According to my experience, it is quite common to conduct the sampling only after the contract has been concluded. The house design, BLV, construction contract, and offer are part of the contract. If you choose a good house builder, where as many things as possible are specifically named in the BLV and are of a higher standard, then very little can go wrong. Additional costs always occur, of course, e.g. more power outlets, more light fittings, more TV wiring, or an extra hot water connection, etc. But a reputable builder like mine also tells you this in advance and specifies the preliminary costs.