I'd like to build a bumpout for a fireplace and have a few questions on the structure requirements.

The bump-out depth needs to be 20" to accomodate the fireplace. The I-beam floor joists (i-joists?) run parallel to the wall which needs the bump out.

What are my option here to support the bumpout if I can't cantilever the floor joists due to the parrellel joists supporting the current floor? I don't really have the option of cutting into these i-joists and doubling them as they are 24' long.

Can I attach the floor joists of the bump out to the house's rim joist with galvanized hangers and then nail on a 2x6 from the furthest edge, 45 degrees back to the current house wall and build the bump out wall around this support?

The fireplace is a mendota dxv45, gas burning. Approximate weight 200lbs.

Another idea, could I sheath the exterior wall of the bump out in plywould instead of OSB and try into an existing stud and use the sheet of ply as a hanger/support for the floor?