How deep is the frost line for footers in Michigan?
Historically, the frost line has been 42" in Leelanau county, in northern Michigan (http://www.leelanau.cc/howdoia.asp?rid=12).  But with the severe cold spells in the past couple of years, where even southerly Kalamazoo, frost has reached down 5 feet in some places, MI municipal code officials may adjust that number downward one day soon.