I have 7 acres and 3 coops. 2 of them are very small, 4x4-ish with attached runs, and they are in my backyard. One has my broody hen and the other has my DD's 3 favorite hens. I put them there because they are very close to electricity and water, and we LOVE sitting in the house and watching chicken tv. The other large coop is about 200 ft away from the house. It sits behind my large chain-link fenced garden area. I chose that area for a few reasons: Access to water, can add electricity later, far enough away from the house for the smells not to be an issue, close enough to the house for chores not to be an issue, ease of chucking vege's over the fence, and I "cheated" a bit on my run fencing by using one side of my chain-link garden fence as a shared side with the run.
As for putting anything over your leach field, even part of the run, I wouldn't do it. As someone else mentioned there are strict rules about septics and leach fields. We have to have ours inspected every couple of years and there's no way we'd pass inspection if anything was that close or over it. Your area may not be as strict but it's worth checking into.