Should I pay my builder a deposit or money up front?

The discipline is money follows work. Some requests for early money have honest roots - long-lead materials, a joinery deposit, a small mobilisation sum - but every pound paid ahead of the work is a pound of your protection handed across on trust, and it should never travel bare. If an advance is genuinely needed: get what it buys named in writing, tie it to the contract rather than a conversation, make sure anything it purchases is identified as yours and covered while it sits off site, and keep the amount small against the whole. A builder who needs your money to start your job is telling you something about their position; a decent one will understand the caution, because they have seen where the alternative ends. When in doubt, the request goes in writing and the answer comes back the same way.