Any of the pole barn manufacturers could design a building with a loft. I have seen horse barns with hay lofts that would work. 1200 sq/ft sounds a little cramped for a family of 4 but it could be done. One thing you would want to do is check on codes in your area, if there was a fire or some other disaster, God forbid, you could be in a lot of trouble. Aside from that, I see no reason why you couldnt do it. You might check with your local building supply, eg. Menards or Home Depot for pole barn plans. they will work with you in designing your barn.
Good luck,
Steve F