Most advice on this tells you to ask whether the builder is insured. That question is close to useless, because almost everyone says yes and the two main covers sit on opposite sides of the law.
The insurance point, stated the right way round
Employers' liability insurance is compulsory where a builder has employees. Gov.uk is unambiguous: you must get it as soon as you become an employer, the minimum cover is at least £5 million, and you can be fined £2,500 for every day you are not properly insured, with a further £1,000 for failing to display the certificate. The statutory basis is the Employers' Liability (Compulsory Insurance) Act 1969. The exemptions are narrow: broadly, where the only employees are family members, or people working outside England, Scotland and Wales. The family exemption has a catch that is widely missed: it falls away where the family business is incorporated as a limited company, so a family firm trading through a limited company needs the cover like any other employer.
Public liability insurance is not legally required. No statute obliges a builder to carry it. It is entirely voluntary. It is also the cover that pays out when someone is hurt on your property or your house is damaged.
So the compulsory cover protects the builder's staff, and the voluntary cover protects you. A great many pages state this backwards, and a homeowner who has read one of them will treat a sole trader with no employers' liability certificate as a red flag when no law has been broken, and treat a firm with no public liability as fine because they have heard employers' liability is the legal one.
The two questions to ask instead
First: will anyone be working for you on this job? A genuine sole trader with no staff has nobody to insure and is committing no offence. A builder who arrives with a team and holds no employers' liability cover is committing an offence carrying a £2,500 daily fine.
Second: can I see the public liability certificate? Then check three things on it: the level of cover, that it is in date across your whole project, and that the named party is the exact legal entity that will invoice you. Trading names and company names differ more often than you would expect.
Certificates are easy to fake. If the job is large, confirm that the insurer is authorised on the Financial Services Register and ask for the broker to confirm cover directly rather than accepting a PDF.
Check the accreditation, do not accept the logo
Citizens Advice puts it plainly: check whether the trader is a member of the approved trader scheme, competent person scheme or trade association they claim, using the relevant scheme's own member list. Every credible scheme publishes a searchable register.
Claimed membership is trivially asserted and lapsed membership is common. Search the exact company name from the quote rather than the trading name on the van.
Three quotes, and none of them the first one
The Federation of Master Builders is direct about this: always get a minimum of three quotes from builders recommended by a source you trust, and never go with the first quote you get. Quotes can vary by hundreds of pounds, and the cheapest is not automatically the best.
Ask each of them whether VAT is included and whether there are costs not mentioned, and ask for labour and materials to be separated out. Those are the FMB's own suggested questions and they are what turns three numbers into three comparable numbers.
A written contract, and why "we never signed anything" is not a defence
The FMB advises always entering into a written contract, and notes that it need not be an expensive legal document. Its checklist covers the parties and contacts, the full agreed price including rates for extras and overruns, whose insurance covers what, how changes will be documented, how disputes will be resolved, how defects will be put right and who is responsible, a pre-agreed rate of liquidated damages for overrun, and a snagging agreement.
Here is the part that surprises people. Citizens Advice states that once you say yes to a quote it is a binding agreement between you and the trader, whether it is written down or not. Homeowners routinely believe that having signed nothing means there is no contract. The opposite is true: the contract formed when you accepted the quote. Writing it down is about proving the terms, not creating them.
That cuts both ways, so be careful what you accept verbally, and get the terms recorded afterwards even if you agreed them on the doorstep.
Before anyone starts
Agree the payment schedule and the deposit in writing, get receipts for anything paid in advance, allow a contingency of at least 10% on the budget as the FMB suggests, and settle who orders the materials.
Checking properly takes an evening and outlasts the job.