I'm building a 16' x 7' shed and chicken coop in Seattle. I want it to have a dirt floor and be safe from predators crawling underneath. I've been struggling with how best to do the "foundation". I saw a post where someone built one like the picture below. Basically an on-grade footer 6" wide, 6" deep, that the bottom plates get bolted to. I suppose this would float with any frost heave(?) I like the idea of something like this to prevent critters from burrowing under-neath and it seems easy enough to do. I wouldn't do it directly on grade, but a few inches below.
My frost line is 18" deep. If I do this with some 2x8 molds and put a piece of rebar in the molds do I need to worry about frost heave causing problems? After I dig the trench and place the molds should I put down a few inches of gravel before the pour? Would it help to have the molds a few inches above the bottom of the trench and let a bunch of concrete come out underneath? Any better ideas?
A lot of other people seem to do something like this with CMUs, but that seems like it would settle unevenly. I know I'm probably way over-thinking this given its intended purpose, but want it to last.