How to choose a builder in London (2026): the paper-trail test

The reliable way to choose a builder in London is to judge the paperwork, not the patter. A trustworthy contractor can show you, on paper, an itemised written quote, a current insurance certificate, the right trade credentials for the gas and electrical work, a proper contract, and a sensible payment schedule — and can give you the numbers of recent clients to call. A risky one stalls on every one of those. This guide walks through each document, what good looks like, the warning signs that should end the conversation, and the two plain questions that tell you most. None of it relies on a glossy website or a single five-star quote.

Start with the quote — itemised, or it isn't a quote

A real quote is broken into lines you can read and challenge. "New bathroom — £18,000" tells you nothing; it's a number with no working shown. A proper version sets out, separately, the strip-out and waste, the first-fix plumbing and electrics, the waterproofing, the tiling labour, the named fittings (make and model, not "sanitaryware"), the second fix, and the snagging. The point isn't the length of the list — it's that an itemised quote proves the builder has actually priced the job rather than guessing a round figure they'll "adjust" once you're committed. If a contractor can't or won't itemise, you can't compare their price to anyone else's, and you can't tell what's been left out.

Insurance — ask for the certificate, today

Two cover types matter for home work. Public liability protects you if the builder damages your property or injures someone — around £1m is standard for ordinary residential refurbishments, more for larger or structural jobs. Employer's liability is a legal must the moment they have anyone working for them. Ask for the certificate, not a verbal assurance: a genuine firm forwards it within the hour, and the one that's vague or "will send it later" is telling you something. On bigger projects, ask about contract works (or "all-risks") cover for the build itself while it's under way.

Credentials — who signs off the regulated work

Some work is legally restricted, and you want to know exactly who is certifying each part:

• Anything to do with gas is restricted by law to a Gas Safe registered engineer — check the person's number at gassaferegister.co.uk rather than trusting the company name on the van.
• Electrical work that's notifiable has to be certified under Part P, via a competent-person scheme such as NICEIC or NAPIT.
• Structural alterations, an extension or a loft each require Building Regulations approval, inspected in stages by council building control or an approved inspector.
• Touch a listed building and listed building consent must be in hand before any work begins.

It's perfectly normal — and fine — for a main contractor to subcontract the gas or the electrics; just ask to see the subcontractor's credentials too, and ask plainly who is responsible for obtaining each sign-off. A builder who treats "who certifies this?" as an awkward question is the wrong builder.

A contract and a payment schedule that protect both sides

For anything beyond a small job, insist on a written contract — for projects under around half a million pounds, a standard form such as a JCT minor-works agreement is the norm and is there to protect you as much as the builder. On payments, the shape that keeps both parties honest is a modest deposit (commonly 10–25%), then instalments released against real progress points — once the first fix is done, again at second fix, and on completion — with a small retention of roughly 5% kept back for a few months after handover against any snags. Two firm rules: never hand over the full amount before the work is finished, and be wary of any deposit above 30% before materials have even been ordered. A payment plan linked to progress means you're never paying for work that hasn't happened.

How to read independent feedback (without relying on it)

Online reviews are one signal, not the deciding one. Treat them as a sense-check rather than proof: a spread of genuine, detailed reviews across an independent platform is reassuring, and the middle-rated ones often tell you more than the glowing extremes. But reviews are easy to curate and hard to verify, so weight them below the things you can confirm yourself — the insurance certificate, the itemised quote, the contract, and a phone call to a recent client. A wall of stars on a builder's own website, with nothing checkable behind it, proves only that they can build a website.

The warning signs that should end it

Walk away on any of these:

• Cash only, or "mate's rates," with no paperwork.
• Pressure to commit fast — "we've a gap next week, decide today."
• Reluctance to put anything in writing, or to itemise the quote.
• No fixed business address, or a brand-new web presence with no traceable history.
• A price dramatically below every other quote — the right reaction is "what have they left out?", not "what a bargain."
• Spec changing mid-job, with charges for things that plainly should have been in the original price.

The two questions that cut through

After the documents, two plain questions tell you most. First, ask the builder for the contact details of their three most recent finished jobs, and then actually ring those owners. Did the team arrive on the days promised? Was the site left clean? Were snags dealt with? And the decisive one: did the bill at the end land where the quote said it would? A good builder hands over those numbers without flinching. Second, ask who your single point of contact will be for the whole job, and what happens to the programme if something unforeseen turns up. The answer reveals whether they run organised projects or improvise — and improvisation is what turns a renovation into a horror story.

Closing CTA

A good builder makes vetting easy, because everything they claim is on paper and checkable. PrimeCraft Surface Solutions quotes to trade level, supplies its insurance and credentials on request, works to a clear contract and a milestone payment plan, and runs each job through one named contact — throughout London and the neighbouring home counties. Want to put us through the test above? Book a no-cost site visit, and the paperwork comes with us.