there's a simple discission of this problem
First, you need to activate
special
menu. For windows:
start - run - regedit
In this window you need to follow the next path:
hkey_current_user\software\graphisoft\archicad\"your version of AC"
Than you find "special menu" in the right side and type 1 there
Now open Archicad and figure out the new "
special
" menu in the menu bar. Here you have to enable "
show penetration lines
" and relax
Now you are ready to build your elevation