An exporter into Second Life You could rapidly construct a mansion in Sketchup, then do all the interior design inside SL.
For bedroom designs that require the alteration of the physical space, such is the case when a closet or trim is added, a quick mock up in AutoCAD or SketchUp may be useful.
Google SketchUp is a free 3D modeling environment where you can build objects, vehicles and even characters.
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