I added storage space to my old garage by putting in an attic. It was much bigger of a job than I thought it would be.
I added 6x 20ft beams. There were originally only 2 beams in the ceiling space of the garage. Then I put down plywood flooring on those beams, and some crude ceiling joists.
There’s not a lot of clearance between the flooring and the roof line, but every little bit of added storage helps.