I am planning to build a 12X12 home office in my attached garage. The room will be between the home wall and the garage wall, in front of the parking area. I want to raise the floor to the level of the house floor and move the fire wall with fire rated door to separate the parking area and the new space. Since the space is between to walls I am thinking of installing a 10 inch ledger on each wall and span 2x10 joists between them. I will use at least 3/8 lag bolts to install the ledgers and joist hangers to attach the joists. Will put down 6 mil plastic over the concrete and install insulation between the joists. My next step is to sketch it up and apply for a permit. Anyone see a problem with my plan?
I'm assuming you're lagging the ledgers into the walls studs? No reason you can't do that, but you might consider something else as an option. If you remove a little sheetrock from the existing walls at the floor level, you could bear the new joists directly on the foundation. Depending on the height, you'd add short studs under the joists up against the wall studs to support the joists, and could nail the joists to the side of the existing wall studs. If the new floor will be low enough, you could bear the joists right on the garage walls' bottom plate. It would require a little blocking and replacement of a small amount of sheetrock either way, but would be a lot stronger in the end.
I wouldn't go with anything larger than a 3/8" lag if you choose the ledger option.
Glad to hear you're aware of the fire membrane requirement between the garage and the habitable space. Good thinking to relocate that fire door.