This was the other winter-project I recently completed. The coop came in a kit and I had assembled it in the fall but quickly found that I have too much wildlife that can dig under and which enjoys chicken dinners to let it simply sit on the ground so in December I took advantage of a good weather weekend and built the foundation for it which is comprised of 2x12 treated lumber that is rated for ground contact and then I poured 3" of concrete at the base of that to ensure nothing could dig under as well as to help anchor it in case of a really bad wind-storm (the 2x12's are anchored into the concrete and the coop is anchored to the 2x12's), then I buried the concrete under dirt and straw.
The hens aren't old enough to lay yet, as I purchased them as chicks back in December after something made chicken dinners out of my more mature flock. They're still pretty small and I'm a little worried they won't make it through winter as small as they are but so far they've been making it, though last night was the first cold night they've had since I put them outside a week ago.
Please pardon the mess of boards and rocks, I initially was going to use 2x6's and after placing them I wasn't happy with them and switched to the 2x12's, so that's why that is there and the rocks were just dug up when excavating for this and I haven't had a chance to move them where I want them.