I've always had another building which I converted into a chicken coop. My present flock occupies a couple of stalls of a horse barn and they are allowed to run in a large-fenced-in pasture by day, large enough so the grass still grows and it doesn't turn into bare ground, not even in the hot dry Georgia summers. The floor of the barn is hard clay, not an ideal surface for cleaning but OK for a backyard flock. Fortunately we don't have weasels and mink here, but it's proof against other predators. And in the event that an occasional hawk does show up, it's large enough to keep the birds penned up in for several days, or theoretically even permanently. If I were to build a coop from scratch, I'd make it as large as I could, large enough so I could keep all my birds in it comfortably if I had to (like if I wanted to go away for a weekend and have a friend care for the flock, or if I had limited run space that got muddy during a prolonged rainy period, etc.). Commercial recommendations call for 4 sq ft of space per bird but I'd shoot for at least twice that. And I'd opt for a solid floor, which gives added protection from digging predators and is easier to clean. I think a raised wood floor would work best, as it wouldn't make an attractive hiding place for rodents. Rats and mice can build tunnels and nests under cement floors. Ventilation is more important than insulation (which mice love to nest in). If you live in a cold area, I'd choose a breed adapted to cold climates. Most American breeds fit this bill nicely, and many of them were developed in parts of the country where the winters are miserable (Rhode Island and New Hampshire Reds, Plymouth Rocks, Wyandottes, Buckeyes, etc.).