24Wx32L Garage Build

im actually in the planning stages of building a carport (maybe convert into shop later). my biggest hurdle is going to be the slab.

i am planning a 24' wide by 28' deep slab...however, from what i have been reading, is it true that it is absolutely crucial to have the building plans FIRST, (where the roll up door will be, and the man doors, building lay out etc, etc???) because, I am not SURE what kind of building I want...

I figure, if I have the slab first, I can build whatever I want on top of it...