I opened up about 300 sq feet over garage by putting a door in the back of a closet. What I found were 6” joists that held up the garage ceiling. I could walk on them but I wouldn't have used the space for living. I beefed up the insulation, laid flooring and covered the rafters with osb to protect my head from the roofing nails. I ran a couple of lights and it was a dandy storage area. I was very careful not to put anything heavy in there.
when we sold the house, I had that space excluded from the inspection. I’m sure it wasn’t up to code. I put a note in the disclosures that the area wasn’t suitable for living and to only store light things in there.