Phase 2 · Pre-Construction · Task 01 of 12
How to Find a Reliable Builder for Your Extension
Where to find builders, what to ask them, and the red flags that save you from a costly project going wrong. Practical vetting guide for UK homeowners.
Prerequisites
Complete these first
The builder you choose will determine whether your extension is a manageable six-month project or an eighteen-month ordeal. Not your architect, not your structural engineer, not your planning authority. Your builder. They control the pace, the quality, the subcontractors, and ultimately the cost. Projects that start at £45,000 and finish at £120,000 are not rare outlier stories. They happen because homeowners picked a builder too quickly, didn't ask the right questions, and didn't check the things that needed checking.
This guide is about reducing that risk. Not eliminating it (no amount of vetting makes a building project risk-free) but stacking the odds in your favour before you hand someone a deposit cheque and the keys to your back garden.
Do this first
Before you start looking for builders, two things should already be done. Your building control application should be submitted (ideally conditionally approved), and you should have read budgeting and contingency so you understand the full cost picture. A builder's quote covers their structural work only, not your total project spend. Knowing this before you compare quotes stops you making decisions based on incomplete numbers.
Where to Look
Start with people you know. If a neighbour or colleague has had an extension built and it went well, that builder goes to the top of your list. You can see the finished work, ask honest questions about timescales and communication, and get the kind of answer you'll never find on a review site.
Most people don't have a ready recommendation though, which means you'll spend a week or two working the phones. That's normal. You'll call builders who never call back. You'll email through website forms and hear nothing. Good builders are busy. Most of their pipeline comes from repeat clients and architect referrals, not homeowner enquiries. Don't take the silence personally, and cast a wide net early.
The Federation of Master Builders directory is the closest thing to a vetted shortlist. FMB membership isn't a paid badge. Every member undergoes credit checks, trading history verification, and an independent on-site inspection. That's why it means something.
External resource
FMB Find a Builder
Vetted builder directory. Credit checks, trading history, and on-site inspections.
fmb.org.uk
Checkatrade and MyBuilder cast a wider net. Checkatrade has verified reviews. Look for builders with 50+ reviews, not five. MyBuilder lets you post your job and receive approaches, but treat every response as a starting point for your own checks, not a pre-qualified lead.
External resource
Checkatrade
Builder reviews and background checks by location.
checkatrade.com
External resource
MyBuilder
Post your project and receive builder quotes.
mybuilder.com
Which? Trusted Traders and TrustMark (government-endorsed) are both legitimate but less commonly used for extension work. Worth a look if you're not finding enough candidates through FMB and Checkatrade.
Your architect is an underrated source. They've seen dozens of builders' work from the inside, who delivers clean blockwork and who leaves gaps that building control flags. Ask which builders they'd recommend and which they'd avoid. Most will be honest, because their reputation is on the line too.
Where Not to Look
Door knockers. Any builder who turns up uninvited offering work is a red flag. Legitimate extension builders don't cold-call. They're booked months ahead.
Be cautious with Facebook marketplace and Gumtree. Zero vetting. That doesn't mean every builder there is bad, but you're starting from nothing.
Main Contractor vs Managing Trades Yourself
Before you start approaching builders, decide what you're actually looking for.
A main contractor (sometimes called a general contractor) manages the whole build. They employ or subcontract all the trades: groundworkers, bricklayers, roofers, scaffolders. You have one point of contact, one contract, and one person responsible for coordination. Most homeowners use a main contractor for a single-storey extension, and that's the approach followed here.
The alternative is managing individual trades yourself: hiring a groundworker, then a bricklayer, then a roofer, then a carpenter, all separately. You save the main contractor's markup on subcontracted work, but you take on all the scheduling, quality control, problem-solving, and rework. If the bricklayer finishes a week late and the roofer can't start on the new date, that's your problem to solve.
Warning
Managing individual trades on a first extension is a high-risk strategy. You don't yet know the sequencing dependencies (which trade needs to finish before the next can start), you don't have relationships with reliable subcontractors, and you won't spot quality issues that a main contractor would catch. The money you save on contractor markup is easily eaten by scheduling gaps, rework, and the cost of your own time.
For your first extension, use a main contractor. You can always self-manage trades on your second project once you understand how a build actually runs.
What this guide covers
- 01How Many Quotes You Need
- 02The Site Visit
- 03Questions to Ask Before Appointing
- 04Checking Credentials
- 05Red Flags
- 06The Deposit
- 07The Uncomfortable Truth
- 08Timing
- 09What Happens Next
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