Ask about the quality of the builders' workmanship, costs, reliability, how long it took them to complete the job and whatever else you feel you need to know. Tradesmen such as plumbers and electricians have invariably built up a network of builders they'll have worked with over the years, so ask for recommendations from tradesmen contacts that you have and trust. If you know anyone involved in the building trade, such as property buyers and architects, then find out whether they have a reputable builder that they use.