Asking for a deposit before a major job is normal and legitimate. The FMB explains why: it covers materials the builder needs to order, subcontractors' fees and equipment. What matters is the size of it, what it is for, and how you pay it.
Two independent bodies, one ceiling
This is unusually well sourced for a consumer question, and the agreement is what makes it strong.
The Federation of Master Builders publishes it verbatim: "A 10% deposit is reasonable but do not pay more than 25%." It adds that you should probably walk away if you are asked for 50% or even more upfront.
Citizens Advice arrives at the same ceiling independently: aim to push the deposit down as much as possible, and do not agree to more than 25%.
A trade body and a consumer body, reasoning from different interests, landing on the same figure is a considerably better foundation than either one alone.
There is no legal cap
The 10% and 25% figures are guidance, not law. No UK statute or regulation limits the deposit a tradesperson may request.
A builder is entitled to ask for 100% upfront. You are entitled to refuse and go elsewhere. The protection here is commercial, not statutory, which is exactly why the number you agree matters. Nothing will reverse it afterwards on the ground that it was too high.
What a legitimate deposit is for
Both the FMB and TrustMark frame the proper upfront payment the same way: a contribution towards materials, not a payment for labour that has not happened yet. TrustMark's advice is to pay for work that has been done rather than by advance payment, while acknowledging you may reasonably be asked to contribute towards materials.
That gives you a question with a right answer. Ask what the deposit buys. "Materials for first fix, ordered on Monday" is an answer. "To secure the booking" is not, and a request for a large sum described that way is asking you to fund the business rather than the job.
Made-to-order items are the clear exception. If you have specified timber windows or a bespoke staircase, expect to pay a deposit against them, because the builder cannot resell them if you change your mind.
Never pay a deposit in cash
If any part of the price goes on a card, make it the money that leaves before the work does.
Under section 75 of the Consumer Credit Act 1974, where a purchase is financed partly by credit, the card issuer is jointly and severally liable with the trader for misrepresentation or breach of contract. The claim covers the whole cash price of the job, not the amount that went on the card. The thresholds attach to the price of the work: it must be more than £100 and not more than £30,000. There is no minimum card payment anywhere in the section.
So a small deposit paid by credit card on a £20,000 job brings the entire £20,000 within reach of a claim against the card company. The Financial Ombudsman states it directly: it is the cash price of the goods or services that matters, not what you paid on your credit card, and section 75 applies even if you only made part of the payment using credit.
This is at its most valuable in the scenario people assume defeats it. If the builder stops trading, the card issuer remains liable and recovers from the trader itself.
Pay the builder's business directly rather than through a third-party payment processor, which can break the chain section 75 depends on. And note that a debit card gives you chargeback, which is a card scheme process rather than a legal right, with a much shorter window and no enforceability.
The same deposit handed over in cash protects nothing at all.
Structure beats percentage
Getting the deposit right matters less than getting the schedule right.
Citizens Advice recommends paying in stages, because it means problems can be put right before you make the final payment. The FMB says the written contract should include the cost and a schedule of payment, and that you should follow it. Subcontractors such as electricians and plumbers commonly want paying in two stages, on completion of first fix and on final completion.
Tie each stage to something you can see and verify, and let the final payment fall due on completion after snagging. Citizens Advice also suggests buying the materials yourself, so that at least you own them if something goes wrong, and asking whether deposit insurance or an escrow arrangement is available.
Get a receipt for everything
The FMB is explicit: ensure you get receipts for all deposits paid, and record deposit payments in writing.
An undocumented advance payment is the hardest money in any dispute to prove you ever made.
A deposit is a contribution to materials, not a leap of faith.