Climate Zone 4, Blue Ridge Mountains of SW Virginia

I’m working with an existing SOG garage with apartment over it. It sits 8′ away from an existing basement which was built with only the first floor framing completed with the anticipation of building over, connecting to the garage/apt. The first floor level of the basement is 12″ higher than the existing garage slab. I want the first floor to be the same level throughout, but wonder what the best approach is to elevate the floor of the garage, assuming the framed walls around the garage and floor framing supporting the apt. above remain. If I wasn’t trying for a flush main floor throughout a vapor barrier, sleepers, and rigid insulation would seem to be fairly straightforward, but creating what would in essence be a 10″± high crawl space or underfloor plenum seems to present problems of various kinds. I admit to being only savvy enough to have an idea of possible problems (pest control, airflow, insulation, dry rot(?), etc. but I’m not savvy enough to know the technical dynamics or possible solutions. The only thing that readily comes to mind are the raised access floors you see in data centers/computer rooms. Any suggestions?