Choosing who builds it
Extension Builders North London
Firtree Build is an FMB member and Which? Trusted Trader based in Enfield, building rear, side-return, kitchen and double-storey extensions across North London. Before you appoint anyone, here's what's actually worth checking, the questions to ask, and the credentials that separate a builder who has genuinely managed extensions before from one who hasn't.
Choosing who builds it
How to Choose Extension Builders in North London
You've decided to extend rather than move. The next decision matters just as much: who actually builds it. An extension is one of the largest sums most homeowners will ever spend on their property, and the gap between a good contractor and a poor one shows up in party wall disputes, missed sign-off, and jobs that overrun by months.
Not every badge is equal
Credentials that actually matter
Anyone can put a logo on a website. What matters is whether it represents an independent check on the business, and specifically what happens if something goes wrong. Most homeowners never look past the badge to ask that second question, which is exactly where a weaker builder's protection turns out to be thinner than it looked.
FMB accreditation
Federation of Master Builders members are vetted before joining and bound by a code of conduct. Crucially, members are required to take part in the FMB's free dispute resolution scheme if a client raises a formal complaint, so the accreditation carries a real mechanism, not just a directory listing. You can check any builder's FMB membership directly on the FMB's Find a Builder register.
Which? Trusted Trader
Involves independent assessment of the business and ongoing monitoring of customer feedback, rather than a one-off badge you can simply display and forget about.
Public liability insurance
Ask to see the certificate, not just be told it exists. £5 million cover is typical for domestic extension work in London.
Structural warranty
A builder confident in their work will offer a guarantee, commonly 10 years, covering structural elements after completion.
Building Control history
Ask for examples of recently completed, fully signed-off projects. A builder who can't point to certified work is a warning sign.
Direct labour on site
Some companies subcontract the entire build after the sale. Ask whether the team on site is direct labour, and who your single point of contact is.
What the badge actually gets you
How FMB's dispute process works, in practice
Most builder websites mention FMB membership without explaining what it means if things go wrong. Here is the actual sequence, so you know what you're getting before you need it, not after.
Raise it directly first
You must put your complaint to the builder in writing first, describing what you're unhappy with and how you want it resolved, and give at least 10 working days for a response.
Apply to FMB
If unresolved, you submit a completed dispute form. FMB confirms your case number, typically within 5 working days of processing.
Free conciliation
Trained ADR officials work to reach an agreed solution, usually by telephone mediation and reference to your written contract and documents. It's free of charge to you as the client.
No binding decision
Conciliation isn't legally binding. If it doesn't resolve things, you retain the right to pursue small claims court or other formal action, exactly as you would without any accreditation at all.
The practical takeaway: FMB membership gives you a free, structured first step that doesn't exist with an unaccredited builder, but it is a conciliation process, not a guarantee of a specific outcome. A written contract that clearly sets out scope, payment stages and how variations are agreed matters just as much as the badge itself, because it's the document the whole process refers back to.
Before you sign anything
Six questions worth asking every builder
These are the questions that separate a builder who has genuinely managed extensions before from one who is learning on your project.
Who manages the party wall process?
On most North London terraces and semis this affects your project timeline directly, and a party wall surveyor's fee for a straightforward agreement typically runs from a few hundred pounds per party upward, more if a formal award is needed. A builder who hasn't mentioned it unprompted may not have planned for it. See our house extensions guide for the full detail.
Is the quote itemised?
A single lump-sum figure with no breakdown makes it impossible to compare quotes fairly, or to see what happens if the specification changes mid-project.
Who handles planning permission?
Agree in writing who submits the application, and who pays if it's refused or needs resubmission, particularly if the property sits inside a conservation area where a refusal is more likely on first submission.
What about unforeseen costs?
Older North London properties, particularly Victorian terraces, frequently reveal surprises once walls are opened up: unrecorded drainage, inadequate foundations, asbestos in older coatings. Ask how these are priced and agreed before work proceeds.
Can I speak to a recent client?
Photos are easy to present well. A conversation with someone who lived through the build tells you far more about communication and reliability.
Who is on site day to day?
Ask whether the team on site is direct labour or subcontracted, and who the single point of contact is throughout the project.
Trust your instincts
Red flags worth walking away from
None of these are automatically disqualifying on their own, but two or more together, especially on an older property, are a strong signal to look elsewhere.
Pressure to pay a large deposit before any design or paperwork has been produced.
Reluctance to put the payment schedule and specification in writing.
No fixed address, or a registered address that doesn't match where the business actually operates from.
Quotes dramatically lower than every other builder you've spoken to are far more often a sign of corners being cut than genuine efficiency.
Unclear answers about who is responsible for planning permission, party wall notices, or Building Control.
A quote for a Victorian or Edwardian terrace that reads identically to a quote for a 1990s house, with no mention of solid-wall construction, older drainage or matching original brick, suggests the survey wasn't thorough.
What we actually do
Working with Firtree, step by step
We are FMB members and a Which? Trusted Trader, based in Enfield and building across North London.
Our team covers both house extensions and loft conversions, including kitchen extensions, so if you're still deciding between options, we can give you a straight comparison for your property. If you're based specifically in Enfield, our Enfield extension builders page covers the local areas we work in most.
See the work, not just the claims
Completed extensions you can inspect
A selection from our project gallery across North London property types, terraces, semis and detached homes.
Choosing a builder FAQs
Questions homeowners ask before appointing a builder
What's the difference between FMB membership and Which? Trusted Trader?
FMB membership is an industry accreditation with vetting and a code of conduct at joining, and it obliges the member to take part in FMB's free dispute resolution scheme if you raise a formal complaint. Which? Trusted Trader involves an independent assessment plus ongoing monitoring of customer feedback. Both give you somewhere to turn if a dispute arises, which a builder with no accreditation does not.
How does the FMB dispute process actually work if something goes wrong?
You first raise the issue in writing directly with the builder and allow at least 10 working days for a response. If unresolved, you can apply to FMB's free Alternative Dispute Resolution scheme, which uses trained officials to attempt telephone conciliation based on your written contract and documents. This isn't legally binding, so if it doesn't resolve the dispute, you retain the right to pursue small claims court or other formal action.
Should I always choose the cheapest quote?
Not automatically. A quote significantly below every other builder you've spoken to is far more often a sign of corners being cut, missing scope, or an intention to add costs later, than of genuine efficiency. Compare itemised quotes rather than headline totals.
Who is responsible for the party wall process, me or the builder?
This should be agreed in writing before work starts. We coordinate the party wall process directly for our clients, but not every builder does this as standard, so it's worth confirming explicitly, along with who covers the surveyor's fee if a formal award becomes necessary.
What happens if hidden problems are found once work starts?
Older North London properties, especially Victorian and Edwardian terraces, often reveal surprises once walls are opened up, such as unrecorded drainage or inadequate existing foundations. Ask upfront how these will be identified, priced and approved before any additional work proceeds, rather than discovering the process mid-build.
Is Firtree Build accredited?
Yes. We are members of the Federation of Master Builders and a Which? Trusted Trader, based in Enfield and building extensions and loft conversions across North London.
Vetting builders for your extension?
Ask us the questions above
We're happy to talk through our credentials, insurance and process before you commit to anyone. A free site visit comes with no obligation.
