As a typical range, a single-storey extension in the West Midlands costs around £2,000–£2,900 per m² in 2026, including VAT. A 20 m² rear extension often comes to £40,000–£58,000 before a new kitchen. Two-storey extensions cost less per m² but more overall. Drawings, approvals and finishes can add several thousand pounds.
How much does an extension cost per square metre?
Builders and cost guides talk in pounds per square metre of internal floor area because it lets you compare quotes. National cost guides published in 2025 and 2026 put West Midlands prices a little below the UK average and well below London. These are typical ranges for a standard specification, including VAT, covering the building work to a plastered and decorated shell with basic electrics, heating and flooring:
- Single-storey rear or side extension: typically £2,000–£2,900 per m².
- Two-storey extension: typically £1,800–£2,600 per m² across both floors, because the foundations and roof are shared.
- Higher-specification work with large glazed openings, roof lanterns, vaulted ceilings or complex steelwork: often £2,900–£3,600 per m² or more.
What do typical extension projects cost?
Small extensions cost more per m² than large ones. Every job needs foundations, a roof, some steelwork and a building control process, and on a small job those fixed costs are spread over fewer square metres. These typical totals include VAT and cover the building work only:
- Small single-storey extension, 10–15 m², such as a utility room or a larger kitchen end: £28,000–£40,000.
- Single-storey rear kitchen-diner extension of around 18–25 m² on a semi: £38,000–£58,000.
- Large or wrap-around extension of 30 m² or more with bi-fold or sliding doors: £58,000–£90,000.
- Two-storey rear or side extension of around 15–20 m² per floor (30–40 m² in total): £68,000–£98,000.
- What these totals usually cover: foundations, walls, roof, windows and doors, insulation, plastering, basic electrics, a radiator or two and building control inspections. They usually leave out the kitchen, floor finishes beyond a screed, decorating, drawings and party wall costs. The gap between 'build cost' and 'all-in cost' is where most budgets go wrong, so check what your own quote covers.
What else do I need to budget for?
Most of the extras are fees, approvals and fit-out. Some quotes include them and some don't, so check line by line. A new kitchen in the extension is usually priced separately: depending on units, worktops and appliances, allow from around £8,000 to £25,000 or more.
- Drawings by an architect or architectural technician: often £2,000–£5,000 for a typical extension, more for complex designs.
- Structural engineer's calculations for steels and foundations: often £400–£1,500.
- Building control charges, which vary by council and project size. For a typical single-storey extension they are often in the region of £600–£1,200. Check your council's published charges.
- Planning fees in England from 1 April 2026: £548 for a householder application, £249 for larger home extension prior approval, or £274 for a lawful development certificate for proposed work.
- Party wall surveyors, if a neighbour doesn't consent: often £900–£1,800 for one agreed surveyor, more if each side appoints its own.
- A contingency of around 10% for things nobody can see until the ground is open.
Why do two similar extensions get very different prices?
Two 20 m² extensions on the same road can be £15,000 apart. These are the usual reasons:
- Ground conditions. Clay soil, nearby trees or made-up ground can mean deeper or specialist foundations.
- Drains. Building over or within 3 m of a public sewer needs the water company's agreement and sometimes a CCTV survey or diversion.
- Openings and steelwork. Taking out most of the back wall of the house needs steel beams, padstones and temporary support.
- Glazing. Large bifold or sliding doors and roof lanterns cost far more than a standard window and door.
- Roof type. A flat roof is usually cheaper than a pitched or hipped roof that ties into the house.
- Access. If materials and spoil have to go through the house or be moved by hand, labour time goes up.
- Finishes. Underfloor heating, porcelain floors and premium electrical fittings add up quickly.
Is VAT included in extension quotes?
Not always. Most extension work on an existing house is standard-rated at 20%, and some traders quote without it. A £50,000 quote plus VAT is a £60,000 bill. Citizens Advice lists 'whether VAT is included' as one of the basics every quote should state.
Ask each builder to price the same drawings and specification, in writing, with a breakdown of work and materials and a clear list of what is excluded. A fixed price for the agreed specification is worth more than a low estimate that can move.
How long does an extension take, and when do I pay?
For a typical single-storey extension, allow a few months for drawings and approvals, then roughly 8–14 weeks on site, depending on size, ground and weather. Two-storey extensions take longer. Lead times for glazing and kitchens often decide the finish date. Weather matters most before the roof is on: heavy rain or frost can hold up groundworks and brickwork, so a winter start may take longer than the same job in spring. Once the extension is watertight, the inside trades follow on quickly.
Payments are normally made in stages tied to progress, such as foundations, roof on and completion, rather than all at once. Citizens Advice recommends avoiding large up-front payments and keeping any deposit as low as possible.
How can I keep the cost of an extension down?
Every figure in this guide is a typical range drawn from published 2025–2026 UK cost guides and adjusted for the West Midlands. Your own price depends on your house, your ground and your specification, which is why a measured survey and a written quote matter. A few choices reliably keep the cost down:
- Keep the shape simple. A rectangle with a flat or lean-to roof is the cheapest form to build.
- Stay within permitted development if you can, to avoid a planning application and the time it takes.
- Decide on your kitchen, flooring and glazing before you get quotes, so prices aren't based on guesswork.
- Avoid changing the design once work has started. Changes are priced as variations.
- Get at least three written quotes for the same drawings and compare like with like.
Sources
- Citizens Advice – Before you get work done on your home
- GOV.UK – Planning fees: annual indexation from 1 April 2026
- Planning Portal – Extensions: planning permission
This guide is general information for homes in England, not legal or professional advice for your property. Rules change — check with your council or the Planning Portal before you rely on it.