In short: a single-storey extension in the UK typically costs £1,500–£2,500 per square metre at standard specification in 2026 — about £30,000–£55,000 for a common 20m² extension. London and the South East run 20–40% higher (roughly £2,500–£3,800 per m²). These figures usually exclude VAT, professional fees and interior fit-out.
Extension prices vary a lot depending on size, specification and where you live, so treat any single number with caution. Below are current 2026 UK ranges, what is and is not included, why costs have climbed, how to keep the price down, and the lead times to plan around.
Cost by size (2026, standard build)
| Extension size | Standard spec (£1,500–£2,500/m²) | Higher spec / South East (£2,500–£3,800/m²) |
|---|---|---|
| 15 m² | £22,500–£37,500 | £37,500–£57,000 |
| 20 m² | £30,000–£50,000 | £50,000–£76,000 |
| 30 m² | £45,000–£75,000 | £75,000–£114,000 |
These are build-cost ranges before VAT and fees. A smaller extension costs more per square metre, not less, because the fixed costs — foundations, a roof, scaffolding and tying into the house — are spread over fewer square metres.
How much does a 20m² extension cost?
A 20m² single-storey extension — a common size for a kitchen-diner — typically runs £30,000–£50,000 at standard specification, or £50,000–£76,000 for a higher specification or in the South East. That covers the build; budget separately for VAT, fees and the kitchen or fit-out.
What is included — and what is extra
A typical per-square-metre build figure covers foundations, the brick and block shell, roof, standard windows and external doors, and basic plastering. Budget separately for:
- VAT — normally 20% on a domestic extension.
- Professional fees — architect or designer, structural engineer and building control, commonly 10–15% of the build cost.
- Planning fee — a householder planning application in England is £528 from April 2026 (you may not need one).
- Party wall surveyor — if a shared wall is affected.
- Fit-out — kitchen, bathroom, flooring, decorating and any knock-through into the existing house.
- Upgrades — bifold or sliding doors, roof lanterns and premium finishes add up quickly.
What is the cheapest way to extend?
You cannot make an extension cheap, but you can avoid paying more than you need to:
- Keep the shape simple — a square or rectangle is cheaper than lots of corners and roof valleys.
- Stick to a standard specification and upgrade finishes later.
- Avoid moving the kitchen, bathroom or soil pipes if you can.
- Stay within permitted development so you skip a planning application.
- Get the drawings and structural design right before you start — changes mid-build are the most expensive kind.
Single-storey or two-storey — which is better value?
If you need more space, a two-storey extension is often better value per square metre, because the extra floor shares the same foundations and roof. The total bill is higher, but the cost of each square metre is usually lower than doing two separate single-storey projects.
Why extension costs have risen
If quotes look higher than you expected, it is not just your builder. UK building materials have climbed steadily: in the year to early 2026, aggregates were up around 8%, structural steel around 8% and imported timber around 7–8%. The main drivers are high energy costs at manufacturing plants, import tariffs (steel especially), lingering supply-chain disruption, and rising labour costs from higher minimum wage and National Insurance. Most are structural rather than temporary, so prices are not expected to fall back quickly. Building in a contingency of around 10–15% is sensible.
Lead times to plan around
Extensions take longer than most people expect — budget time as well as money:
- Design and drawings: 4–8 weeks.
- Planning decision: around 8 weeks from validation (16–20+ in conservation areas or on listed buildings).
- Building regulations: 3–6 weeks.
- Booking a builder and ordering materials: 4–8 weeks — good builders are booked months ahead, and bespoke items like bifold doors can take 8–12 weeks.
- Construction: 8–16 weeks for a single-storey extension.
From first idea to finished room, a single-storey extension usually takes 6–9 months. Our guide on the order to do things and who to contact first walks through it.
Where the brickwork sits in the cost
The bricklaying is labour-only — you or your main contractor buy the bricks, blocks, sand and cement, and we lay them. See what a bricklayer charges for an extension for how that part is priced.
Figures are 2026 UK market guides drawn from published cost data and our own experience; treat them as ballpark. Your real price depends on your site, spec and where you are.
Frequently asked questions
How much does a single-storey extension cost in the UK?
In 2026 a single-storey extension typically costs £1,500 to £2,500 per square metre at standard specification. For a common 20m2 extension that is roughly £30,000 to £55,000. London and the South East run 20 to 40 percent higher. These figures usually exclude VAT, professional fees and interior fit-out.
How much does a 20m2 extension cost?
A 20 square metre single-storey extension typically costs about £30,000 to £50,000 at standard specification in 2026, or £50,000 to £76,000 for higher specification or in the South East. That is before VAT, fees and fit-out such as a kitchen.
Is a small extension cheaper per square metre?
No. Smaller extensions cost more per square metre, not less, because fixed costs like foundations, a roof, scaffolding and connecting to the house are spread over fewer square metres. A 10m2 extension can cost noticeably more per m2 than a 30m2 one.
What is the cheapest way to extend a house?
Keep the shape simple (a square or rectangle), stick to a standard specification, avoid moving the kitchen or bathroom services, and use permitted development so you skip a planning application where possible. A simple single-storey rear extension is usually the best value per square metre.
Do builders charge VAT on an extension?
Usually yes. Most extension quotes are shown before VAT, and you normally pay 20 percent VAT on labour and materials for a domestic extension. Always check whether a quote includes VAT before comparing prices.
Is a two-storey extension better value than single-storey?
Often, yes, per square metre. A two-storey extension roughly doubles the floor area for a much smaller increase in cost, because you share the same foundations and roof. If you need the space, it can be the better value option even though the total bill is higher.
Why have extension costs gone up?
Building materials have risen sharply — aggregates, timber and structural steel were all up 7 to 8 percent in the year to early 2026 — driven by energy costs, import tariffs and supply-chain pressure. Labour costs have risen too with higher minimum wage and National Insurance. That feeds straight into extension prices.
Getting a price for your extension
Every extension is different, so the brickwork is priced after we have seen the job. If you are planning an extension in Hampshire, Berkshire or Surrey, see our extension work or send us the details and we will come and look.