The short answer: Vetting a builder properly means checking three things before you sign anything: who they really are, what they've actually built, and how they handle your money. Knowing how to vet a builder is what separates a smooth extension from a half-finished shell and a bank balance you can't get back. For a big job — an extension, a loft, a full rewire, a structural opening — the cost of getting this wrong in 2026 runs from £5,000 to well over £50,000, so an afternoon of checks is the best-value time you'll spend on the whole project.
This guide is for the jobs that matter: the ones with scaffolding, building control sign-off and a five-figure quote. A £300 tap change doesn't need this. A £45,000 rear extension absolutely does. Work through the checks below in order, and don't pay a deposit until every one is green.
Start with identity, not the quote
The mistake almost everyone makes is leading with price. You get three numbers, pick the friendly one in the middle, and only later ask who you actually hired. Reverse it. Before a builder sets foot in your home, confirm they are a real, traceable business.
Ask for the full registered company name and number, then check it free on Companies House. You're looking for how long they've traded, whether accounts are filed on time, and whether the company has been dissolved and reborn under a new name — a classic way to shed bad reviews and unpaid debts. A sole trader won't have a company number, and that's fine, but you still want a real name, a fixed address (not just a mobile and a Gmail), and a VAT number if they're turning over more than £90,000.
Then check trade-body membership against the body's own register, not the logo on the van. TrustMark is the government-endorsed scheme and lists vetted firms. For specific trades, verify the badge at source: Gas Safe Register for any gas work, NICEIC or NAPIT for electrics, FENSA or CERTASS for windows. A logo on a website is worth nothing until you've found the firm on the official list.
Insurance and qualifications: the paperwork that protects you
A big job introduces real risk — to your home, your neighbours' homes, and the people working on site. The paperwork below isn't bureaucracy; it's the only thing standing between you and a five-figure bill if something goes wrong.
The three documents to see before work starts
• Public liability insurance — ask for the certificate and check the cover level (£2m–£5m is normal for a domestic build) and that it's in date. This pays out if the build damages your property or injures someone.
• Employer's liability insurance — legally required if they have employees or labour-only subcontractors on site.
• A structural warranty or insurance-backed guarantee for the finished work — so a defect that appears in year three is still covered if the builder has stopped trading.
For anything structural — removing a wall, a loft conversion, an extension — you also need building regulations approval, signed off by your local authority building control or an approved inspector. Ask the builder directly: who is submitting the building control application, and will they hand you the completion certificate at the end? That certificate is the document your conveyancer and your buyer will ask for when you sell. No certificate, no proof the work was legal.
Here's the line I'll hold to: if a builder hesitates, bristles or gets vague when you ask to see their insurance certificate and a building control plan, the interview is over. It doesn't matter how good their Instagram looks or how warm the recommendation was. A professional who does five-figure jobs has this paperwork ready and is glad you asked — it's the awkward ones who don't.
Check the work, not just the words
Reviews and recommendations are a starting point, not proof. In 2026 a five-star profile can be bought, seeded or quietly cleansed of complaints, so treat any review you can't trace to a verified, completed job with caution. The reviews that count are the ones tied to a real transaction on a platform that verifies it.
Ask to see two finished jobs and one in progress
Anyone can photograph a finished kitchen. Far more telling is a live site: is it tidy, are materials stored properly, is there a visible plan? Ask for the contact details of two clients whose jobs completed at least a year ago — long enough for any settlement cracks, damp or snagging to show — and actually phone them. Ask the questions that matter: did the final bill match the quote, did they turn up consistently, and how did they handle the thing that inevitably went wrong?
Match the builder to the job
A reliable kitchen fitter is not automatically the right person to put a steel beam in your ground floor. For structural work, you want a builder who does that work regularly and who'll happily involve a structural engineer. Specialism beats general friendliness on a big job.
The quote and the contract: where money goes wrong
Get at least three written quotes for the same defined scope, so you're comparing like with like. A quote that's 30% below the others isn't a bargain — it usually means something has been left out, to be added back as an “extra” once you're committed. A good quote itemises labour, materials, a provisional sum for the unknowns, and VAT, and it states what is excluded.
Never pay for a big job in full upfront, and be wary of a large deposit. A staged payment schedule tied to completed milestones — for example a modest mobilisation payment, then payments as the foundations, the structure, the roof and the fit-out each finish — keeps the builder's incentive aligned with yours and keeps your money safe if things stall. Always keep a final retention (typically 5%) held back for a few weeks after completion, released only once snagging is done.
Get the whole thing in a written contract: scope, start and finish dates, payment stages, who supplies what, and how variations are priced. For larger projects the JCT Home Owner contract is a recognised, plain-English option. A contract feels heavy-handed until the week you need it, at which point it's the difference between a conversation and a court.
What to do in the next 30 minutes
• Look up your shortlisted builders on Companies House and note how long each has traded and whether accounts are filed.
• Check every trade badge at source — TrustMark, Gas Safe Register, NICEIC/NAPIT or FENSA — not just the logos on the website.
• Email each builder asking for their public liability certificate, two recent client references, and confirmation of who handles building control.
• Write a one-page scope of works so all three quotes price exactly the same job.
• Draft your staged payment plan now, before any conversation about deposits.
Use Trade Pilot's verified-review filter to start from builders whose reviews are tied to real, completed jobs in your area — then run the checks above before you commit. It's a faster way to get to a shortlist you can actually trust, without the guesswork of an open search.
Frequently asked questions
How do I check if a builder is legitimate in the UK?
Confirm the business on Companies House, verify any trade-body membership on the official register (TrustMark, Gas Safe, NICEIC, FENSA), ask to see in-date public liability insurance, and phone at least two clients whose jobs finished over a year ago. A legitimate builder provides all of this without resistance.
What insurance should a builder have?
At minimum, public liability insurance (commonly £2m–£5m of cover for domestic work) and, if they employ anyone on site, employer's liability insurance. For structural work, also ask about an insurance-backed guarantee or structural warranty on the finished job. Always ask to see the certificate and check it's current.
How much deposit should I pay a builder?
As little as possible, and never the full amount. For a big job, tie payments to completed stages rather than handing over a large lump sum upfront. A small mobilisation payment plus milestone-based instalments, with a 5% retention held back until snagging is finished, keeps your money protected.
Do I need building regulations approval for an extension?
Yes — extensions, loft conversions and structural alterations all need building regulations approval, signed off by local authority building control or an approved inspector. Agree in writing who submits the application, and make sure you receive the completion certificate at the end. You'll need it when you sell.
Are online builder reviews trustworthy?
Treat them as a starting point, not proof. Reviews that can't be traced to a verified, completed job are easy to fake or curate. Favour platforms that tie reviews to real transactions, and always back up a profile with your own reference calls and a site visit.
Authority sources to check before you commit: Companies House (gov.uk) to confirm the business is real and trading, and the TrustMark register to verify a builder is part of the government-endorsed scheme.
