Basically, your builder has a mark up on all the costs that he spends, you will have no control of the final outcome. Better enter into a fixed price lump sum at the start based on tender drawings
Unless you are an expert in building costs your self, this is asking for trouble.
Cost plus contracts can be useful where there is some major uncertainty in the work, between parties where both are equally able to assess a bill of quantity.
Why would a builder even suggest it to a retail customer ?