What I want to accomplish is to lift the entire plan 13 feet above the ground and then place my triangles and other terrain objects around the elevated house.

Nice design [:)] Maybe I miss something, but it looks to me as if you can go in and add your 13 feet to the elevation in all level tabs. Insert a new level 0, on which you insert a triangle for the slope (and box for the flat terrain 13'). If there is a basement, the geo-shapes should go around the foundation. ok