The basement will be considered the 1st floor. Whatever sets on top of it will be the 2nd floor.

 

EXAMPLE:

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).