As I hate the SEOs ...
Use the Primitives, definitely. Set the basic parameters, including the shell thickness. Then, marquee the area that is your actual plan. THe marquee can be any shape - if you can't draw it, use Magic wand. See it in 3D from the top , in plan view, save as object.
Archicad will remember what is in the marquee, the rest will not be seen.
You can do this method to do any cookie cutter crazy object anyway ... I seem to remember that there is a small article by Dwight of the LightWorks fame in the first CookBook? ...
Djordje
ArchiCAD since 4.55 ... 1995
HP Omen