George,

The easiest way to create a multi-level house, is to install and use the CopyAsNewFurniture plug-in. See the tutorial.

Draw each floor separately, then use Tools --> CopyAsNewFurniture to copy an entire floor and stack it on top of another floor.

There is a tutorial on how to build multi-level houses, but it's a bit outdated. I just finished revising it, but it hasn't been published yet. Send me your email address and I will send you the revised tutorial.

Hans