House extension cost per m² in the UK (2026)
For a standard-specification extension in 2026, most UK homeowners should budget £1,800 to £3,200 per m² for the build itself. The spread is wide because the rate depends on where you live, how the extension is built, and - more than anything - what you have decided counts as "included". This guide unpacks the rate properly so you can tell a realistic quote from an optimistic one.
- Single-storey rear: £2,000–£3,200 per m²
- Double-storey: £1,800–£2,700 per m² (cheaper per m², dearer in total)
- London premium: commonly 20–35% above the national mid-point
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The 2026 headline rates
The table below gives realistic 2026 build rates per square metre of new internal floor area. They assume a competent regional contractor, a standard specification, and a straightforward site with reasonable access. They exclude VAT variations, professional fees, kitchens and bathrooms, and anything unusual under the ground.
| Extension type | Basic spec | Standard spec | High spec |
|---|---|---|---|
| Single-storey rear | £1,800 – £2,200/m² | £2,200 – £2,700/m² | £2,700 – £3,600/m² |
| Side return infill | £2,000 – £2,500/m² | £2,500 – £3,100/m² | £3,100 – £4,000/m² |
| Wrap-around | £1,900 – £2,300/m² | £2,300 – £2,900/m² | £2,900 – £3,700/m² |
| Double-storey rear | £1,600 – £1,950/m² | £1,950 – £2,400/m² | £2,400 – £3,100/m² |
| Over-garage | £1,700 – £2,100/m² | £2,100 – £2,600/m² | £2,600 – £3,300/m² |
| Orangery / garden room | £1,700 – £2,100/m² | £2,100 – £2,800/m² | £2,800 – £3,800/m² |
Why double-storey is cheaper per m²
Foundations, groundworks, scaffolding, the roof and the site set-up are largely fixed costs. Build two floors instead of one and you spread those costs over twice the floor area. That is why a double-storey extension usually shows a lower rate per square metre while costing considerably more in total.
Regional variation in 2026
Labour is the main driver of regional difference; materials vary far less. As a working guide, apply the following adjustments to the standard-spec mid-point:
| Region | Adjustment | Typical standard-spec rate |
|---|---|---|
| London | +20% to +35% | £2,900 – £3,600/m² |
| South East | +10% to +20% | £2,600 – £3,100/m² |
| South West, East of England | +0% to +10% | £2,300 – £2,800/m² |
| Midlands | -5% to +5% | £2,150 – £2,650/m² |
| North of England, Wales | -10% to -5% | £2,000 – £2,450/m² |
| Scotland, Northern Ireland | -10% to +5% | £2,000 – £2,600/m² |
What the rate normally includes
- Groundworks, foundations and drainage connections within the footprint
- Structure: walls, steels, roof, insulation to current Building Regulations
- Standard windows and one set of external doors
- First and second fix electrics and plumbing
- Plastering, screed or floor build-up, and basic decoration
- The builder's overheads, site management and profit
What it almost never includes
- Professional fees: architect, structural engineer and planning or building control fees, commonly 8–15% of build cost
- Kitchens and bathrooms: priced separately, from £5,000 to well over £40,000
- Premium glazing: large sliding doors and structural glass can add £8,000–£25,000
- Landscaping and making good outside the extension footprint
- Unknowns below ground: shared drains, tree roots, poor sub-soil, existing shallow foundations
The single most common budgeting error
Multiplying floor area by a headline rate and calling that the project cost. On a typical £60,000 build, fees, kitchen, glazing upgrades and a contingency can add £25,000 to £40,000 more. Always budget the project, not just the build.
What pushes your rate up
If several of these apply to you, expect to sit in the upper half of the range rather than the middle:
- Poor access: no side gate means everything goes through the house, by hand
- Deep or reinforced foundations: trees, made ground, or building near a drain run
- Party wall work: awards and surveyor fees on terraced and semi-detached properties
- Long structural spans: big openings need big steels and sometimes padstones and props
- Conservation area or listed status: materials and detailing are dictated, not chosen
- Changing your mind mid-build: the most expensive variable of all
Worked examples for 2026
| Project | Area | Build cost | Realistic all-in budget |
|---|---|---|---|
| Side return, Midlands, standard spec | 9 m² | £23,000 – £28,000 | £30,000 – £40,000 |
| Single-storey rear, North West, standard | 20 m² | £42,000 – £52,000 | £55,000 – £75,000 |
| Single-storey rear, London, standard | 20 m² | £58,000 – £72,000 | £75,000 – £105,000 |
| Double-storey rear, South East, standard | 36 m² | £78,000 – £96,000 | £100,000 – £135,000 |
The gap between build cost and all-in budget is not padding - it is fees, fit-out, VAT on professional services and a contingency of 10 to 15 per cent. Projects that run into trouble are usually the ones that never made room for it.
How to compare quotes on rate
Divide each quote by the new internal floor area and you have a comparable rate. If one builder comes in far below the others, the usual explanation is not efficiency - it is a thinner specification, an excluded item, or a provisional sum waiting to grow. Ask each builder to price the same written specification, and ask what their figure assumes about foundation depth, because that is where the surprises live.
For idea-level inspiration before you commit to a footprint, see our insights guides; for whether your scheme needs consent, start with the planning section.
FAQs: extension cost per m² (UK, 2026)
What is the average cost per m2 for a house extension in the UK in 2026?
Around £2,200 to £2,700 per square metre for a standard-specification single-storey extension, with a full national range of roughly £1,800 to £3,200 per square metre depending on region, type and specification.
Is a double-storey extension cheaper per square metre?
Yes, typically £1,950 to £2,400 per square metre at standard spec, because the fixed costs of foundations, scaffolding and roof are spread over two floors. The total cost is still higher because you are building far more floor area.
Does the per square metre rate include a kitchen?
No. Build rates cover structure, weathertight envelope, first and second fix and basic finishes. Kitchens, bathrooms, premium glazing, landscaping and professional fees are all additional and commonly add 30 to 50 per cent to the build figure.
How much more expensive is an extension in London?
Commonly 20 to 35 per cent above the national mid-point, which puts a standard-specification London extension at roughly £2,900 to £3,600 per square metre in 2026. Labour rates and site access constraints drive most of that difference.
How much contingency should I allow?
Ten to fifteen per cent of the build cost for a straightforward extension, and closer to twenty per cent for older properties, sloping sites, or anywhere the drainage and foundations are unknown before work starts.
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