Make sure, too, that the builder has the correct experience and qualifications to take on the task. Agree costs. Make sure as well that you're paying just for the builder you have chosen and not for sub-contractors (unless agreed), as otherwise this could have an impact on the final bill. Ask about VAT, and make sure you know where the builder is buying his materials from for the job. If it suits you to do so, pay for work either in stages as it's completed, or pay for the whole job in a final settlement after completion. Always ask for receipts and keep a written record throughout the course of the work.
