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getting the steels in
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.
I guess the big issue is getting the steels in. That's going to take some fairly substantial work regardless of the rest.