Thanks for the advice!

The slab is about 12-14 inches above grade and feels like it averages
only 4 inches below it. I'm shooting at 16 as a typical depth, although
i really haven't done enough excavation to know that for sure.

I didn't think it was structural and doesn't appear to have piers under
it (like my front porch does) although, again, i've only done a cursory
check for such things.

We're just outside of boulder colorado and for foundations i'm pretty
sure that the frost line requires that foundations be 32" below grade.

Graham