There are some stickies at the very top of this section of the forum that might help you.
https://www.backyardchickens.com/t/206328/byc-coop-section-helpful-thread-index
And here are a couple of articles I think anyone building a coop should read. I’ll throw in the winter coop one although in Missouri your problem is not cold. The heat of summer will be much more dangerous to your chickens.
Pat’s Big Ol' Ventilation Page
https://www.backyardchickens.com/web/viewblog.php?id=1642-VENTILATION
Pat’s Cold Coop (winter design) page:
https://www.backyardchickens.com/web/viewblog.php?id=1642-winter-coop-temperatures
Pat’s Big Ol' Mud Page (fixing muddy runs):
https://www.backyardchickens.com/web/viewblog.php?id=1642-fix-a-muddy-run
You’ll see a whole lot of stuff. Remember that the vast majority of this is personal preference. Chickens need feed, water, protection from the elements though how much depends on your conditions and where you live, and protection from predators. You’ll see where people feed all kinds of weird things to their chickens, put strange things in the water, or add all kinds of things in the coop. The chickens really don’t need any of that. Humans want to put them in there.
I’ve got a lot of things in my coop for my convenience. But that is for me. The chickens really don’t care.
What you need in a coop is roosts, nest boxes, ventilation, a way for them and you to get in and out, and …………………… Well, that is about all you need. I have more stuff, but that is for me, not them. And the stuff I have for me is not fancy or pretty.
About space. That’s a subject I have some feeling s about. I don’t believe in magic numbers for anything to do with chickens. We keep them in so many different conditions, in different climates, with different flock make-ups, with different goals, using different management techniques; there cannot be one magic number that covers all of us. I know if you have no experience with chickens you need a starting point. That’s where the guidelines or rules of thumb come in. These are numbers or suggestions that will keep most of us out of trouble most of the time considering the wide range of conditions and management techniques we use. They are a good starting point but following them does not always guarantee total success and violating them does not always guarantee total absolute failure.
A rule of thumb often used on this site is 4 square feet per chicken in the coop and 10 square feet in the run. Chickens need a certain amount of space. How much depends on a whole lot of different things. It does not matter if that space is available in the coop, coop, and run or if they sleep in trees and are never fenced. Your management techniques really come into play here. Commercial operations have proven you can get by with a lot less space, but they work hard at it and use techniques the vast majority of us don’t.
Chickens have learned to live together in a flock. A basic way they do this is that when there is a conflict, the weaker runs away from the stronger or just avoids them to start with. They need enough space to run away or avoid or it can get violent.
I find that by adding more space I have to work less hard. Poop management is an example. I cleaned out my coop for the first time in four years last November, not because I needed to but because I wanted that stuff in my garden. I scrape my droppings board every two to three weeks. Some people, especially in suburbia, clean their coops or scrape their droppings boards on a daily of not weekly basis. Size has a lot to do with it but there are other considerations.
I find that by having more space than the absolute minimum I have a lot more flexibility in dealing with problems. If I have a predator problem, I can lock them in the coop and run until I deal with the predator. If a hen decides to hide a nest outside, I can lock them all in the coop and run until she learns where I want her to lay. If I have a broody, she has room to raise her chicks with the flock. If I raise chicks in a brooder, integration is much easier if I have lots of room. If I want to wait until fairly late in the morning to let out of the coop, no big deal. I have enough room in the coop that they won’t start killing each other. If the college girl that takes care of my chickens when I go to see my granddaughter waits until pretty late to let them out, they are fine. I really like the extra flexibility extra room gives me.
I don’t know what you mean by roost box. A rule of thumb is that they need 9” of roost space per adult bird but I really prefer more. Again flexibility. I find the time mine are most brutal to each other is when they are on the roosts. During the day, they have room to get away from each other. I try to give them that same option on the roosts. This becomes especially important to me when I have younger chicks in there, either broody raised or brooder raised. A broody practically always teaches her chicks to roost on the roosts before she weans them and protects them while they are on the roosts. But after she weans them, it is not unusual for another hen to make life brutal for them as they are settling down to sleep. I’ve had some chicks leave the roosts and go sleep somewhere else because of that brutality. I put in a separate roost a little away from the main roosts and lower down so they have a safe place to go that is not my nesting boxes.
As far as nest boxes, a rule of thumb is one nest for every 4 hens. They will mostly lay in one or two of these but the extra ones gives you what? Flexibility. If a hen goes broody and won’t let other hens lay in her nest, they have options for example. The more hens you have the more you can relax the hens to nest ratio. If you have 40 hens you don’t need ten nest boxes. Seven or eight would be plenty. That’s another thing about these guidelines. They are often set up for a relatively small number of chickens in a suburban setting. They mean a lot less if you have a lot more chickens.
I’ll mention that most building material comes in 4’ or 8’ sections. I know you said you are getting most of it for free, but if you plan around those dimensions you can usually get by with a lot less cutting and less waste. For what you described and knowing you plan to expand later, the 8’ x 12’ sounds real good to me.
That’s enough typing. Good luck.
