I was having the same problem when trying to build sunken rooms...the stairs either go miles into the ground or give me the 'terrain' constraint terraintile' message. After messing around with it, I accidentally discovered that it works for sunken rooms if you delete 1 grid of wall (any wall in the room with your stairs; you can replace it after finishing them) to delete the ceiling over that room. You can replace that part of ceiling by drawing walls on top of that room to cover the entire open room. (Those walls can also be deleted afterward if necessary.) Without the ceiling/foundation, it seems to give you more freedom without the funky problems! Hope that helps!
