It's not really authorized by code to have combined attic spaces of homes with attached garages because of the danger of automobile fumes reaching the living space. If I decide I need the combined space with the garage it means any carbon monoxide from a running engine would automatically be pumped into the living space. Soooo, I would only do it because the garage is used for storage only, as most one car garages are around here. Once I convert that attic space then no running internal combustion engines would ever be allowed in there. As is often said, kids, don't do this at home.
PS. I would only do it if it was found that the removal of roof space for solar electric required it. It might not be needed.