macfrog, I had a similar idea in mind for a time because if you drop a chair on a tree, its elevation will be equal to the tree height making it disappear most of the time from the 3D view. My idea would be to add a "preferred elevation for objects placed on top" property to each 3D model available in the catalog: for a tree this value would be 0, for computer desk that includes shelves, it would be equal to the desk elevation itself.