Best extension builders in the UK: how to choose (2026)
There is no national league table of extension builders, and anyone who publishes one is usually ranking whoever paid them. The best extension builder for you is a local firm with the right accreditation, relevant recent work, a clear written quote and availability when you need them. This guide shows you how to identify that firm yourself - what to check, what to ask, and what should make you walk away.
Get 3 Free Extension Quotes
Tell us about your extension and we will match you with up to 3 vetted local builders. You compare; you decide.
- ✓ FMB, CIOB or TrustMark-registered builders
- ✓ Local to your postcode
- ✓ Free, no-obligation quotes
Why we do not publish a ranked list
Extension building is an intensely local trade. A firm that is outstanding in Leeds is irrelevant to you in Exeter, and a builder who is excellent on Victorian terraces may be the wrong choice for a double-storey on a 1970s estate. National "top ten builders" lists are almost always advertising in disguise, and star ratings collected without verification are trivially easy to game.
What genuinely helps is knowing the tests a good builder passes. Apply the checks below to three local firms and you will end up with a better shortlist than any published ranking could give you.
Accreditations that mean something for extensions
| Scheme | What it signals | How to verify |
|---|---|---|
| FMB (Federation of Master Builders) | Independently inspected and vetted building firm; members must meet quality and financial checks | Search the FMB's own member directory by company name and postcode |
| CIOB (Chartered Institute of Building) | Individual professional qualification in construction management; strong sign on larger or complex projects | Ask for the member's name and grade, then check the CIOB register |
| TrustMark | Government-endorsed quality scheme covering technical competence, trading practice and customer service | Search the TrustMark directory - registration is per business |
| NHBC or similar warranty provider | Relevant where structural warranty on new build elements is offered | Ask which warranty is offered and who underwrites it |
| Gas Safe / NICEIC | Applies to the sub-trades doing gas and electrical work, not the main contractor | Ask who is doing that work and check their individual registration |
Always verify at source
Logos on a website or van cost nothing to copy. Take the company name and registration number and check it on the scheme's own directory. Lapsed and borrowed memberships are one of the most common forms of misrepresentation in the trade.
The paperwork to insist on
- Public liability insurance - £2m minimum is common; £5m for larger schemes. Ask for the certificate, check the dates.
- Employers' liability insurance if they have employees - legally required.
- Contract works / all-risks cover - protects the partly built extension itself. Confirm whether their policy or your buildings insurer covers it.
- A written contract. For extensions, a JCT Homeowner contract or the FMB's equivalent is entirely normal and protects both sides.
- Company details - registered number, VAT number if they charge VAT, and a trading history you can look up at Companies House.
How to compare quotes like for like
The most useful thing you can do is write a one-page specification and give the identical document to every builder. Without it, you are comparing three different projects and the cheapest quote simply describes the least work.
- Divide by floor area. A comparable rate in 2026 is roughly £1,800–£3,200 per m² depending on region and spec.
- Hunt the provisional sums. Every "PS" or "allowance" is a number that can grow. Ask what happens if it does.
- Check the exclusions list - it is where the real differences hide: fees, glazing, kitchens, making good, skips, welfare, scaffolding.
- Ask what foundation depth is assumed and who carries the risk if the ground says otherwise.
- Confirm the programme in weeks, and what happens if it overruns.
An outlier low quote is rarely a bargain. Far more often it is a thinner specification, an omission, or a firm buying the job with the intention of recovering margin through variations. Our cost guides give the per-square-metre benchmarks to sanity-check any figure.
Questions worth asking before you sign
- Can I see two extensions you finished in the last twelve months, ideally similar to mine?
- Will you give me contact details for those clients - and may I visit?
- Who will actually be on site, and who is the single point of contact?
- How many other jobs will you be running at the same time as mine?
- Who deals with building control inspections and sign-off?
- What is the payment schedule, and what triggers each stage?
- What is your defects period after completion, and what does it cover?
The answer to the site-visit question tells you the most. A builder proud of their work is usually happy to show it; a builder who deflects is telling you something.
Red flags
Walk away if you see these
- A large deposit demanded up front. Staged payments against completed work are normal; a big cash advance is not.
- Pressure to decide today, or a "discount if you sign now".
- Cash-only, no VAT receipt, no written quote.
- No fixed address, no company registration, no verifiable insurance.
- Reluctance to use a written contract.
- An offer to "handle building control informally" or skip it altogether.
Payment structure that protects you
For a typical extension, payments in stages tied to completed milestones - foundations complete, structure to damp course, roof watertight, first fix, second fix, practical completion - keep the incentives aligned. Hold back a retention of around 2.5 to 5 per cent for the defects period, and never pay ahead of work that has actually been done. Pay by bank transfer with a record, and treat any request to change bank details mid-project as a fraud attempt until you have confirmed it by phoning the number you already had.
Getting to a shortlist quickly
Three quotes is the sweet spot. One gives you no comparison; five wastes everyone's time and builders notice when they are one of a crowd. Give each firm the same specification, the same drawings if you have them, and the same deadline. Then judge on the quality of the quote as much as the number at the bottom - a builder who prices carefully usually builds carefully too.
If you would rather not chase firms yourself, we will pass your project to up to three vetted local builders and let them come to you.
FAQs: choosing an extension builder (UK, 2026)
What accreditation should an extension builder have?
FMB membership, TrustMark registration, or CIOB qualification for the individual running the project are the most meaningful in UK extension work. Gas Safe and NICEIC apply to the gas and electrical sub-trades rather than the main contractor. Always verify registration on the scheme's own directory rather than trusting a logo.
How many quotes should I get for an extension?
Three, all priced against the same written specification. Fewer gives you no basis for comparison, and more tends to reduce the effort each builder puts into pricing. Compare the exclusions and provisional sums, not just the headline total.
Why is one extension quote much cheaper than the others?
Usually because it covers less work. Check the exclusions list, the provisional sums, the assumed foundation depth and whether fees, scaffolding and making good are included. A quote that is 30 per cent below the others is normally an incomplete specification rather than a bargain.
How much deposit should I pay a builder?
Little or none up front. Staged payments against completed milestones are the norm, sometimes with a modest advance for materials ordered specifically for your job. A demand for a large percentage before work starts is a serious warning sign.
Do I need a written contract for a house extension?
Yes. A JCT Homeowner contract or the FMB equivalent sets out the price, programme, payment stages, variations process and defects period. It protects both parties and costs nothing to use. A builder who resists one is not a builder you want.
Let three vetted builders come to you
Describe your extension once and we will pass it to up to three accredited local builders. Free, no obligation, and you stay in control of who you speak to.