Add On to Existing Building

We currently have ten hens in two chicken tractors, but the chickens free range in my pasture all day long. We'd like to build a proper coop so that we can add more chickens later. I'm thinking that I'd like something in the 120 sq. ft. range.

I have an existing shop that is 40 feet long and I'd like to add a coop onto the back of it. The shop is just a wood-sided pole building. I'll probably get a concrete pad poured because rodents will be a problem if I don't. Does anyone have any examples of coops like this that are built onto existing structures? I'm trying to figure out what materials I'll need and what kind of construction techniques to use.

Thanks!