We typically suggest building on a crawlspace or, with a little more effort, on a full basement. These foundations create a warmer floor and ensure year round comfort in the new space. Slab on grade foundations also work well, but we insulate the foundation and often install "in floor heat" to truly make a comfortable living space. Piers usually are reserved for porches and decks that do not need to be insulated to the same degree as a house. If budget dictates an addition to be built on piers, we refer to it as 3-1/2 season porch. It’s difficult to maintain comfortable floor temperatures in Minnesota winters in additions built on piers.