It's so easy!
Creating floating walls and rooms has long been a favorite “hack” for Sims players. In The Sims, it could only be done with the use of third party mods. In The Sims 2 and The Sims 3, you could create floating rooms by creating a room, building a room on top of it, and then deleting the first story. This would leave your second story hanging in mid-air.