What do you have under the topsoil? Is it clay or sand or rocky loom? Will you be putting 3/4 crushed rock and prepping it or just pack and build? Where does the water go when it rains and will you need drainage to keep it from under your foundation? What are you building your house with, is it going to be conventional or a out of the box build (earthbag or strawbale or something like that.) Save that topsoil to help build up your garden.