well basically. the red line is the section plane. what it does in sketchup anyway, is everything behind it (as in everything on the side of the camera) dissapears, so you can see into the building as well as being able to see the thickness of the building.
id rather not do this through boolean modifier as id like to remove the section after i render it so im left once again with a whole building that i can make a plan view (this would mean making a horizontal slice through the building and removing the top. again id rather not do this through boolean modifier as id like to take it of again.)