House Extension Cost Calculator
Estimate the 2026 cost of a UK house extension by type, size, region and specification — with fees, VAT and contingency included.
How much does a house extension cost in the UK?
In 2026, a standard single-storey rear extension costs roughly £1,900 to £2,600 per square metre to build, before VAT, professional fees and contingency. A basic specification starts nearer £1,700/m²; a premium, glass-heavy build with structural glazing, underfloor heating and a vaulted ceiling reaches £2,800–£3,500/m². On that basis a typical 20 m² extension comes to around £40,000–£60,000 for the build alone, or closer to £55,000–£85,000 all-in once fees, VAT and a contingency are added.
The calculator above turns those benchmarks into a figure for your project. Pick the extension type, enter the floor area, choose your UK region and specification level, and it derives a cost-per-m² rate, then adds a 12% contingency, your professional fees and a VAT line, and shows a ±15% likely range. You can type your own rate into the override field if you already have a builder's figure.
Cost by extension type (2026, standard spec, ex-VAT)
| Extension type | Typical £/m² | Typical project total |
|---|---|---|
| Single-storey rear | £1,900 – £2,600 | £40,000 – £60,000 (20 m²) |
| Side return | £1,900 – £2,200 | £15,000 – £30,000 |
| Kitchen extension (+ fit-out) | £1,900 – £2,600 + £8k–£15k kitchen | £30,000 – £48,000 |
| Double / two-storey | £2,000 – £3,000 | £70,000 – £120,000 (40 m² total) |
| Wrap-around | £2,400 – £3,000+ | £50,000 – £90,000+ |
Cost by size — single-storey, standard spec
| Floor area | Build cost (ex-VAT) | All-in guide (fees, VAT, contingency) |
|---|---|---|
| 15 m² (small) | £28,000 – £39,000 | £40,000 – £55,000 |
| 20 m² (3m × 6.5m) | £38,000 – £52,000 | £55,000 – £75,000 |
| 25 m² | £48,000 – £65,000 | £68,000 – £92,000 |
| 30 m² (medium) | £57,000 – £78,000 | £80,000 – £110,000 |
| 50 m² (large) | £95,000 – £130,000 | £130,000 – £180,000 |
Regional price adjustment
Build costs vary by where you are in the UK. The calculator applies a multiplier to the base rate:
| Region | Vs UK average |
|---|---|
| London | +25% to +40% |
| South East | +10% to +25% |
| South West | +5% to +15% |
| Midlands | around average |
| North England | −5% to −10% |
| Scotland | around average (cities higher) |
| Wales | −10% to −15% |
| Northern Ireland | below average |
Professional fees and other costs
The headline £/m² is the build cost only. On top, budget for:
| Item | Typical 2026 cost |
|---|---|
| Architect / designer | £1,500 – £3,000 (or 8–15% of build cost) |
| Structural engineer | £400 – £2,000 |
| Planning application (England) | £258 – £548 |
| Building Regulations | £400 – £1,000 |
| Party wall surveyor (if needed) | £700 – £2,000 |
| VAT on build | 20% (extensions are standard-rated) |
| Contingency | 10 – 15% of build cost |
Is it cheaper to build up or out?
A single-storey rear extension is the cheapest way to add space in total, because it is the smallest job. But a double-storey extension is usually cheaper per square metre of new space — you share one set of foundations and one roof across two floors, so the second storey is close to "free" structurally. If your budget is fixed and your footprint is limited, building up often gives more room for the money.
How this calculator works
It multiplies your floor area by a cost-per-m² rate (derived from the extension type, your region multiplier and the specification level, or your own override), then by the number of storeys. It adds a 12% contingency, your professional-fees figure, and shows VAT on the build separately. The result is a mid-point with a ±15% range, because real quotes vary with ground conditions, access, roof complexity and how busy local builders are.
Frequently asked questions
How much does a house extension cost in the UK in 2026?
A standard single-storey rear extension costs roughly £1,900 to £2,600 per square metre to build, so a 20 m² extension typically comes to £40,000–£60,000 before VAT, fees and contingency. Basic specs start nearer £1,700/m² and premium builds reach £2,800–£3,500/m².
How much is an extension per square metre?
Around £1,700–£2,000/m² for a basic single-storey build, £1,900–£2,600/m² for standard spec, and £2,800–£3,500/m² for premium finishes. Double-storey work is roughly £2,000–£3,000/m² of total floor area. London and the South East add 15–40%.
Is it cheaper to build up or out?
A double-storey extension costs more in total but usually less per square metre of new space, because you pay for one set of foundations and one roof across two floors. Building up is often better value where the footprint is limited.
Do I need planning permission for an extension?
Many single-storey rear extensions fall under Permitted Development rights, subject to size, height and boundary limits, so they do not need a full planning application. Building Regulations approval is separate and applies to almost all extensions.
What professional fees should I budget for?
An architect or designer (£1,500–£3,000 or 8–15% of build cost), a structural engineer (£400–£2,000), the planning fee (£258–£548), Building Regulations (£400–£1,000) and a party wall surveyor where needed (£700–£2,000) — usually £3,000–£6,500 in total.
Does the estimate include VAT?
Most builder quotes exclude VAT. Extensions to an existing home are standard-rated, so add 20% to the build cost unless your project qualifies for a relief. The calculator shows VAT separately.
How much contingency should I add?
Add 10–15% on top of the quoted build cost. Unexpected groundwork, drainage diversions and structural surprises are common once walls come down.
How long does a house extension take to build?
A single-storey extension typically takes 10–14 weeks on site; a double-storey extension 12–20 weeks. Design, planning and Building Regulations sign-off beforehand can add two to four months.
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A quick note
This is a rough estimate built on 2026 UK benchmark rates. Actual costs depend on specification, ground conditions, access, roof design and local labour rates — get quotes from local builders for an accurate figure. See our full disclaimer.