How to Choose a Builder for a London Extension (And 10 Red Flags to Avoid)

Choosing the right builder is the single most consequential decision you'll make on your extension project. Get it right and you'll end up with a well-built, on-budget home improvement that adds real value. Get it wrong and you could face abandoned work, structural problems, disputed payments, and years of stress to fix it.

The difficulty in London is that the market is enormous and largely unregulated. Anyone can call themselves a builder. There's no licensing requirement for general construction work, no government register you can check, and plenty of operators who use slick websites to mask a thin track record. That's why the vetting process matters so much — and why cutting corners on it so often ends badly.

This guide walks through exactly how to find, vet, and appoint a builder for your London extension — and the ten warning signs you should never ignore.

Start With Recommendations, Not Search Engines

The most reliable starting point is still word of mouth. If a neighbour has recently had an extension built, knock on the door and ask how it went. Ask your architect or structural engineer — they work with builders regularly and will tell you honestly who delivers and who causes headaches. Local Facebook groups and Nextdoor threads are also useful sources of real, recent experience.

Beyond personal recommendations, look at trade accreditation bodies. The Federation of Master Builders (FMB) vets members on financial stability, quality of work, and insurance. TrustMark is a government-endorsed quality scheme covering a wide range of trades. Membership of either is not a guarantee, but it is a meaningful filter — rogue operators rarely bother with the scrutiny these schemes involve.

Services like My Local London Builder work by connecting homeowners with pre-vetted local builders, saving the time and risk of researching from scratch. Rather than giving you a fixed price (every project is different), the aim is to get you in front of the right builders quickly, so you can compare properly. Whether you use a matching service or find builders yourself, the vetting steps below apply regardless.

What to Check Before You Invite Anyone to Quote

Before you even pick up the phone, do a basic background check on any builder you're considering. This takes ten minutes and filters out the obvious risks.

Questions to Ask at Quote Stage

When you invite builders to quote, treat it as a structured interview as much as a price exercise. The questions you ask — and the quality of the answers you receive — tell you as much about a builder as the number at the bottom of the page.

Ask for proof of public liability insurance (minimum £2 million, ideally £5 million for a large extension). Ask who will be on site every day and who is the named project manager. Ask how they handle planning permission and building regulations sign-off — will they manage it or do you? Ask about their subcontractor relationships: are they long-term working partnerships or do they use whoever is available?

A good builder will also ask questions back. They should want to understand the design, the site access, any structural complexities, your timeline, and how you'll be living during the works. A builder who doesn't ask anything before quoting hasn't thought about the job properly.

How to Compare Quotes Fairly

Always get a minimum of three quotes for any London extension. Aim for quotes that cover the same specification — if you're comparing apples with oranges, the cheapest quote is meaningless. Ask each builder to break down their costs: labour, materials, groundworks, structural work, and a contingency provision.

Be cautious of quotes that are significantly below the others. In London, reputable builders with full books don't need to price aggressively to win work. A quote that's 30–40% below market rate almost always means something has been left out — or that the builder is struggling for work and may have cash flow problems mid-project.

Once you have your quotes, don't just compare totals. Compare what's included and excluded. Ask each builder: what would cause the price to go up? A builder who has thought through the risks will have a clear answer. One who hasn't will give you a vague shrug — which typically turns into a running series of variation orders once work begins.

If you're unsure how extension costs in London compare broadly by project type, our guide to kitchen extension costs in London gives a detailed breakdown of what to expect at different price points and specifications.

What a Good Contract Covers

Never proceed without a written contract. This is non-negotiable. A proper building contract should include: a detailed scope of work, the agreed price and payment schedule, a timeline with key milestones, a process for handling variations, who is responsible for planning and building control, insurance obligations on both sides, and a dispute resolution process.

The JCT Minor Works Building Contract is commonly used for residential extension work and is widely understood by builders and solicitors. Your architect can administer it if you have one appointed. If the builder offers their own contract, read it carefully — particularly the clauses on payment, variations, and termination.

Payment should always be tied to milestones, not to arbitrary dates. A typical structure might be: 10–20% deposit on signing, then stage payments at foundations complete, structure complete, roof watertight, first fix complete, second fix and finishes complete, and a retention of 2.5–5% held for six to twelve months after practical completion.

The 10 Red Flags — and Why They Matter

The following warning signs are not automatically disqualifying on their own, but each one should prompt a conversation. If you encounter several of them with the same builder, walk away.

• Asks for cash only, or offers a significant discount for cash. This is the clearest indicator that a builder is operating outside the system — no VAT, no paper trail, no recourse.
• No written contract offered. A handshake deal protects nobody. A builder who resists putting things in writing doesn't want to be held to them.
• Cannot provide proof of public liability insurance. If something goes wrong on your property — a structural collapse, a fire, an injury — you need to know a real insurer is behind the builder. Ask for the certificate and verify it directly with the insurer.
• Quote with no breakdown or very little detail. "Extension: £65,000" is not a quote. It's a guess. A professional builder itemises labour, materials, and subcontract work.
• Pressure to sign quickly or a "deal" that expires soon. Legitimate builders don't need to pressure you. Urgency is a sales tactic, not a sign of quality.
• No traceable business address or online presence. If you can't find them independently — on Companies House, Google, or trade directories — that's a problem.
• Offers to start immediately with no other commitments mentioned. Good builders in London are typically booked 2–4 months ahead. If someone can start next week, ask why.
• Negative pattern in reviews, or none at all. One bad review is noise. A pattern of complaints about finishing, communication, or overpayment requests is signal.
• Vague answers about subcontractors. On most large extensions, specialist trades (structural steel, electrical, plumbing) will be subcontracted. A builder should be able to name the firms they use and confirm they are covered by the main contract's insurance.
• No interest in your drawings, planning status, or structural engineer's report. A builder quoting without wanting to understand what they're building hasn't thought about your job. They've thought about getting the deposit.

Checking References Properly

Most builders will give you references if asked. The value of those references depends entirely on how you use them. Don't just accept a name and number — ask the reference specific questions. Did the builder finish on time? How did they handle problems when they arose? Was the final cost close to the original quote? Would they use them again, and have they already recommended them to anyone else?

The best reference check is a site visit. Ask to visit a completed project that is similar in scope and scale to yours. See the quality of the finishes. Ask the homeowner whether the site was kept clean, whether the team was professional, and whether there was a single point of contact who was responsive. These details matter just as much as the technical quality of the brickwork.

Working With My Local London Builder

My Local London Builder connects London homeowners with pre-vetted, experienced local builders. We don't set fixed prices — every project is priced by the builder based on your specific drawings, site conditions, and specification. What we do is make sure that the builders you speak to have a verified track record in London extensions, carry adequate insurance, and are accustomed to working on occupied homes.

When you submit your project details, we'll match you with suitable builders who can provide comparable quotes. That means you get the benefit of competitive pricing without the risk of finding unknown contractors yourself.

Once you've received your quotes, this guidance — and our article on how many quotes to get for a London extension — should help you make a confident, informed choice.