Instructions
Click on the "Build Mode" icon, denoted with a saw and paint roller, in the lower left corner of the screen. Select the picture of the stairs to open options for building stairs in your house.
Click on the connecting stairs to attach stairs to a raised outdoor porch. The connecting stairs are indicated by a dotted line leading up to three stairs. Select the style of stairs you want for your house and click on the porch where you want the stairs to lead down from.
Place stairs inside the home by clicking on the full staircase icon in the stairs options. Select a style of staircase and click inside the house where you want the stairs to lead up from.
Click on the picture of the top house floor with an up arrow in the lower left corner of the game screen to move up one floor. Return to the building tool and click on the "Flooring" icon to place a floor on the second level. The flooring will automatically leave a space above the stairs.
Complete the second floor by adding walls, windows and other features until it looks the way you want it to. Click the icon to move up a floor and place the roof by clicking on the roof icon in the building tools.