It is usual practice for the project design to be finalised and agreed before tenders are obtained or a building contract is signed. The advantage of this is that the owner knows what they are entitled to expect; contractors know what they are to provide and a lump-sum price can be set. However, it is almost inevitable that there will be changes to the original scope of work. For this reason most building contracts allow for variations and adjustments to the lump-sum price.
Related note: Latent conditions