Extension builders near me: how to choose one in 2026 (UK)
There is no official national league table of “best extension builders”, and any site claiming one is usually just ranking whoever paid the most. What actually protects you on a project worth tens of thousands of pounds is a short list of checks: is the builder registered with a recognised scheme, do they carry the right insurance, will they give you a fixed written quote with a payment schedule, and can they prove they have handled party wall and structural warranty paperwork before. Get those right and the specific firm matters far less than the process. This guide shows you exactly what to look for and what a fair 2026 price looks like.
- Check registration: FMB (Federation of Master Builders) and TrustMark, plus a structural warranty provider
- Typical 2026 rate: £2,200–£3,200 per m² for a standard single-storey extension
- Always compare: three written quotes, like for like
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Why we do not publish a “top 10 extension builders” list
A house extension is a bespoke build. The firm that produced a flawless rear kitchen extension in one town might be the wrong choice for a two-storey side return with tricky foundations three streets away. A single national ranking cannot capture that, and it cannot see whether a builder is quietly overstretched this quarter or has just lost a key site manager. Rankings also go stale the moment they are published, while the checks below stay true.
So we teach the method instead of selling you a list. Verify the credentials, compare three genuine quotes, and read how each builder behaves during the quoting stage — that process travels with you to any town and any type of extension.
The accreditations that actually matter
General building is not a licensed trade in the UK, so anyone can legally call themselves an extension builder. That makes voluntary schemes your main quality filter. None of them guarantee a perfect job, but membership means the firm has been vetted, carries insurance, and can be escalated to if things go wrong.
Verify the card, not the logo
A logo on a van or website proves nothing. Ask for the firm's FMB or TrustMark membership number and check it on the scheme's own online register. Confirm the company name on the register matches the name on your quote and contract — not a similar-sounding trading name.
What extension builders charge in 2026
Extensions are usually priced per square metre of new floor area, then adjusted for spec, access and groundworks. Below are realistic 2026 build costs excluding VAT, professional fees and fit-out extras like high-end kitchens.
For a full breakdown see our cost guides.
How to compare quotes without comparing apples and oranges
The cheapest extension quote is rarely the cheapest extension. Differences usually hide in the spec — foundation depth, glazing quality, whether making good and decoration are included. Put every builder on the same footing before you compare a single number.
- Insist on it in writing. A verbal figure is an opinion, not a quote
- Check labour and materials are split out. You want to see where the money goes
- Ask whether it is a quote or an estimate. A quote is a fixed price; an estimate can drift
- Confirm VAT. Is it included, and is the firm VAT-registered at all?
- Match the spec. Same foundation type, same roof, same glazing, same finish — or you are not comparing like for like
Questions worth asking before you book
About the business
Are you FMB or TrustMark registered, and what is your membership number? Who provides your structural warranty? Can I see two recent extensions and speak to those owners? What insurance do you carry, and are you VAT-registered?
About the job
Have you dealt with a party wall agreement before? Will you handle Building Control sign-off? What is your payment schedule tied to — stages, not dates? What is your realistic start date and duration, and who is my day-to-day site contact?
Red flags to walk away from
- Cash only, no paperwork. No contract, no VAT, no comeback if it goes wrong
- Large deposit up front. Stage payments tied to completed work are normal; a big cash lump before anything starts is not
- Pressure to decide today. A genuine builder’s quote is not a one-day flash sale
- Vague about party wall or structural sign-off. On extensions this paperwork is not optional, and dodging it is a warning sign
Party wall experience is the check most people skip
If your extension is built up to or near a shared boundary, or you are cutting into a party wall to insert a steel beam, the Party Wall etc. Act 1996 almost certainly applies. That means serving formal notice on your neighbours and, if they dissent, appointing a surveyor. A builder who has done this many times will steer you through it early; one who waves it away can leave you exposed to a legal injunction that halts the build.
Ask directly how many party wall situations they have handled and what happened. Local independent firms often know the specific quirks of your area’s housing stock — Victorian terraces, 1930s semis, new-build estates — which matters more than the size of the company. A big national contractor is not automatically safer; a well-reviewed local builder with a track record on your street type is often the better bet.
FAQs: finding a good extension builder in the UK (2026)
How much does a house extension cost in 2026?
Most single-storey extensions cost £2,200–£2,700 per m² to build in the regions, rising to £2,800–£3,600 per m² in London and the South East. A typical 20m² rear extension therefore lands around £44,000–£72,000 depending on location and spec, before VAT, fees and fit-out.
What accreditation should an extension builder have?
Look for FMB (Federation of Master Builders) and TrustMark membership, plus a 10-year structural warranty on the finished build. The structural design itself should come from a chartered structural engineer. Always verify membership numbers on the scheme’s official register rather than trusting a logo.
Do I need a party wall agreement for an extension?
Very often, yes. If you build up to a shared boundary or cut into a party wall for a beam, the Party Wall etc. Act 1996 requires you to serve formal notice on affected neighbours. Choose a builder who has handled this before, and allow several weeks for the process.
Should I use a big national firm or a local builder?
Neither is automatically safer. A well-reviewed local independent who knows your area’s housing stock often outperforms a large contractor. Judge each on accreditation, written references and how clearly they quote — not on company size.
How much deposit is normal for an extension?
Payment should be tied to completed stages — for example foundations, superstructure, roof, first fix, completion — not paid as one big lump up front. A modest deposit to secure a start date can be reasonable, but be wary of any builder demanding a large cash payment before work begins.
How do I compare extension quotes fairly?
Make sure every builder is pricing the same specification: same foundation depth, glazing, roof type and level of finish. Get everything in writing, check labour and materials are itemised, and confirm whether VAT is included. Only then does the headline figure mean anything.
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