If it is over 200 square feet you are not legally using a 12" footer.Now if you consider a 10' X 20' single story building "large" your statement is correct but here is the Michigan state building codes and a copy and paste if you don't want to scroll down thru the whole thing.
42" deep footer is what is needed to pass code in Michigan for buildings over 200 square feet.
https://www.michigan.gov/documents/lara/lara_bcc_2015_residential_code_502813_7.pdf
R 408.30522 Minimum depth.
Rule 522. Section R403.1.4 of the code is amended to read as follows:
R403.1.4. Minimum depth. All exterior footings and foundation systems shall extend 42
inches below actual grade. Where applicable, the depth of the footings shall also conform
to section R403.1.4.1 of the code.
Exception:
Upon evidence of the existence of any of the following conditions, the building official
may modify the footing depth accordingly:
(a) Freezing temperatures (freezing degree days).
(b) Soil type.
(c) Ground water conditions.
(d) Snow depth experience.
(e) Exposure to the elements.
Buildings under 200 square feet are not held to these requirements.
No inspector is going to go from 42" to 12" unless you are sitting on bedrock and I still doubt it then.If you hit good solid rock that requires a breaker to excavate around 30" they will often let that go but most likely will make you drill and rebar into that rock before pouring a footer.There is not a snowballs chance in **** are you getting away with only digging a foot deep.
I have some experience with building design and construction also and knew right away something was off about 12" footers in a frost area.