Use "all floors" marqueetool to choose your building.
Turn on / off the layers you want to have in your object.
Select "Use selection / marquee in 3d" (Button F5 on win)
Open "Perspective Setting" (button ctrl+shift+F3 on win)
Change to parallell projections
Change to top view
Myself I aslo place my camera Azimuth 270 degree.
And at last .
File -> Libraries and objects -> Save 3d model as ...