Where I live, in the mountains of Southern
California, just about every building site is sloped.
In 30-odd years of building foundations here, working on
hillsides has become second nature to me.
In this climate, the ground never freezes more than a
few inches deep, so there's no need for a deep frost footing.
On flat sites, all you need is a shallow trench around
the perimeter of the building, a simple perimeter form,
and you're ready to pour the slab and footing in one shot
(see "Monolithic Slabs on Grade," 3/95).
On many sloped sites, we do much the same thing on
the side of a hill. We build stemwall