I am very suspicious of the process you describe. If the "rooms" arrive in pre built wall sections that would be one thing, but if he is building entire rooms and trucking them up to your house, I really have to question the integrity of the construction. Make sure you have contacted your local building code enforcement agency and discussed the process with them before you start spending any money.
I don't know how to give you the name you are looking for, but that was my 2 cents worth.