And... finally... here's the great feature awaited for years: multiple levels management [:)]
Hope you'll like this first beta version which contains the following features:
- New Plan > Create level / Modify level / Delete level menu items to support multiple levels with possible intermediate levels,
- Made floors and ceilings transparent with the same factor used for walls,
- Improved magnetism of walls during drawing,
- Fixed minimum sizes change during importation avoiding to loose proportions.

This system doesn't support negative levels (below the ground level) yet, and I foresee to add menu items to choose between assembled levels and currently selected level in the 3D view.
I sincerely hope you'll find it easy enough to use. I'll update help later and will try to post a few examples.