Framing dropped addition subfloor using hangers off sill plate – GreenBuildingAdvisor

I’m putting an addition on a house that has a “dropped” subfloor. The original floor joists do not sit on the stub wall. The exterior wall sill plate sits directly on the stub wall. The subfloor assembly is entirely supported on interior piers, and the top of the floor joists is flush with the top of the sill plate. The decking stretches uniformly across the subfloor joists and the sill plate. So the subfloor assembly is “Floating” inside the concrete stub wall, except for the decking which extends over it. 

The addition is a simple 16×24 U-shape against the house, but the house foundation in that location is a complicated shape because of a bay window. 

For my addition, I need to match the stub wall height because of the site elevation, so conventional platform framing is not an option…the wood would be in the dirt. So I also need to have a “dropped” subfloor. I don’t want to use piers on a tiny 300sq ft foundation, so I need to fasten the joists to the concrete wall somehow.

System 1: Use ledgers bolted to the concrete all the way around, then use face-mount hangers on that. The city already approved that because bolted ledgers are prescriptive. They only said I need to use 1 bolt per 11 inches (which is a lot) and approved hangers.

System 2: hang off sill plate. Use an oversize sill plate, 8″ wide instead of 5.5″ wide, so it covers the whole top of the concrete stub wall. Use as the sill plate for the exterior wall in the normal way. And then hang the floor joists off of the same board using top-mount hangers. The problem with this is it’s unconventional.  Simpson specifically lists the hangers for “2x nailers on top of concrete walls”. But it doesn’t say you can use a single board both as the sill plate for your wall and as the nailer for the floor joists. The permit office does not find this prescriptive so I would need an engineer to approve it. I’m completely confident in this, but I’m not the one who has to be convinced. I’m trying to get this approved prescriptively without hiring an engineer for a small project.

System3: Use a pony wall inside the concrete foundation bearing on the footer. This would actually be cheaper and easier than bolted ledgers.

The problems with system 2 and 3 is that system 1 (ledgers bolted to the concrete) is the only system guaranteed to work on the old-work side, to attach floor joists to my existing foundation. If I have to use bolted ledgers on that side anyway, I may as well stick with the bolted ledgers on both sides. 

Other ideas are to pour a completely new wall (doubled wall) on the house side of the foundation, then we can put in a brick ledge for a pony wall. Or we can cast in an interior brick ledge and set the joists directly on the that instead of needing a pony wall up from the footer, but again, that’s only relevant to the new wall…I still need a way to connect the joists to the house unless I pour a new wall on that side as well (which is a complicated shape). So even though a bolted ledger is my least-favorite option, it works on both the new and old work, and it’s approved without engineering, so I’m leaning toward that. Does anyone know how hard it is to drill 50 3/4″ holes in concrete, if you rent the right drill and bit? I honestly don’t know.