1. Payment structure: how much they want, and how
A deposit above roughly a quarter to a third of the total contract value, requested before any work has started, shifts nearly all of the financial risk onto you before you've seen any evidence of how this particular builder actually performs on your particular job. It's the single most common feature of a job that stalls or vanishes partway through, because once a large deposit has changed hands, there's very little left to lose by walking away from a difficult or unprofitable site.
Insistence on cash only, with no invoice, no paper trail and pressure to avoid anything that looks like a formal record of what was agreed, is worth treating with real caution. Beyond the obvious tax and VAT implications for the builder, a job with no paper trail leaves you with nothing to point to if a dispute arises later over what was actually included in the price, what stage the work had reached, or what was promised verbally at the outset.
A sensible structure stages payment against progress: a modest amount to secure the booking and any materials that need ordering in advance, followed by payments tied to completed milestones, first fix, second fix, practical completion, rather than one large sum upfront or, at the other extreme, nothing until the very end. Either extreme leaves one side carrying almost all of the risk, which isn't how a well-run contractor typically prefers to operate either.
2. Documentation: is there a written, itemised quote and contract
A verbal 'about £15,000' is not a quote, it's a guess, and it gives you nothing to hold either side to once work is underway and the figure inevitably starts to move. A proper quote is written down, itemised by area of work or trade, and specific about what's included and what isn't: labour, materials specification, waste removal, and making good after each trade, rather than a single figure that could mean almost anything once you start asking what it actually covers.
Vague, single-line quotes are one of the most common sources of dispute in construction, usually because both sides had a genuinely different idea of what was covered from the outset, and there's nothing written down to settle the disagreement either way. It's entirely reasonable to ask a builder to clarify anything in a quote that isn't clear before signing anything, and a reliable one will do this without pushback, because a clear quote protects them from scope disputes just as much as it protects you.
Beyond the quote itself, a written agreement covering start and finish dates, how variations to the scope will be priced and agreed before they happen rather than discovered on the final invoice, and the payment schedule tied to milestones, is what turns a friendly conversation into something enforceable if things go wrong. A builder unwilling to put any of this in writing is keeping their options open at your expense.
3. Pricing: suspiciously cheap quotes and quotes you can't actually compare
A quote that undercuts every other one you've collected by a wide margin often isn't a better deal, it's a way to win the job cheaply and make the difference back later through extras once you're committed and the walls, floors or roof are already open. By the time an unexpectedly large 'discovery' appears on the invoice, switching to a different contractor mid-job is far more disruptive and expensive than it would have been at the outset, which is exactly the point in the job where a scope-creep-based pricing model does its damage.
It's also worth checking that quotes are actually pricing the same scope before comparing the bottom line. One builder's figure might include skip hire, making good, a contingency allowance for unexpected findings in an older property, and full waste removal, while another's excludes all of that and simply looks cheaper on paper as a result. A lower headline number sometimes just means a shorter list of what's actually included, not a genuinely cheaper job for the same outcome.
The practical fix is to ask each builder the same specific questions in writing, what's included, what's excluded, how variations are priced, what the payment schedule looks like, and compare those answers directly rather than the total figure alone. A builder with a straightforward, honestly priced quote will answer plainly; one relying on a bare number to win the work often starts hedging as soon as the detail is asked for.
4. Credibility: insurance, trade body membership, and a real business record
Ask to see a public liability insurance certificate, not just a verbal assurance that cover exists. This matters practically: if something is damaged during the works, a neighbouring property is affected, or someone is hurt on site, insurance is what stands behind the builder's ability to put it right without the cost simply landing on you. A legitimate builder has the certificate ready and won't treat the request as unusual.
Trade body membership, such as the Federation of Master Builders or TrustMark, isn't a guarantee of quality on its own, but it does mean a builder has gone through some level of vetting and has a recognised route for a dispute to be raised if things go badly wrong. No membership doesn't automatically mean a builder is dishonest, plenty of good tradespeople work without one, but it does remove one of the easier ways to check them, so it's worth weighing alongside the other signs here rather than in isolation.
For a limited company, a Companies House search is free and takes a couple of minutes: it confirms the company genuinely exists, how long it's actually been trading, and whether accounts have been filed recently. A company incorporated days before it quoted you, or one with lapsed filings, isn't automatic proof of a scam, but it's a reason to ask more questions rather than fewer, alongside asking for photos or addresses of similar completed work and, ideally, speaking to a past customer directly rather than relying on a curated gallery of best-case photos alone.
5. Behaviour: pressure tactics, cold-calling, and suspicious availability
Someone who knocks on the door claiming to have been working nearby and to have 'noticed' damage from the street, a slipped roof tile, cracked render, a chimney issue, is one of the oldest tricks in the trade, and it particularly targets older homeowners who may feel less able to challenge an unsolicited caller. A genuine assessment needs someone actually on the roof, in the loft, or examining the wall up close, not a glance from the pavement, and reputable builders generally don't solicit work door to door in the first place.
Claims that something will fail imminently, or a discount that only applies if you agree today, are a classic pressure tactic designed to stop you getting a second opinion or a comparison quote. A genuine problem, in a roof, a wall or anywhere else, will still be exactly as genuine tomorrow or next week, and any builder worth using lets the survey findings speak for themselves rather than rushing you into signing before you've had time to think it over.
A contractor who can start literally tomorrow, with apparently nothing else booked in, may simply be quiet for a reason worth asking about. A genuinely busy, well-regarded builder is usually booked weeks or months ahead, since good trades don't sit idle waiting for the phone to ring. Suspiciously instant availability, especially when paired with pressure to book immediately to lock in a discount, is worth treating with more caution than reassurance.
Use this as your starting checklist
None of these five checks needs specialist knowledge, and asking about them upfront costs you nothing beyond a slightly awkward conversation, which any reliable builder is used to having. If you're weighing up a specific trade, our guides on bathroom renovation scam red flags and roofing scam traps apply these same principles to the specific mechanisms that show up in those jobs. Our separate guide on how to find a reliable builder in London takes the same checks and turns them into a positive vetting process, including for us. Our construction company London and property refurbishment London teams are happy to be checked against every point covered here before you commit to anything.