It was a surprise to us to realize that the chicken run is more of a challenge than the coop itself, which we build inside our red barn by converting horse stall space. We were able to divide that space and had concrete floors with olrd horse planks over it. So the major challenge was sealing it up and installing accessories like the roosts, nest boxes, platforms, feeders, porch, feed room and water supply.
You have to be very cautious when building a run. It needs to keep predators out and that means no opening larger than 1/2". We roofed ours with clear vinyl and added an underground predaor flange. We also use snow boards and lock the run on both sides.