Near as I can figure it, the difference between the high corner and the low corner on the northern elevation is 7'8". If the concrete pier at the high corner is 4" above grade, then the wall supporting the undercroft will be 8' above grade. Which means that the floor of the undercroft will be at grade level -- just walk right in, no steps down from the outside.
But I also measured the fall across the face of the building site, from the low corner on the north to the low corner on the south, and here the ground falls another 11" plus a hair. Since this area is enclosed by the retaining wall I've been building, it'll be easy to backfill with dirt, raising the ground level to the same height as on the northern side of the building site.
So, it's all good. Next up, I'll dig out the hillside, level with the future floor of the undercroft. Then I'll dig out the trenches for the footers. Should be fun.