The basement will be considered the 1st floor. Whatever sets on top of it will be the 2nd floor.
When you have your walls drawn on the 1st floor do this
Edit> Select All
Edit> Set Object Elevation. Note: (Mac Computers: Edit> Elevate Object)
Type in -7-0 and click OK
What this does is take out the 8ft default wall setting and sinks them into the ground 7ft.
This leaves 1 ft of the walls above ground.
After you draw the 2nd level its elevation would be set to 1-0 ( 1Ft above ground).