The idea of LEGO CAD is quite amazing. CAD refers to Computer Aided Design. In terms of designing Lego structures, machines, and objects on your computer, it is very possible using several software applications created by both LEGO and third parties.
LEGO's own software
, Lego Digital Designer is no longer being supported. Yes, it could still be downloaded (with errors until you enter offline mode). If you want to avoid errors etc.. we recommend LEO CAD (open source alternative)