How to Choose the Right Contractor for Your London Home Extension

Introduction

Choosing a contractor for a home extension is one of the most significant decisions you'll make during the project. Get it right and the build runs smoothly, comes in on time, and meets your expectations. Get it wrong and you're dealing with disputes, poor workmanship, programme delays, and in the worst cases, projects that are never completed.

In London's residential construction market, homeowners are often presented with a wide range of options — from small local builders to larger design-and-build firms. Comparing them meaningfully isn't always easy, especially when the information being offered varies significantly between quotes.

This guide sets out eight things to check before you appoint a contractor for your extension.

1. Are They Registered with a Trade Body?

The most credible trade registrations in UK residential construction include:

Federation of Master Builders (FMB) — members must pass an independent assessment, carry appropriate insurance, and commit to a code of practice
TrustMark — the government-endorsed quality scheme for home improvement work
Checkatrade — verified reviews and background checks
TrustATrader — independently reviewed tradespeople

None of these guarantees perfect work, but they do provide a baseline of accountability. An unregistered contractor with no verifiable track record carries more risk.

2. Can They Provide Evidence of Recent Similar Work?

Ask specifically for examples of extensions similar to yours — similar size, similar project type, similar property. Better still, ask whether you can speak to a previous client or, where appropriate, visit a recent project.

Any contractor confident in their work will welcome this request. Reluctance to provide references is a red flag.

3. Do They Carry the Right Insurance?

A contractor working on your home should carry:

Public liability insurance — protection if property or people are damaged during the build
Employers' liability insurance — required if they employ staff
Contractors' all-risk insurance — covering the works themselves while under construction

Ask to see the certificates before appointing. Verifying that the policy is in date and covers the type of work being carried out takes five minutes and is worth doing.

4. Do They Provide a Proper Written Contract?

A verbal agreement or an invoice-and-quote chain is not adequate protection for a significant building project. You should expect a written contract that clearly sets out:

The scope of works
The programme and key milestones
The payment schedule (linked to milestones, not time)
How variations will be handled
What happens if there are delays or disputes

If a contractor is uncomfortable providing a proper written contract, do not proceed.

5. Is Their Quote Detailed Enough to Be Meaningful?

A quote that says "extension — supply and install — £X" tells you very little. A quote that breaks down demolition, foundations, structural work, first-fix, second-fix, glazing, roofing, and finishing separately — with inclusions, exclusions, and assumptions clearly stated — tells you a great deal.

Be very cautious about the cheapest quote if it is also the least detailed. A low price based on assumptions that don't hold will become an expensive price by the time the project is finished.

6. Who Is the Principal Contractor?

Under CDM 2015, any project involving more than one contractor must have a principal contractor appointed in writing. Ask who will fill this role, what it means on their projects, and how they manage coordination between trades.

A contractor who doesn't know what CDM 2015 is, or who dismisses it as irrelevant to your project, is not operating with the level of professional awareness you should expect.

7. How Do They Communicate During the Build?

Ask how updates will be communicated during the project, how often you'll hear from them, and what happens when an issue arises on site. A professional contractor will have a process for this — a named contact, an agreed update schedule, and a clear approach to raising variations or problems.

Contractors who communicate well before you've appointed them tend to communicate well during the build. Those who are hard to get hold of in the sales process are often harder to get hold of when there's a problem on site.

8. Is the Programme Realistic?

Ask for a written programme with milestones. An experienced contractor will give you realistic timescales, flag likely pinch points, and explain what the build sequence involves. An optimistic programme that doesn't account for material lead times, Building Control inspections, or coordination between trades will almost certainly overrun.

How RCB Approaches Contractor Selection

At RCB Design & Build, we operate as a full principal contractor and are transparent about our process, our accreditations, our contract terms, and our project communication standards from the first conversation.

We believe the right contractor should make the selection process easy — by providing the information you need to make a confident decision.

Ready to compare properly?
Contact RCB Design & Build to discuss your extension project. We'll explain our process clearly and give you the information you need to make an informed choice.

📞 07359 872594 | contact@rcbgroup.co.uk | www.rcbgroup.co.uk