How can I determine if a prexisting slab on grade is frost protected?

Question

I have a detached garage that was built sometime in the 50's. At some point it seems that an addition was made to it to create a shop. There was an old oil burning stove in there as well on the garage side.

I would like to convert the garage to an ADU but the county says I probably can't do it if the slab on grade (they don't think it is) isn't a frost protected one. I'd like to know if there is any way that I can simply check to see if it was in fact built with frost protection (but I doubt it). I have attached a picture of the building to give you an idea of what it looks like (yes it has asbestos siding I know).

I'm in climate zone 5b. Thank you for your help and consideration.

Picture:

Your picture didn't get added. Please [edit] to include it. – FreeMan Nov 16 '22 at 18:21 — FreeMan, Nov 16 '22 at 18:21
Sorry about that, picture now included. Thank you. – LuckyURE Nov 17 '22 at 19:59 — LuckyURE, Nov 17 '22 at 19:59

score 1 · Accepted Answer · answered Nov 16 '22 at 19:55

"Built sometime in the 1950's" == almost certainly NOT frost protected.

Dig at an edge and observe the lack of insulation. If a flock of pigs happens to have been flying North by Northwest overhead when it was built in the dark of the moon, observe the insulation. That's for a "frost protected shallow foundation" nobody was building 70 years ago.

Depending how your county defines "frost protected" the other thing you could more plausibly find in a 1950's slab would be a frostwall to a footing below frostline. You could even excavate all around and insulate that to make it more to modern standards.

How can I determine if a prexisting slab on grade is frost protected?

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