I often use Alternativeto.net to search for opensource/free software similar to commercial software i am familiar with.

http://alternativeto.net/software/omnig ... rm=windows

Although i have used Inkscape before and recommend it also for SVG, though i have never impmorted a 3D / 2D floor plan into it...

perhaps also this thread may help regarding the idea of 'layers'

http://www.sweethome3d.com/support/foru ... hread,3563