How long does it take to find and vet a builder for an extension?

Allow two to eight weeks. Good builders are booked weeks ahead, so expect unanswered calls early on. The vetting itself, checking references, visiting past jobs, and comparing quotes on the same specification, is where the time goes.

Should I use the FMB, Checkatrade, or MyBuilder to find a builder?

Use all three as a starting point, not a shortlist. FMB membership involves credit checks and an independent on-site inspection, so it carries the most weight. Checkatrade and MyBuilder cast a wider net, but you still run your own reference checks on anyone they surface.

What insurance should a builder have before I hire them?

Public liability cover of at least two million pounds is the baseline. Ask to see the certificate and check it is current. A builder who cannot produce proof of cover is one to walk away from.

Is the cheapest builder quote usually the right choice?

Rarely. A quote well below the others usually means something has been left out or the builder has misread the job. Compare quotes line by line against the same specification before you judge on price.

Can my architect recommend a builder for my extension?

Yes, and it is an underrated route. Architects see builders' work from the inside and can tell you who delivers clean work and who cuts corners. Ask which they would use and which they would avoid.

Back to Single Storey Rear Extension

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.

13 min readUpdated 2026Free with email

Builder vetting process infographic showing five stages from sourcing candidates through credential checks, site visits, quote comparison, and final appointment, with red flag indicators at each stage

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.

What this guide covers

01Where to Look
02Main Contractor vs Managing Trades Yourself
03How Many Quotes You Need
04The Site Visit
05Questions to Ask Before Appointing
06Checking Credentials
07Red Flags
08The Deposit
09The Uncomfortable Truth
10Timing
11What Happens Next

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