Garage ceiling joist support for studio apartment?

I bought a house with a 24×24′ attached garage. The rafters above the garage has a cape cod style. There’s already a door in the attic space leading to the outside (2 stories up with no staircase….) The previous owner told me he was planning to build an apartment space up there, complete with mini kitchen and bathroom. Seemed like a fun project for me to finish. However, looking at the garage ceiling it appears the joists are only 2x6s spanning 24′ running parallel with the garage door opening, and I don’t see how there’s any center support. I can’t fathom how the ceiling isn’t sagging or how someone planned to build a whole apartment on top of that. Does this sound right to anyone?