Extension work is the largest sum most households ever hand to a small business, and it is almost always paid in stages against work that is hard to inspect. The checks that matter are unglamorous and take an evening: who the company legally is, what insurance stands behind it, what the contract says happens when something goes wrong, and whether the payment schedule ever puts your money ahead of the work.
Home extension builders: find out who you are actually contracting with
Get the full legal name and company number and read the record. A company incorporated last month with a trading name that has been on vans for years is a different risk from an established one, and neither is disqualifying, but you should know which you are dealing with. Check the registered address, the filing history and whether the directors have a string of dissolved companies behind them. This is public, free and takes five minutes.
Insurance and the work that needs a scheme
Public liability covers damage to you and your property. Contractors all risks or a joint names policy covers the works themselves while they are half finished and uninsurable under your own buildings cover, which is the gap most homeowners never think about. Separately, electrical work, and certain other notifiable work, has to be either signed off by building control or certified by someone on a competent person scheme, so ask which route the builder is using before the first fix rather than after.
The contract and the payment schedule
Use a written contract with a scope, a programme, a variations procedure and a retention. Stage payments should follow completed work, never precede it, and a large deposit before anything arrives on site is the single clearest warning sign in domestic building. Where the contract was agreed in your home rather than at the builder's premises, cancellation rights normally apply, and the builder is required to tell you about them in writing.
References from kitchen extension builders that are worth taking
Ask for two projects of your shape finished more than a year ago, not the newest one. A build looks perfect on handover day; what you want to know is whether the flat roof held, whether the snagging list was finished, and whether the builder came back. Ask the previous client what went wrong, because something always did, and how it was handled. That answer tells you more than any photograph.
Questions people ask about house extension builders
Is a trade association membership a guarantee?
No, but a scheme with a real vetting process and a complaints route is better than none. Check what the scheme actually promises rather than assuming.
How much deposit is reasonable?
Enough to cover materials genuinely ordered for your job, released against evidence. A large upfront payment for work not yet started is the most common way homeowners lose money.
What if the builder stops turning up?
Your rights sit under consumer law and your contract. Document everything in writing from the start, because a dispute is won on the paper trail.
Do extension builders in London really cost more?
They are usually pricing tighter access, longer material handling, parking and permit costs and more party wall work, which are real differences in the job rather than a different rate for the same job.