Walk around the area (if other people have already built) and if...

Walk around the area (if other people have already built) and if they are willing to chat ask them if any unplanned costs incurred for the site.

As an example most build contracts will charge you more if they have to "Break Rock" for the foundation

Most builders in large scale land releases will know if it likely that there will be a lot of rock in the ground beforehand but probably won't tell you until it becomes a cost variation