What I started doing with SU was to draw my plan out in PS, saved as jpg, then import that into SU (make sure you size it correctly) so I knew exactly where everything was and just drew out the outer lines and inner wall lines (that form the rooms and corridors), and then lastly use that raise tool thingy and just pull the walls up leaving the rooms on the ground. Works great if everything is on one level but adding more levels adds complexity and confusion so I did other levels as new models then grouped everything and imported that back into the first model.