Never pay ahead of the work

The one rule that protects a self builder more than any other: money follows work, never the other way round. Pay for what is built and stands on site, or what the contract genuinely makes yours, and treat every request to pay further ahead - a large deposit, next month's materials, a lump to keep things moving - as a question that needs a proper written answer first. A builder paid ahead of the work has your money and their own priorities; a builder paid in step with the work has a reason to keep building. If a request will not survive being put in writing, that is the answer.