Splitting up complex object into elements for manipulation

Hi there,

I am trying to simplify an existing complex 3D building model most likely created in AutoCAD.

When I select and highlight for example just a single wall it selects multiple walls and other building elements. This very complex object consists of several shared cells with meshes, lines, etc. spanning several levels.

My objective is to simplify the drawing, delete most of the elements and then work with the bare minimum of building parts (walls, ceiling, floor, etc).

How can I split up such a complex object and then delete individual parts?

Thanks in advance for your help!

Kurt