Good Builder or Cowboy? What to Look for When Choosing an Extension Builder

There is no formal qualification required to call yourself a builder in the UK. Anyone can set up a building company tomorrow. That makes choosing the right person for a £40,000 to £100,000 extension project one of the most consequential decisions you will make as a homeowner. Knowing what to look for before you sign anything is the most effective protection you have.

Where Should You Start Looking for a Builder?

Word of mouth from people whose judgement you trust is the most reliable starting point. If a friend or neighbour has had extension work done and is genuinely happy with both the quality and the experience, that is worth more than any online review.

Beyond personal recommendations, the Federation of Master Builders operates a vetted directory at fmb.org.uk where members are independently inspected. TrustMark is a government-endorsed scheme covering a wider range of trades. Both are worth using to build a shortlist, not as a guarantee of quality but as a meaningful filter.

Get at least three written quotes before making a decision. Three quotes give you enough to identify outliers without wasting builder goodwill.

What Are the Green Flags That Indicate a Reliable Builder?

They visit the site before quoting.

A builder who quotes without visiting the property is guessing. A builder who asks to see your drawings, discusses the ground conditions and takes time to understand the project before pricing it is planning properly.

They provide a detailed written quote, not an estimate.

A quote specifies exactly what is included and is legally binding for that scope of work. An estimate is an approximation and can vary significantly from the final bill. A professional quote for an extension runs to multiple pages and specifies materials, labour, a payment schedule and what is excluded.

They are easy to contact and communicate clearly.

If a builder is slow to return calls before you have signed anything, they will be slower once you have. Responsiveness during the quoting stage is a reliable indicator of how they will communicate during the build.

They can provide references from recent comparable projects.

Ask for the contact details of two or three clients from the past 12 months who had similar work done. Then call them. Ask about quality, programme, communication and how problems were handled.

They carry adequate insurance.

Every builder working on a domestic extension should carry public liability insurance of at least £2 million and employers liability insurance of at least £5 million. Ask to see the certificates, not just a verbal confirmation.

They are VAT registered.

Any builder with a turnover above £90,000 must be VAT registered. A builder carrying out significant extension work who is not VAT registered warrants further investigation.

They are registered at Companies House or can provide a UTR number.

If they claim to operate as a limited company, their details should be searchable on Companies House. Sole traders should be able to provide their Unique Taxpayer Reference.

What Are the Red Flags That Should Give You Pause?

They ask for a large deposit before work starts.

Reputable builders do not need 40% or 50% of the project value upfront. A reasonable deposit is 10% to 15% with payments tied to completed stages of work. A builder demanding a large sum before breaking ground is a warning sign.

They pressure you to decide quickly.

“This price is only valid today” or “I’ve got another job starting next week” are pressure tactics. A legitimate builder with a full order book does not need to rush you. Take the time you need.

Their quote is vague or significantly lower than others.

A quote that is dramatically cheaper than others is not necessarily a bargain. It may mean work is excluded that you expect to be included, materials are below specification, or costs will be added later as the project progresses.

They cannot provide references or avoid the question.

Every established builder should be able to point you to recent satisfied clients. Hesitation on this point is a red flag.

They insist on cash payment.

Cash-only arrangements mean no paper trail, no VAT and no protection for you if something goes wrong.

They are slow to communicate before you have even appointed them.

Poor communication before the contract is signed will not improve once the work starts.

They cannot show proof of insurance.

Verbal assurances are not sufficient. Ask to see actual certificates. If they cannot or will not produce them, do not appoint them.

What Questions Should You Ask a Builder Before Appointing Them?

Ask these before signing anything. A professional builder will answer all of them without hesitation.

Are you insured, and can I see your public liability and employers liability certificates? Can you provide references from recent extension projects similar to mine? Is this a fixed-price quote or an estimate? What is the payment schedule? Who will be on site day to day, you or subcontractors? How will you communicate with me during the build and how often? What happens if problems arise that affect the programme or cost? Are you a member of any trade associations?

What Contract Should You Expect?

A reputable builder will provide a written contract before work starts. For domestic extension projects, the JCT (Joint Contracts Tribunal) Minor Works Building Contract is widely used and gives both parties clear rights and obligations. At a minimum, the contract should set out the full scope of work, the fixed price, the payment schedule tied to construction stages, the programme, what happens in the event of variations and how disputes are resolved.

Never start work on a verbal agreement alone.

Frequently Asked Questions

Is there a qualification required to be a builder in the UK? No. There is no legal requirement to hold any formal qualification to operate as a builder. This is why checking trade body membership, insurance, references and business registration is so important before appointing anyone.

What is the maximum deposit I should pay a builder upfront? No more than 10% to 15% of the total project value. Payments should be tied to completed stages of work. Any builder requesting 40% to 50% upfront before starting should be treated with caution.

How do I check if a builder is a member of the Federation of Master Builders? Search the builder’s name or location at fmb.org.uk. Members are independently inspected and vetted. Membership is not a guarantee but it is a meaningful indicator of legitimacy.

Should I get a written contract before work starts? Yes, always. A written contract specifying the scope of work, fixed price, payment schedule, programme and variation procedure is essential. Never start a project on the basis of a verbal agreement.

What insurance should a builder have for an extension project? Public liability insurance of at least £2 million and employers liability insurance of at least £5 million as a minimum. Ask to see the actual certificates before appointing anyone.

Use Your Space carries out extensions across Solihull, Warwick, Knowle, Dorridge, Bentley Heath, Shirley, Balsall Common, Leamington Spa and Kenilworth. We are fully insured, provide detailed fixed-price contracts and can point you to recent clients across the area.

Contact Use Your Space today to discuss your project.