Build a large amount for stilts in a square pattern at least 10 wide and 10 long. Click the second level and put flooring on top of the stilts and walls around the top. Place stairs going up to the second level. Have your Sim go up the stairs, then remove the stilts. Enter build mode and delete the stairs and the house will disappear. Rebuild the stairs and click on them. The game should tell your Sim to go down. When have him or her go back up, they will disappear at the top.