How to choose a builder you can trust (and the red flags to avoid)

A loft conversion or extension is likely one of the biggest cheques you’ll ever write outside your mortgage. So a bit of nervousness is healthy. The good news is that the builders who cut corners almost always give themselves away early, if you know what to look for. Here’s how to separate the safe pair of hands from the chancer.

Start with how they actually win work

The single best predictor of a good job is an unprompted recommendation from someone you trust. A neighbour in Bishopston who’ll happily walk you round their finished loft is worth more than any glossy advert. Word-of-mouth matters because the builder’s reputation is on the line in a community they keep working in — they can’t afford a bad job following them around Redland or Southville.

So ask around first. Then build a shortlist of two or three firms and put each through the same checks below. Comparing like-for-like is the whole point.

How to vet a builder properly

Don’t take anything on trust. Ask to see the evidence, and don’t feel awkward about it — a good builder expects these questions and answers them without flinching.

Insurance and the paperwork

Public liability insurance (typically £1m–£5m of cover). Ask for the certificate and check it’s in date. This protects you if something goes wrong on site.
Employer’s liability insurance if they have employees on the job.
For loft conversions and extensions, you’ll need building control sign-off — either through Bristol City Council or a private approved inspector. A proper builder will talk you through this as standard. Be wary of anyone who shrugs it off.
If you share a wall with a neighbour (most Bristol terraces do), the Party Wall Act may apply. Your builder should know this and flag it early.

References and seeing real work

Ask for two or three recent customers you can actually phone — and call them. Ask: did it finish on time? Did the price hold? Were there surprises? Would you use them again?
Ask to visit a finished job, or at least a current site. Seeing the standard of work in the flesh tells you more than any photo gallery.
Look at how a live site is run. Is it tidy and safe, or chaos? It’s a good proxy for how they’ll treat your home.

Accreditations (helpful, not gospel)

Memberships like the Federation of Master Builders (FMB), TrustMark, or relevant trade bodies are a reassuring sign — many vet their members and offer dispute help. For anything involving gas or heating, the engineer must be Gas Safe registered by law. Treat accreditations as a green flag, not a guarantee; references and your own eyes still matter most.

The right questions to ask

Before you sign anything, get straight answers to these:

Who exactly will be on site, and will the person I’m speaking to be managing it?
Are you using subcontractors, and are they insured too?
What’s the realistic start date and how long will it take?
What’s included in the price — and just as importantly, what isn’t?
How are payments structured against stages of work?
What guarantee do you offer, and is it backed (e.g. an insurance-backed warranty)?
How do you handle changes or unexpected problems once we’ve started?

A trustworthy builder gives clear, unhurried answers. Vagueness or irritation at fair questions is itself a red flag.

Red flags to walk away from

Some warning signs should end the conversation, no matter how charming the person is:

A large upfront deposit. A modest deposit to secure materials can be reasonable, but anyone wanting a big chunk of the total before lifting a tool is a worry. Genuine firms have the cash flow to start work.
Cash only, or “no VAT if you pay cash”. It dodges tax, leaves you no paper trail, and you’ll have no comeback if it goes wrong.
No written, itemised quote. A scribble on the back of an envelope or a single round number is not a quote. You want it in writing, broken down, so you can compare and hold them to it.
No fixed price, only “day rates” for a defined job. Day rates have their place for small or open-ended work, but for a £40k–£70k project you want a fixed price for a clear scope.
Pressure selling. “This price is only good today” or “I’ve got a gap next week if you decide now” is a tactic, not a favour.
Won’t put guarantees or timescales in writing. If they mean it, they’ll write it down.
No fixed address, only a mobile number, and a company you can’t find on Companies House. Reputable Bristol firms are easy to trace.

A note on price

The cheapest quote is rarely the bargain it looks like. A price well below the others usually means something’s been left out, and you’ll meet it later as an “extra”. Costs genuinely vary with access, spec, and the age and quirks of your house — a Victorian terrace throws up surprises a 1990s semi won’t — so honest builders give ranges and explain what could move the number. What you’re really buying is certainty: a clear scope, a fixed price, and someone who turns up.

If you’d like a straight conversation with no sales pressure, we’re happy to visit your home, talk through your ideas, and give you a free, no-obligation fixed written quote. Have a look at our recent work and the areas we cover first — and if your project touches heating or a new boiler, see our heating services. No hard sell, just honest advice.