Build a house on the wooden pillars and make sure you cover the entire area where you are building. After you are done, destroy the pillars underneath it and build stairs to get to your front porch.
Make two stories the same size as you want the top floor to be. The first has to be totally enclosed. Make stairs to the top floor then create the walls, roof, etc. You will not be able to change it later. Finally, delete the walls of the bottom floor, then delete the stairs. Your Sim will be on the top story by himself with no bottom floor.
Use pillars on the first floor to create the shape of the house. Fill in every space inside the shape, except a spot for a staircase. On the second floor, build the house. Enable the move_objects on code, go to the first floor, and delete the pillars. You should now have a staircase going up to a house that is floating.
Place a lot of columns around the ground and build your house on top of it. When you are done, delete all the columns. Note: Make sure you have a way to get to the house, or no one can get in.