You need a 'pad'. Pads in Revit cut the topo uniformly to a certain flat depth to signify the building 'pad', which is typically under the floor slab and foundation and is made up of gravel or tamped earth.
Look on the Site tab on the Design Bar and you'll see a pad tool there. Try drawing some over your topo and you'll see what I'm talking about. Then put a pad under your house's foundation, and the ground will be cut properly for your basement (just like real life 🙂 )
Good luck!