If you're pricing an extension for a client — or budgeting one for yourself — the headline number you'll see everywhere is £1,800 to £3,000 per m². That's broadly correct for a standard single-storey rear extension in most of the UK. But it doesn't tell you much about what's included, what drives that figure up or down, or what your specific project will actually cost.
This guide breaks it down properly. We've used data from real BOQs we've produced, current trade rates, and material pricing from UK suppliers — not just rehashed averages from aggregator sites.
The Quick Answer: Extension Costs per m² in 2026
| Extension Type | Cost per m² (excl. VAT) |
|---|---|
| Single-storey rear | £2,200 – £3,200 |
| Double-storey | £2,000 – £2,800 |
| Side return | £2,500 – £3,500 |
| Wraparound | £2,400 – £3,300 |
| Kitchen extension (with fit-out) | £2,800 – £4,000 |
| Garage conversion | £1,200 – £1,800 |
What Actually Drives the Cost?
1. Foundations
This is the first big unknown on any extension. Standard strip foundations (600mm × 250mm) are straightforward and affordable. But if you're near trees, on clay soil, or building near a drain, you might need deeper foundations, piling, or engineered solutions — and that can add £5,000 to £15,000 to your build before you're even above ground.
2. Structural Steel
Opening up an existing wall into the extension almost always requires an RSJ (rolled steel joist) or similar steel beam. A single steel beam, supplied and fitted, typically costs £1,200 to £3,500 depending on span and loading. Larger openings or load-bearing wall removals push this higher.
3. Glazing
Bifold doors, skylights, and floor-to-ceiling glass look fantastic but account for a disproportionate share of the budget. A set of 4m aluminium bifolds will cost £4,000 to £8,000 installed. Rooflights add £800 to £2,500 each depending on size and whether they open.
4. Roof Type
A flat roof (typically EPDM or GRP) is the cheapest option for single-storey. A pitched roof with tiles adds 15–25% to the superstructure cost but gives better aesthetics and longevity. A lantern roof or glazed roof is a premium choice at £3,000 to £8,000 on top.
5. Your Location
Labour rates vary significantly across the UK. London and the South East commands a 20–40% premium over the national average. Scotland and the North of England are generally at the lower end of the scale.
Regional Cost Breakdown
| Region | Single-Storey per m² |
|---|---|
| London & South East | £2,800 – £4,500 |
| South West | £2,300 – £3,200 |
| Midlands | £2,100 – £2,900 |
| North West | £2,000 – £2,800 |
| North East | £1,900 – £2,600 |
| Scotland | £2,000 – £2,800 |
| Wales | £1,900 – £2,700 |
| Northern Ireland | £1,800 – £2,500 |
The Costs Most People Forget
The build cost per m² only tells part of the story. Here are the extras that consistently catch people out:
Professional fees — architect, structural engineer, and building control. Budget 10–15% of build cost. On a £60,000 extension, that's £6,000 to £9,000.
Party wall agreements — if you're building on or near a boundary. Typically £1,000 to £2,500 depending on complexity and whether your neighbour appoints their own surveyor.
Planning permission — many extensions fall under Permitted Development, but if you need full planning, the application fee is £206. Your architect will charge £500 to £1,500 to prepare and submit it.
Building regulations — separate from planning. Application fee around £500 to £900 depending on your local authority.
Kitchen or bathroom fit-out — a mid-range kitchen installation adds £10,000 to £25,000 on top of the build cost. Bathrooms add £5,000 to £12,000.
Drainage — if your extension sits over or near existing drains, you may need to divert them. Build-over agreements with the water company are straightforward but diversions cost £2,000 to £6,000.
Landscaping — your garden will take a beating. Allow £2,000 to £5,000 to put it right afterwards.
Real-World Example: 30m² Single-Storey Rear Extension
| Item | Cost (excl. VAT) |
|---|---|
| Foundations & groundworks | £8,500 |
| Superstructure (walls, steel, roof) | £24,000 |
| Windows & bifold doors | £7,500 |
| Electrics | £3,200 |
| Plumbing & heating | £2,800 |
| Plastering & decoration | £4,500 |
| Flooring | £2,400 |
| Preliminaries & waste | £3,600 |
| Build total | £56,500 |
| Professional fees (12%) | £6,780 |
| VAT @ 20% | £12,656 |
| Total all-in | £75,936 |
That works out at roughly £2,530 per m² all-in including VAT and fees — comfortably within the expected range for a mid-spec extension outside London.
How to Get an Accurate Price for Your Extension
Online cost guides (including this one) give you a ballpark. But every extension is different — your site conditions, your spec choices, your location all shift the numbers. The only way to get an accurate figure is with a proper Bill of Quantities (BOQ) based on your actual drawings.
A BOQ measures every element of the build, prices it with current market rates, and gives you a line-by-line breakdown you can use to get competitive quotes from builders. It's the difference between "probably around sixty grand" and knowing exactly where every pound goes.
Get a Professional BOQ for Your Extension
Upload your drawings and get a fully itemised Bill of Quantities with current UK market rates — typically within 2 hours.
Get Your BOQ — From £99 →Is Extending Worth It?
In most cases, yes. Industry data consistently shows that a well-designed extension adds 10–20% to property value, typically returning £1.20 to £1.50 for every pound spent. Kitchen-diners and extra bedrooms with en-suites deliver the strongest return. That said, there's always a ceiling price for your postcode — over-specifying an extension in a lower-value area won't get your money back.
Compared to moving, an extension almost always works out cheaper when you factor in stamp duty (£7,500 on a £300,000 purchase), estate agent fees, solicitors, removals, and the general chaos of moving house. If you like where you live, extending is usually the smarter financial move.