I build on my table, or on the large, hard bed in my room, depending on how long I am going to build. Since my room is at the same time my TV room, computer room, bedroom, study room, and Lego room, where I keep all the sorted parts in pretty organized drawers it is very hard to prepare surrounding for building.
But when I do, I sit on the table, place my drawers around me, so I do not have to get up during building.
Or if I do not have much time, I just place MOC on the bed and some most necessary parts for building it.
Of course, I spend a lot of designing (80%) in MLCad.
I keep my MOC on the closed shelf and covered with something, to ensure no dust will catch on it.