How to Choose a Builder in London: Complete Guide
How to find, vet, and hire the right builder for your extension. Avoid cowboys, understand contracts, and protect your investment.
TL;DR - The Quick Answer
London homeowners should collect 3 to 5 quotes from builders working from the same specification, then check each one against a standard vetting list: Companies House registration, public liability insurance of at least £2 million, and references from at least three similar projects. Deposits should never exceed 10 to 15 percent upfront. Personal recommendations and architect referrals are the most reliable starting points. Once a builder is selected, a JCT Minor Works contract sets out scope, payment stages, and a defects liability period.
Choosing the right builder is one of the most important decisions you'll make for your extension project. A good builder delivers quality work on time and budget. A bad one can cause stress, delays, cost overruns, and poor workmanship that affects your home for years.
Where to Find Builders
Personal Recommendations
Best source. Ask neighbours, friends, colleagues who've had similar work done. You can see the finished quality and ask honest questions.
Architect Recommendations
Your architect or designer will have builders they've worked with before. They know who delivers quality and who to avoid.
Trade Associations
Federation of Master Builders (FMB), TrustMark, Checkatrade. Membership requires vetting and provides some consumer protection.
Local Building Projects
See skips or scaffolding nearby? Ask the homeowner how their builder is performing. Builders working locally reduces travel costs.
Vetting Checklist
Getting Quotes
Get 3-5 quotes minimum. Ensure all builders are quoting from the same specification-ideally detailed drawings and a schedule of works from your architect.
| Quote Element | What to Check |
|---|---|
| Fixed price vs estimate | Fixed is safer; estimates can increase |
| What's included | Skip hire, scaffolding, building control fees? |
| Payment schedule | Stage payments, not large upfront deposits |
| Timeline | Start date, duration, completion target |
| Variations process | How are changes priced and agreed? |
| Professional | What They Do | Typical Cost |
|---|---|---|
| Architectural designer | Design, planning drawings, application management | From £1,295 fixed fee |
| Main contractor (builder) | Full construction, coordinating trades | £40k–£120k+ (project cost) |
| Structural engineer | Beam calculations, foundation design | £600–£1,500 |
| Party wall surveyor | Neighbour agreements under Party Wall Act | £800–£1,500 per neighbour |
| Building control | Inspect and certify building regulations compliance | £800–£1,500 |
Source: Mayfair Studio project data • Data as of 2025
Red Flags to Avoid
Large Upfront Deposits
Never pay more than 10-15% upfront. Stage payments should match work completed.
Cash Only
Reputable builders accept bank transfers. Cash-only suggests tax avoidance.
No Written Quote
Everything should be in writing. Verbal agreements lead to disputes.
Pressure Tactics
"Price only valid today" or "I have another job waiting" are pressure tactics.
Unusually Cheap Quotes
If one quote is 30%+ cheaper, they've either missed something or plan to cut corners.
No Insurance Proof
Ask to see current insurance certificates. If they won't provide them, walk away.
Contract Essentials
Use a proper building contract. The JCT Minor Works or FMB contract are industry standards that protect both parties.
- Scope of works: Detailed description of all work included
- Contract sum: Fixed price with clear variation process
- Payment terms: Stage payments linked to milestones
- Start and completion dates: With provisions for delays
- Defects liability: Typically 6-12 months to fix issues
- Retention: Hold back 2.5-5% until defects rectified
- Dispute resolution: Adjudication or mediation clause
Get 3 to 5 quotes from builders working from the same specification. Check Companies House registration, public liability insurance of at least £2 million, and references from at least three similar projects. Never pay more than 10 to 15 percent upfront. Use a JCT Minor Works contract setting out scope, payment stages, and a defects liability period of 6 to 12 months.
Frequently Asked Questions
Frequently Asked Questions
How much deposit should I pay a builder?
Maximum 10-15% upfront for materials ordering. The rest should be stage payments as work progresses.
Should I use the cheapest builder quote?
Rarely. Understand why it's cheaper. Usually middle quotes from well-referenced builders offer best value.
What if things go wrong with my builder?
Document everything in writing. Use your contract's dispute resolution process. Citizens Advice and trading standards can help with serious issues.
How many builder quotes should I get?
Get 3-5 quotes minimum. Ensure all builders are quoting from the same specification-ideally detailed drawings and a schedule of works from your architect.