This video is dedicated to my 6 year old nephew Samuel that called me today on Facetime asking me how to build a second story on his house.
To be able to perform this action you have to be on level 17 and complete the mission multi story renovations and you can build it.
Click on the chair icon to open the home store. Go to the house section and select the floor construction tab just right of the room tab. The icon look like three sheets of paper floating on top of each other - the icon is trying to represent the three floors you can build.
A blue grid will appear marking out the new floor, it will also show the cost for the new floor.
Once you buy it you can place rooms on the second floor.
To reach the second floor you need a staircase in place. It is find just right of the floor construction tab in the home store and has a stair icon :)
The cheapest stair will set you back 30,000 simoleons. When you place the stairs make sure your sim can reach the second floor by marking any spot on the second floor. If the sim cannot reach it make sure the entrance to the stairs are not blocked by any furniture.