Extension costs are one of the most searched and most misleadingly quoted figures in UK home improvement. A builder’s quote of “around £2,000 a square metre” can be meaningless without knowing whether that includes VAT, professional fees, kitchens, or even the glazing. This guide unpacks the real 2026 figures — by extension type, specification level, and region — so you can build a realistic budget before approaching contractors.
What the Per-m² Rate Actually Covers
A credible per-m² build cost for an extension should include: demolition and strip-out, groundworks and foundations, structural frame, external walls, roof, windows and external doors, first-fix services (electrics, plumbing, heating), internal walls, insulation, first and second fix joinery, plaster, and decoration. It typically excludes VAT (20%), professional fees (architect, structural engineer, planning), kitchen or bathroom fit-out, furniture, landscaping, and Party Wall surveyor costs.
Always clarify scope when comparing quotes.
Cost per m² by Extension Type (2026)
| Extension type | Standard spec (£/m²) | Good spec (£/m²) | High spec (£/m²) | Notes |
|---|---|---|---|---|
| Single-storey rear (cavity wall) | £1,800–£2,200 | £2,200–£2,800 | £2,800–£3,800 | Most common; straightforward groundworks |
| Single-storey rear (structural glazing / aluminium) | £2,400–£3,000 | £3,000–£3,800 | £3,800–£5,000 | Steel frame, bifolds or sliders, complex roof |
| Two-storey rear | £1,600–£2,000 | £2,000–£2,600 | £2,600–£3,400 | Lower per m² as foundation/roof costs spread |
| Side return (infill) | £2,200–£2,800 | £2,800–£3,600 | £3,600–£5,000 | Often requires structural steelwork; London common |
| Wrap-around (rear + side) | £2,000–£2,600 | £2,600–£3,200 | £3,200–£4,500 | Complex geometry; single contract |
| Over-garage / above existing | £1,800–£2,400 | £2,400–£3,000 | £3,000–£4,200 | Must check existing structure; often lighter construction |
| Basement extension | £3,500–£5,000 | £5,000–£6,500 | £6,500–£9,000+ | Tanking, underpinning, pumped drainage |
All figures exclude VAT and professional fees. London and South East add approximately 15–25% to build costs; North of England and Wales may be 10–20% below the midpoint figures.
Worked Example: 25 m² Single-Storey Rear Extension
A typical 5 m × 5 m rear extension in the Midlands, standard specification:
| Item | Cost |
|---|---|
| Build cost (25 m² × £2,000/m²) | £50,000 |
| VAT @ 20% | £10,000 |
| Architect (design + contract admin, ~10%) | £6,000 |
| Structural engineer | £900 |
| Planning application fee (England, 2026) | £258 |
| Building Regs application (Full Plans) | £1,200 |
| Party Wall surveyor (if required) | £1,000–£1,800 |
| Total | ~£70,000–£72,000 |
In London, the same project would likely land between £85,000 and £100,000 including fees.
What Drives the Per-m² Cost Up
1. Structural complexity. Any extension requiring a steel RSJ or flitch beam to open up the rear elevation — typically needed when merging kitchen and dining space — adds £2,000–£8,000 in steel supply, installation, and temporary propping, which is hidden in the per-m² figure.
2. Groundwork conditions. Poor ground (made ground, trees within 5–10 m requiring deeper or piled foundations, high water table) can add £5,000–£25,000+ depending on severity. A ground investigation report (£500–£1,500) is wise on any site with uncertainty.
3. Roof type. A pitched tiled roof costs less than a flat roof with standing-seam zinc or aluminium, and far less than a frameless glazed lantern roof. Allow an extra £200–£400/m² for premium roof finishes.
4. Glazing. Bifold or sliding door sets in aluminium cost £800–£2,500 per running metre depending on width and specification; a fully glazed wall with roof lights can add £15,000–£30,000 to a project.
5. Utility diversions. Drainage rerouting, gas pipe diversions, or adding underfloor heating can add £3,000–£10,000 to any project.
What Drives the Per-m² Cost Down
Adding floor area generally reduces the per-m² rate because fixed costs (scaffolding, foundation mobilisation, roofing setup) are shared across more space. A 15 m² extension is rarely meaningfully cheaper than a 20 m² one, but the 20 m² extension costs less per square metre.
Two-storey extensions share the same foundation and roof over twice the floor area, hence the lower per-m² rate — but the total project cost is of course higher.
Regional Adjustments (2026)
| Region | Adjustment vs national midpoint |
|---|---|
| London (inner) | +20–30% |
| London (outer) | +15–20% |
| South East, East | +10–15% |
| East Midlands, West Midlands | 0 to +5% |
| Yorkshire, North West | −5 to +5% |
| Wales, North East | −10 to −5% |
| Scotland | −5 to +10% (varies by city) |
Professional Fees to Budget
Professional fees are often under-budgeted. Allow:
- Architect (design + planning + building regs + contract administration): 8–15% of build cost
- Structural engineer: £800–£2,000 for residential extensions
- Planning fee (England): £258 for a householder application (2026)
- Building Regs (Full Plans, England): £900–£1,800 depending on local authority
- Party Wall surveyor: £800–£2,500 if applicable
Getting Accurate Quotes
Per-m² rates are a planning tool, not a contract figure. To get accurate quotes:
- Have architectural drawings prepared before approaching builders — quoting from outline descriptions produces wildly varying prices.
- Get at least three competitive tenders from similar-tier contractors.
- Use a fixed-price JCT Homeowner or Minor Works contract rather than a day-rate arrangement.
- Include a contingency of 10–15% in your budget for unforeseen groundwork issues, price variations, and design changes.
