The next thing is to build in flexibility. Things never work out exactly as you plan. You will be back in there adding things and changing things until you get it the way you want it. So stay flexible in your thinking. By building in flexibility I mean that you give yourself some options in how you manage them. I sometimes need to isolate a chicken or such. I made my nests so I can lock a chicken in there if I want to, that’s come in handy in many ways. I built my brooder into the coop, an elevated brooder with a wire floor. The top acts as a droppings board. With a wire floor it can be used as a broody buster. When I have chicks in there I insert something to give them a solid floor. I can isolate adults in there if I need to, and sometimes I do. Since I integrate younger chickens a lot I put an extra roost, lower than the main roost and horizontally separated but higher than the nests to give the younger chickens a safe place to roost that is not my nests. That’s actually over my nests. The flat nest tops act as a droppings board. A lot of this depends on how you manage your chickens, your goals, and your flock make-up. In a 10’ x 20 you have a lot of room to be flexible, so you are in good shape, but some built-in place to isolate a chicken is really handy.
Realize that we are all unique. There are so many variables in our goals, climate, rural versus urban, flock make-up, and our facilities that what works for one will not work for another. There is a big difference in someone keeping four hens in a small urban back yard in Florida versus someone having a flock with many hens and roosters in the country in Ontario. One of your biggest challenges on here when you see a suggestion or recommendation from me or anyone else is whether that actually fits your unique situation. There are a lot of different people on here with a lot of experience in different circumstances. Not everything applies to everyone.
Don’t believe magic numbers with anything to do with chickens. That includes coop space, run area, roost length, brooder space, hen to rooster ratio, hen to nest ratio, temperatures in a brooder, really anything. Consider most of what you see as guidelines, a starting spot for someone with no experience but don’t consider them absolute laws of nature. I’ve seen recommendations on coop space anywhere from 1 square feet per chicken to 16, sometimes considering run space, sometimes not. Which number is really the magic one? How big of a brooder do you need? Are the chicks moving out at 4 weeks or are you keeping in there until they are practically grown? There are so many variables with all of these that there is no real magic number that covers all of us in all conditions.
I’ll add one more and quit. If anyone on here, me or anyone else, ever comes across as you absolutely have to do something the way I tell you or civilization as we know it will forever be altered, get a second opinion. I regularly integrate five-week-old brooder raised chicks with my adult flock with no problems. Some people will tell you that you have to wait until they are 16 weeks to do that. I don’t but for some people and their unique circumstances 16 weeks might be the right answer, five weeks could lead to disaster. That’s an example of trying to match the recommendations to your unique situation.
I’ve probably made it sound harder or more complicated than it really is. There are so many different options that can work that it seems hard. You are probably going to do fine. Good luck!