The short answer
For a traditional build, little or nothing beyond specific materials ordered for you: pay in stages for work that is done. A modular maker has a fair reason to ask for more, because it builds your room before it reaches your house. Pay that deposit only against a written contract and a payment schedule, and keep a good share back until the room is installed and signed off.
The safest rule for paying a builder is simple: pay for what has been done, not for what has been promised. Modular extensions bend that rule a little, and it helps to know why and by how much.
A traditional build: stages, not deposits
With a brick and block extension, the builder spends money on site as the work goes on. A sensible schedule ties each payment to a stage you can see: footings poured, walls to plate, roof on, first fix done, finished. A request for a large share before anyone has dug a hole is a warning sign. The exception is a specific item ordered for you that cannot be returned, such as made-to-measure glazing, and even then you can ask to pay the supplier directly.
A modular build: why the deposit is larger
A modular maker builds most of your extension in its factory. It has to buy timber or steel, insulation, windows and doors, and book a slot on its production line, all for a room that fits only your house. It is fair for it to want a meaningful payment before it starts.
What matters is how that payment is protected and how the rest is spread.
A payment shape that protects you
- On signing: a deposit to start design and order materials, set out in the contract.
- During production: a payment when the module is substantially built, ideally after you or your representative have seen it in the factory.
- On delivery and installation: a further payment once the room is craned in and weathertight.
- On completion: the balance once snagging is done and building control has issued its completion certificate (or, in Scotland, the council has accepted the completion certificate).
Keep a real sum, not a token one, for that last stage. It is the only lever you have to get the final details finished.
Before you hand over any money
- A written contract that names the price, what it includes, the payment stages and what happens if either side pulls out.
- Deposit protection. Ask whether the deposit is held in an escrow account or covered by an insurance-backed scheme, and get the answer in writing.
- Who owns the module. Ask whether the materials or the part-built room become yours once paid for. If the maker failed mid-build, this matters.
- A company you can check. Look it up on Companies House, and ask whether it belongs to a scheme such as TrustMark or a trade body such as the Federation of Master Builders.
Card or transfer?
Paying part of a deposit by credit card can give extra protection under consumer credit law, within limits. Bank transfers to a business account named in the contract are normal for larger sums; never pay into a personal account because you were asked to.
The modular extension cost pages set out what a typical job costs, so you can see whether a deposit is a sensible share of the whole.