Quintain,
If your purpose with the model is to place it on GE there's no real need to model it from inside, too (navigation in GE won't really let you "go in" anyway). Therefore there's no real need to model the floors either.
Also, since you cannot "cut through" the GE Terrain, moƶdeling a basement (or any kind of undergound structure) is simply useless.
If you want to model "realistic" buildings however, with the floors and walls (thick walls - not just single faces) as well as inside walls, have a look at these tutorials here: http://www.sketchucation.com/forums/scf/viewtopic.php?f=20&t=723 and http://www.sketchucation.com/forums/scf/viewtopic.php?f=20&t=752