This isn't always necessary, and something I'd want to avoid: old houses aren't designed to carry additional steel beams, getting them in is disruptive and expensive, it's a big material/resource impact. I've worked on quite a few loft conversions with reinforcement in small plywood and timber elements (joists, floors, webbing where bits of trusses are removed) and it's a much more pleasant process when all the pieces can be carried manually. You do need a willing structural engineer whose solution does not default to steel beams.
Regardless of steels or whatever, once you're getting a structural engineer in, reinforcing joists, etc then the cost is starting to head up for a not that usable space.
If you want storage then just board it over, add a striplight and don't put anything too heavy up there.
This isn't always necessary, and something I'd want to avoid: old houses aren't designed to carry additional steel beams, getting them in is disruptive and expensive, it's a big material/resource impact. I've worked on quite a few loft conversions with reinforcement in small plywood and timber elements (joists, floors, webbing where bits of trusses are removed) and it's a much more pleasant process when all the pieces can be carried manually. You do need a willing structural engineer whose solution does not default to steel beams.