Large second-floor extensions to a house, or even an entire house in the air, can be built totally unsupported by the following method. First, lay out a complete ground layer of pillars. You'll need lots to form a surface to build on. Next build your second-floor home on top of these pillars. It's important that you do ALL the structural work at this point; get all the walls placed and floors put in. Don't forget the stairs! Next, simply switch to the ground floor, and delete all the pillars. The house will remain standing, completely unsupported by anything except thin air! Voila! A castle in the clouds for your Sims!