I’m trying to get a gauge on how far I need to take the foundation / stem walls on a mudroom retrofit / rebuild.
Climate Zone 6 / Montana
Other context info here:
https://www.greenbuildingadvisor.com/question/mudroom-retrofit-questions

BUT, the gist of it is this:
– I am adding a foundation stemwalls under a mudroom that had been built on a slab. The slab extended from interior to exterior on all three exterior sides, going past the exterior face of framing by anywhere from 1′-5′.

– None of the studwalls going on these new stemwalls are going to really be loadbearing.

– On the driveway side which has a ±1′ roof overhang above it (single story), I was able to dig down 24″-28″ below grade, pour an 8″ stemwall (no footer) that extends 6″-8″ above grade. Not ideal and not perfect, but much better than what was there. It’s getting 3.5″ of XPS on the foundation and 3″ of EPS or Polyiso over the sheathing, with a rainscreen gap.
‘Scenario A’ in the attached image.

– On the remaining two sides, the slab extends out past exterior face of framing to form a concrete sidewalk. The slab has a few big cracks on the interior side of the mudroom, and the exterior side slopes away from framing. probably 1″ over 5′. These last two walls are protected by a ±5′ overhang on one side, and ±14′ overhang on the other (under a covered porch).
‘Scenario B’ in the attached image.

Question:
How necessary do you think it is to dig 2′-3′ down on the sides that are covered by massive roof overhangs?
I can’t see that soil ever getting wet – if it does I have other big problems – so it seems like the odds of frost heaving are low.  It would be amazing if I could save the backbreaking labor of digging 2′-3′ deep for an 11 1/2″ wide trench cut in between the slabs. I’m thinking I could basically make it an 18″ trench or something, and it would act like a grade beam.

Am I totally off here? Or should I just commit to the fact that my life is now digging and concrete until I die?

Caveat: This is a retrofit on an 1880 house that has been neglected for 144 years but is mostly in okay shape that I’m rehabbing at a glacial pace. Not an ideal situation, but I’m one guy working nights and weekends on this, so any place I can save time without it getting too half-assed makes a huge difference. To me, at least.