What kind of objects were used to make the AutoCad model? Was it made in AutoCad Architecture? (formerly Architectural Desktop) In that case it has to be "exploded" before importing it. It is done by using the Export to AutoCad function from inside a 3D window (using a plan view generates a 2D plan). Or, if your application has it, use the "AECOBJEXPLODE" command.
When importing, use the "Merge coplanar faces" option in SU to avoid triangulation, and be sure you have the correct import units.
Anssi