a full engineer's design vs a guess
- VAT status — non-VAT registered builders can appear cheaper
- Provisional sums for kitchens, bathrooms and finishes
- Unclear or missing allowances for prime-cost items
- Programme length and how overheads are recovered
The Homeowner Quote Checker Pack gives you a structured way to compare quotes line-by-line so you can spot the gaps before you sign a contract.