Finding a good builder in London is genuinely difficult. There are excellent contractors and there are very poor ones, and on the surface they can look identical — both will give you a quote, both will say they are experienced, and both might have a handful of photos on Instagram.
This guide explains what actually separates reliable from unreliable, and what to check before appointing anyone.
Start with verified credentials — not self-reported ones
Anyone can claim to be experienced. The following credentials are independently verified and worth checking:
Checkatrade — reviews are collected directly from clients by Checkatrade, not submitted by the contractor. A score of 9.5+ from 50+ reviews is meaningful. Below 50 reviews, treat the score cautiously.
Federation of Master Builders (FMB) — membership requires inspection and insurance verification. Not all builders are eligible. Check the FMB member finder at fmb.org.uk.
TrustMark — a government-endorsed quality scheme. TrustMark-registered businesses have been assessed for technical competence, customer service, and trading practices.
Gas Safe Register and NICEIC/NAPIT — for any work involving gas or electrics, ensure the relevant trades are registered with the appropriate scheme. Ask to see the registration number.
Ask for evidence of similar completed projects
Generic project photos are easy to fake or source online. Ask specifically:
- Can I see photos of a similar project (loft conversion, rear extension, or refurbishment) to mine?
- Can I contact a client who had this type of work done?
- Can you show me the Building Control completion certificate from a recent project?
A contractor who is reluctant to provide any of these is a contractor to be cautious about.
Get at least three quotes — but compare them properly
Getting multiple quotes is good practice. But comparing quotes from different contractors is harder than it looks, because contractors scope projects differently.
When comparing quotes, ask each contractor:
- Does your price include the structural engineer's fees?
- Does it include Building Control management and fees?
- Does it include the staircase (for loft conversions)?
- What are the payment terms and are they tied to project milestones?
- What happens if the cost goes above the quoted price?
A lower quote that excludes the structural engineer, Building Control, and a contingency for unknowns is not a lower quote — it is an incomplete quote.
Be cautious of these red flags
- Pressure for a large upfront deposit. A reputable contractor will ask for a reasonable mobilisation payment (10–20%) but not 50% upfront before a brick is laid.
- No written contract or scope of works. Every project should have a written document specifying exactly what is included and what is not. "We'll sort it on site" is not a substitute.
- No Building Regulations mentioned. Any contractor who does not mention Building Control for structural or extension work is either planning to skip it or is inexperienced.
- Vague about subcontractors. A good principal contractor is transparent about who is doing which trade and has established relationships with reliable subcontractors.
- Too good to be true pricing. Prices significantly below market rate are almost always a sign that something is excluded, the quality will be poor, or the contractor will run into financial difficulty mid-project.
Ask about project management and communication
This is often overlooked. The build quality matters — but so does how the project is managed. Ask:
- Who is my single point of contact during the project?
- How do you communicate progress — daily updates, weekly calls?
- What happens if there is a problem on site?
- How do you manage changes to the scope?
A principal contractor model — where one person takes accountability for the whole project — is very different from a loose arrangement of separate trades where no one is in charge.
What to look for in a contract
Before signing anything, make sure the document includes:
- A detailed scope of works specifying exactly what is and is not included
- A payment schedule tied to completion milestones (not dates)
- A clear variation clause explaining how changes in scope are priced
- A defects liability period after completion
- References to Building Regulations compliance
If a contractor wants you to sign a one-page document with a single lump sum figure and nothing else — walk away.
Our approach at RCB Design & Build
We are Checkatrade-verified (9.96/10, 114+ reviews), FMB-registered, and TrustMark-verified. Every project starts with a free site survey, proceeds with a detailed written scope and fixed price, and is managed as a principal contractor project — one point of contact, all trades coordinated, Building Control managed from start to finish.