We bought a 10 x 10 wooden shed and modified it. It is placed on a sloped part of our property, so we have a walk in coop with space underneath it for the chickens to get under to get out of the heat. I can crawl under it ( when I have to to get an errant chicken or chick that didn't go in at night) but I don't like too because it is usually snowy or muddy when a chicken decides to stay out all night.

In this coop we put in interior nest boxes ( my hubby wanted exterior nest boxes but I said no). Our other coop has exterior boxes. We only have 2 windows. I wish we had either made the windows larger or put in more of them. We have added a ventilation fan on the roof. I wish we had put poop boards in under the roosts too. We did have a friend that wired the coop for electric, so we have lights and during winter I can use a heated dog water bowl in there. In the summer, I also use an oscillating fan to get the air moving. I used a waterproofer on the floor and insulated the ceiling and walls. Mice have gotten into the walls though, so I wish I had done something that would have kept the mice out.
I wish I had an automated pop door!!Also, more ventilation would be great!
Our 2nd coop, my hubby built from scratch. It has a dirt floor. 2 large windows and a lot of narrow ventilation under the eaves. It has an attached small ( 5 ft x 5 ft or so) run. I

that aspect of it!! That way when the weather is bad and we don't let them out into the fenced in chicken yard, they still get some "outside" time. One mistake that we made was that on the attached run we did not use hardware cloth. We used hog fence (or something) that has bigger wholes. We have had raccoons reach through and try to pull chickens back through the holes

and have tried to grab them off the roost when they decided to sleep in the enclosed run.
Another coop we built, but have since quit using and are in the process of dismantling, was built on a lovely flat concrete pad. We thought that would be great, but we were very wrong!! The chicken droppings just stayed were they fell. Water didn't run off either. The only way to clean it was to hose it down but that made the dirt part gross & muddy. The coop smelled all the time (except in the winter when everything was too frozen to stink).