Plan...? In Space Engineers I've never had more then a vague idea of what I wanted to build before I build it, this has resulted in... mixed levels of success. But I kinda work that way, once I've got a base model that works I tend to reproduce that one with modifications depending on what I need it to be.
However, building giant capital ships/stations with lots of people will probably require a more focused vision (or not, that could produce something interesting too!) and I'm hoping there is an in game option for planning purposes to do just that. Something akin to creative mode, I believe the devs mentioned this as a possibility but I could be wrong.