Tonight's Beta 9 adds support for levels at a negative elevation. Hope you like it. [:)]
As soon as you place an object, a room or a surface surrounded by walls at a level with a negative elevation, Sweet Home 3D will compute a hole that will surround the added item. Multiple underground levels containing items of different sizes are supported.

Note that the virtual visitor can't go below elevation 0, and I'm not sure I'll add that in the next update (keeping the visitor location in the home when underground isn't so easy).