Some time last year, I was taken by a bug (this seems to happen often) to design some 6mm-scale buildings that could be easily stacked to reach any height desired by the user. Attached are some photos of how that initial effort turned out.
The black infantry bases seen in the pics were designed digitally by me, but are the exact dimensions of both the old and new Epic 40K infantry bases (20mm square and 10x40mm). I designed the buildings to allow them to accommodate both types.
For a lark, I added a fire escape to this one:
I also have made some modular staircase-strips for modelers to use in converting everyday objects to 6mm terrain pieces. These will be available on my Shapeways store soon. The silhouettes are 6mm from nose to toes.