The more floors your house has, the more skill is needed; see the table below for the required skill levels. Only the first (ground) floor planning needs 1 [[carpentry]] skill per wall and 1 skill per tile. With 20 skill a roof may be placed over the first floor, but not an opening or floor above. For upper floors, the roof counts as a floor (although it may have less of a skill requirement than an opening or floor above - unconfirmed).
*update stone wall require [[masonry]] to make upper floor.
*You may have a total of 16 floors (counting ground level as the first floor)