Do you have the coop already? I can see a problem with moving the coop. You want 18 chickens, and that means a larger coop.. especially lets assume you go over and end up with 20 chickens. So this coop will be around 10X10 to house that many birds- I suppose you could fit them into an 8X8 since the grazing area will be so large. But even with an 8X8, think about how HEAVY that coop will be! I'm not so sure you'll be moving that thing around- if at all. That is a lot of lumber, and depending on what kind of walls and floor you put in- I couldn't even begin to guess the weight of that thing. I really like the suggestion to put the coop in the middle and either make 4 doors to open access to whatever area you want opened to- or to just open the entire area up to them. Maybe even build the coop a bit larger than you intended to accomodate this.