BlueHen and Rosemary and Thyme,
Thank for sharing your knowledge. My coop is a very similar size to the one in the link one of you sent, and probably could be modified in a similar way, but there are several reasons I don't want to do that right now. The main one is that we have had many days this summer over 105 degrees, and that under coop run is the best spot for deep shade to keep them and their food and water cool in these high temps. During the day I leave it open to the larger run, and they often retreat in there during the hottest part of the day. Even with the shade tarps and the tree, I can't duplicate that deep shade in the bigger run, which is 87sq ft, including the under coop part. If I enclosed the bottom to make more coop space, I think it would be hotter in there. Second, in winter we have few days (SoCal) or nights below 40 degrees, so there really isn't a need for more warm indoor space. They really only use the coop house part for sleeping out of sight and one more layer of security from predators. Of course, we haven't been through a winter yet. If I can easily get all the poop out of their sleeping area (with the mats) every day, why wouldn't I? I live in a tract home in a city, so I will never be allowed to have more than 4 chickens, and the coop/run can never be anywhere but exactly where it is (25 ft. from all the neighbors). I hope to soon get a chicken tractor that will allow me to pasture them on the back lawn part of every day, and this will further diminish the concentration of poop. If they destroy the lawn, I plan to replace it with California native grasses, which take less water and require less mowing. So far I haven't found one with enough grazing space, and I'm afraid if I made a DIY one it would be to heavy for me to move around. I did put the excelsior mats in one nest box and in the pet crate nest box, and I can see that they've gone in to look around (feathers), but I don't see any poop in either of them. Interestingly, in the last few nights when I come out to remove the flashlight (they don't like to go up the ramp into the house if it is completely dark), I have found three of them on the skinny roost. Although there is plenty of room, the last one won't go up there, nor will she let me put her up there. Somehow they do learn what they need to know without anybody to teach them.