I have some suggestions: first is gm_mobenix_v3_final, wich is an awsome map for building/partying and more, tough its a heavy and big map.
second is gm_atomic, wich has huge build place, and its pretty awsome too, and also this map is huge and it doesnt have water so it isnt perfect for build.
i know also more maps that would be good like:
gm_imagination
gm_construct_2009
gm_botmap_v2
and a fun nice map that isnt rlly for building: gm_RDCinema_b4 =D
every map has its + and - so you decide lawl
A*B