I have found quite a bit of info on FPSFs and there is always a concrete slab included. I’m wondering if I can pour a Frost-protected shallow footing without a slab? I would build a small sealed crawl space inside the footing and build a wood-framed floor just above that?

I’m exploring options because I just took apart a floor in a timber-framed addition in climate zone 6. I found that the sills are on dirt or piles of rocks, and are rotted mostly beyond repair. One of the corner posts is fully floating above beams that have totally rotted away from the corner to nonexistent. I need to remove the sills and install a foundation, and I’m exploring options that leave at least the roof and some walls intact. The floor structure is on or barely above grade, so I don’t have much room for foundation options.

I can of course pour traditional footings at frost depth, but I was hoping there was something a little less invasive. I can’t raise the framing because it’s attached to the house on one side with a full field stone basement, and an apartment addition on a slab on the other end. The goal is one continuous floor throughout, with the kitchen spanning the connection to the main house.

I don’t think I have the clearance for piers, as there would be little to no open air underneath and likewise not enough space to build skirting and seal a crawlspace. But if I tried to pour a mono-slab FPSF I think it will end up a full step too low to match the floor in the rest of the house. If it’s too low, how high could I build up the interior grade? Could I just pour the slab and then wood frame a floor on top of it instead?