How much is a 4m x 5m extension?

Around £36,000–£50,000 for the build at a standard specification (£1,800–£2,500 per square metre), before VAT, professional fees and any kitchen or bathroom fit-out. With those included, a realistic all-in budget is £50,000–£65,000 — and more in London and the South East.

Does the price per square metre include VAT?

Almost never. Published extension costs are quoted excluding VAT, and most extension-sized firms are VAT-registered, so add 20% to the build cost. A very small builder trading under the £90,000 VAT threshold doesn't charge it — but check rather than assume.

Does an extension add more value than it costs?

Often not immediately. Widely-cited Nationwide research puts the uplift at roughly 5% of house value for every 10% of floor space added — which at 2026 build costs frequently comes in below what you paid. Extensions reliably pay their way as space you actually use, and only dependably turn a profit where house prices per square metre are highest, such as London and the South East.

How long does a single-storey extension take?

Typically 8–12 weeks on site for the build itself, stretching to 3–4 months where there's a kitchen and a knock-through into the existing house. Door to door — first design conversation, planning, tendering, then the build — allow closer to a year.

How much deposit should a builder ask for?

Consumer guidance puts a fair deposit at 5–10%, with up to 15–20% sometimes seen on smaller jobs. Anything over 25% up front is a walk-away signal, and a demand for half the money before work starts is the biggest red flag in domestic building. The rest should be staged against completed, inspectable milestones — never dates.

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From the blog

How much does a single-storey extension cost? Honest 2026 numbers

13 August 2026 · Robert McLaggan

A single-storey extension in the UK typically costs £1,800–£2,500 per square metre at a standard specification, rising to £2,500–£3,000 for a higher finish and £3,000–£4,000+ for premium, glass-heavy designs — all before VAT. A 4m × 5m (20m²) extension therefore lands around £36,000–£50,000 for the build itself; a 6m × 5m (30m²) around £55,000–£75,000, and more in London, which runs roughly 20–25% above the national range. The per-metre figure excludes the things that hurt most: VAT at 20%, professional fees (£2,000–£7,000+), and the kitchen, which can add £10,000–£22,000 on its own. Expect 8–12 weeks on site and closer to a year door-to-door including design and planning.

Extension prices get quoted in pounds per square metre, and that single number hides most of what you'll actually spend. Here are the honest 2026 figures — the per-metre bands, what they quietly exclude, and how the money should change hands.

The going rates

Across the major UK price guides, a single-storey extension in 2026 costs:

Standard specification: £1,800–£2,500 per m²
Good specification: £2,500–£3,000 per m²
Premium / glass-heavy / architect-led: £3,000–£4,000+ per m²

All of those are before VAT — more on that below.

Put against real sizes:

4m × 5m (20m²): roughly £36,000–£50,000 for the build
6m × 5m (30m²): roughly £55,000–£75,000, with well-specified projects in the guides reaching £85,000+

London and the South East run 20–25% above these ranges — a 30m² extension quoted at £90,000 in London is not an outlier. Elsewhere in the UK, including Scotland, prices tend to sit inside or below the national band, though no major guide publishes a reliable Scotland-specific figure.

The guides themselves disagree more than you'd expect — one respected source puts its floor at £2,200/m² while another's budget band starts at £1,250/m². Most of that gap is what each figure includes, which is the real lesson of this guide.

What the per-metre price doesn't include

Four things sit outside almost every £/m² figure, and together they can add half again to the number you first wrote down:

VAT. Published figures are almost universally ex-VAT, and most firms doing extension-sized work are VAT-registered — so add 20%. (A sole trader under the £90,000 turnover threshold legitimately doesn't charge it; check rather than assume.)
The kitchen. If the extension is for a kitchen — and most are — the fit-out is priced separately. A mid-range kitchen alone can run £10,000–£22,000 on top of the shell.
Professional fees. From around £2,000 for structural calculations and drawings at the modest end, to £7,000+ for a full architect service on a larger project (typically 10–12% of build cost). A structural engineer is £1,500–£2,500 of that.
Statutory costs. A householder planning application in England is £528 plus a portal fee. Building regulations sign-off runs £600–£1,200. In Scotland the system is different: you need a building warrant before work starts, with fees scaled to the value of the works — and warrant fees rose again in April 2026.

There's also the party wall. If you're building on or near the boundary in England or Wales, the Party Wall Act applies: it costs nothing if your neighbour consents to the notice, but a disputed notice means appointed surveyors, and that can run £1,000+ per neighbour.

What actually drives the price

Within the bands, five things separate a £1,800/m² job from a £3,500/m² one:

The ground. Foundations are £200–£350/m² in normal conditions — and unknowable until dug. Poor ground, drains to move, or a tree with a preservation order nearby can add thousands before anything shows above the surface.
The opening into the house. The steel for a knock-through and the structural work around it is commonly £5,000–£6,000, and it's usually excluded from the per-metre figure.
Glass. Bifolds, sliders and rooflights are the fastest way up the scale — a good aluminium bifold set is £2,000–£3,500 supplied before fitting, and heavily-glazed designs push the whole project toward the premium band.
Water in the new space. A kitchen or bathroom in the extension brings plumbing, drainage and a heating extension with it — £5,000+ of trades beyond the shell.
The roof. A simple warm flat roof is the budget option; a pitched, tiled roof or a roof lantern moves the number up.

How long it takes

Plan on 8–12 weeks on site for the build, stretching to 3–4 months where there's a kitchen and internal alterations. Door to door — design, planning (about 8 weeks for a decision), tendering, then building — a year is realistic. Anyone promising the whole journey in a season is skipping a stage you'll miss later.

Will it add more value than it costs?

Honestly: often not, at least not immediately. The widely-cited Nationwide research puts the uplift at about 5% of house value for every 10% of floor space added. On a £300,000 house, a 20m² extension might add £20,000–£30,000 of value against a £50,000+ all-in cost. The maths only reliably turns profitable where house prices per square metre outrun build costs — London and the South East. Everywhere else, an extension is best justified the honest way: as space you'll live in, not an investment.

How the money should change hands

This is where extensions go wrong more often than in the brickwork. The consumer guidance is consistent:

Deposit: 5–10% is fair. Up to 15–20% appears on smaller jobs. Over 25% is a walk-away signal, and a builder wanting half the money up front is the single biggest red flag in domestic building. If materials genuinely need pre-funding, pay the supplier directly.
Stage payments tied to milestones, not dates. A typical structure: deposit, then payments at foundations passing inspection, walls up, weathertight, first and second fix complete, and completion — each released when the stage is inspectably done.
Retention. It's normal to hold 2.5–5% for 6–12 months after completion against snags.
Paperwork. A written contract (the JCT Homeowner or FMB domestic contract are the standards), itemised quotes, and proper invoices. Cash-only deals lose you the paper trail and any card-payment protection — and a builder who works properly will want the paperwork too, because staged payments and deposits protect them as much as you.

A builder who proposes this structure unprompted is telling you something about how the rest of the job will run. So is one who doesn't.

Common questions

How much is a 4m x 5m extension?: Around £36,000–£50,000 for the build at a standard specification (£1,800–£2,500 per square metre), before VAT, professional fees and any kitchen or bathroom fit-out. With those included, a realistic all-in budget is £50,000–£65,000 — and more in London and the South East.
Does the price per square metre include VAT?: Almost never. Published extension costs are quoted excluding VAT, and most extension-sized firms are VAT-registered, so add 20% to the build cost. A very small builder trading under the £90,000 VAT threshold doesn't charge it — but check rather than assume.
Does an extension add more value than it costs?: Often not immediately. Widely-cited Nationwide research puts the uplift at roughly 5% of house value for every 10% of floor space added — which at 2026 build costs frequently comes in below what you paid. Extensions reliably pay their way as space you actually use, and only dependably turn a profit where house prices per square metre are highest, such as London and the South East.
How long does a single-storey extension take?: Typically 8–12 weeks on site for the build itself, stretching to 3–4 months where there's a kitchen and a knock-through into the existing house. Door to door — first design conversation, planning, tendering, then the build — allow closer to a year.
How much deposit should a builder ask for?: Consumer guidance puts a fair deposit at 5–10%, with up to 15–20% sometimes seen on smaller jobs. Anything over 25% up front is a walk-away signal, and a demand for half the money before work starts is the biggest red flag in domestic building. The rest should be staged against completed, inspectable milestones — never dates.

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