There are at least two ways to work around the fact that a Plan Region will not affect the viewing of Topography elements. From the AUGI forums (2007):
What about having a ‘cut-plane’ of the view above the topography and doing a plan region around your house? (ie: reverse the logic)
You could try creating a view of the toposurface from a higher vantage point that doesn’t include showing the building. Then overlay this view with the view of the building (overlay on a sheet that is) at the level you want without the toposurface. This way the two views give you the whole but permit the two different cut planes you really need. If you set the toposurface view to wireframe it should give you the look you want, guessing what that really is, though.