How Much Does an Extension Cost Per Square Metre in the UK?

The ‘cost per square metre’ is the estate agent’s favourite statistic and the homeowner’s most misleading one. It suggests extensions are commodities — but a simple single-storey rear extension and a double-storey side extension with structural gymnastics are different species, and the per-metre rate swings wildly between them.

Here’s how much an extension costs per square metre in the UK honestly: the real ranges by type, what the rate includes (and doesn’t), and why your project will land where it lands.

How much does an extension cost per square metre: the numbers

Single-storey rear extension: £2,000–3,000/m² outside London, £2,800–4,000/m² in London and the South East. Double-storey extension: £2,500–3,500/m² (the foundations and roof are shared across two floors, so per-metre it’s better value). Side return / wraparound: £2,800–4,000/m² — the fiddly geometry and structural work push the rate up. Garage conversion (not technically an extension, but the comparison everyone makes): £1,000–1,500/m².

So a 20m² single-storey rear extension costs £40,000–60,000 outside London, £56,000–80,000 in London. These are 2026 build costs including VAT but excluding the fit-out variables below — treat any quote dramatically below these ranges with deep suspicion. If it sounds too cheap for building work, it’s either missing something or someone’s cutting something.

• Single-storey: £2k–£3k/m² (£2.8k–£4k London) | Double-storey: £2.5k–£3.5k/m².
• Side return/wraparound: £2.8k–£4k/m² | Garage conversion: £1k–£1.5k/m².
• 20m² rear extension: £40k–£60k (regional), £56k–£80k (London).

What the per-metre rate includes (and what it hides)

The headline rate typically covers: foundations, structure, roof, windows and doors, basic electrics and plumbing, plastering and a standard finish. It usually excludes: the kitchen or bathroom fit-out (£8,000–20,000 — see our guide to kitchen costs), premium glazing (a 4m sliding door wall can be £8,000–15,000 alone), underfloor heating, landscaping the wrecked garden, and professional fees.

Professional fees are the forgotten 10–15%: architect (£3,000–8,000 for a typical extension), structural engineer (£1,000–2,500), building control (£800–1,500), party wall surveyor(s) (£700–1,500 per neighbour if needed), and possibly planning application fees. Budget the fees separately from the build cost or they’ll ambush you. And always keep 10–15% contingency — groundworks surprise everyone, because nobody knows what’s under your garden until they dig.

• Headline rate excludes: kitchen/bathroom fit-out, premium glazing, fees.
• Professional fees run 10–15% on top: architect, engineer, building control.
• Keep 10–15% contingency — groundworks always surprise.

What drives your project up or down the range

Ground conditions dominate: standard strip foundations are cheap; piled foundations (needed on clay, near trees, or on made-up ground) add £5,000–15,000. Drainage: if a sewer runs under your extension, you’ll need a build-over agreement and possibly diverted drains (£2,000–8,000). Access: terraced houses with no side access pay more — everything goes through the house by hand.

Specification choices swing the total enormously: standard uPVC windows versus aluminium bifolds (£5,000–12,000 difference on a typical rear), basic bathroom versus luxury wet room, standard radiators versus underfloor heating throughout. Decide these before getting quotes — ‘builder’s choice’ specifications are where budgets quietly double. And complexity: every corner, level change and structural beam adds cost. The cheapest extension per metre is a simple rectangle; every architectural flourish has a price.

• Foundations: piling adds £5k–£15k; sewers underneath add £2k–£8k.
• Glazing spec is the biggest variable — bifolds vs uPVC swings £5k–£12k.
• Simple rectangles are cheapest; every corner and beam costs.

Extension cost per square metre: planning and neighbours

Many rear extensions fall under permitted development (up to 3m deep for terraces, 4m for detached, or 6m/8m under the larger home extension scheme with neighbour consultation) — but PD doesn’t remove building regs, party wall obligations or the need for good design. In conservation areas or for larger schemes, full planning permission takes 8–13 weeks and costs £500+ in fees.

Party walls are the London terrace special: if you’re building on or near the boundary, you must serve notice on neighbours 2 months before work. Most neighbours consent; some don’t, and then you need surveyors (£700–1,500 per side). Start this process early — it’s the most common cause of extension delays. And talk to neighbours before the formal notice lands: a cup of tea and the plans does more than any legal letter. Compare with the alternative of going up instead: our guide to loft conversion costs covers the vertical option.

• Check permitted development limits; building regs always apply regardless.
• Party wall notices: serve 2 months early; budget £700–£1,500 per neighbour.
• Talk to neighbours with tea and plans before the formal notice.

Will it add value? The honest maths

An extension typically adds 10–20% to a property’s value — but only if it’s well designed and appropriate for the area. The critical concept is the ceiling price: the maximum any house on your street will sell for. If neighbouring 4-beds sell at £500,000 and your 3-bed is worth £400,000, spending £80,000 to create a 4-bed makes sense. If 4-beds on the street sell at £450,000, you’ve spent £80,000 to add £50,000.

Check sold prices (not asking prices) for extended houses on your street and adjacent streets. And consider the alternative uses of the money: in some markets, moving to the bigger house costs less than building it, once stamp duty and fees are weighed. Extensions make most sense when you love the location and the street supports the value — building the biggest house on the street is how money gets buried in bricks.

• Adds 10–20% typically — but only up to the street’s ceiling price.
• Check SOLD prices of extended neighbours, not asking prices.
• Sometimes moving is cheaper than building — do the full comparison.

An extension costs £2,000–3,000 per square metre outside London and £2,800–4,000 in it — plus 10–15% in fees, 10–15% contingency, and the fit-out on top. The projects that go well share three traits: honest budgeting from the start, professional designs before builder quotes, and neighbours who were consulted early. Get those right and the per-metre rate is just arithmetic; get them wrong and it’s the most expensive lesson in homeownership. For the definitive rules, GOV.UK’s planning permission guidance is the official source.