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.
We have a bazillion Cape Cod style houses around us. They went up like weeds in the post WWII building boom.
They were originally two bedrooms and a bath downstairs and an unfinished attic upstairs.
The joists on the 2nd floor are also 2X6.
About 7/8ths of them have had the attic finished off. Plywood over the joists & carpet on top - drywall right over the wall studs & on the ceiling - with little to no insulation - heating was cheap in the 40s/50s and 60s.
You bounce as you walk in the upstairs....
My granddad always said, :As one door closes, another opens".
Wonderful man, terrible cabinet maker...