Remember to make a chicken door as well as a people door (at least one) that will be convenient for you. You will probably want a people door on the run as well.
Ventilation is something that is often tricky for new designers. It needs to be at as high a point in the coop as you can manage. If you're going to build a square or rectangular walk-in building, I feel the simplest is to have the roof slanting from one side to the other, so the ventilation can be on the high side. In a walk in coop, it should be relatively easy to have enough ventilation and still have no draft on the chickens.
You will probably want a couple of windows. Actually they can simply be holes in the wall covered with hardware cloth, with a simple hinged wood shutter on the outside.
You're planning way more than the minimum space, and if you live in Alabama, you can actually have a smaller coop or even a 3 sided structure attached to a fenced run or yard. In the South, heat is much more of a problem than cold. Actually, what we have down here is never "winter" to a chicken; they tolerate temps well below zero but are in danger of dying at 100 degrees, and must have both shade and breeze --- and never run out of water.
There doesn't have to be anything in the run, but they do enjoy things to jump on, hide in or under, and even roost on.
A rule of thumb often quoted here is 4 sq ft per chicken in the coop and 10 sq ft in the run. So for 15 chickens, that's maybe an 8' X 8' coop and a 10' X 15' run. Particularly in the South, outdoor space is more important than indoor, and they will benefit from as large a run or yard as you can give them, while they probably don't need that much coop space. Mine (10 chickens now, more in the past) live in a fenced yard about 70' X 70' which they use, and even keep the grass pretty well eaten out of. There are bushes and clumps of tall weeds in there. Of course it is just fenced so it is not predator proof.
If i were starting over in Alabama (if that's where you are) I would build a 3 sided structure with a secure attached pen and probably a larger not secure fenced yard. What I have is a large 4 sided building with gobs of ventilation -- two walls are partly hardware cloth, and there is air space at the top of the walls all the way around. It's so breezy it's their preferred cooler, shaded spot in the heat of summer. They have a corner with solid walls they roost in, so little or no draft in winter. I've had chickens in this setup for about 4 years and they do fine.
Here is a thread showing the sort of structure I'm talking about:
https://www.backyardchickens.com/t/163417/please-show-me-your-hot-weather-coops
You can put your general location (maybe just your state) in your postbit (under your user name.) Just go to your profile (brown bar at the top of the page) and look for "location." Helps people a lot in answering lots of questions.
Nest boxes should be about 12" on each side, or a little larger. You can use anything for a nest box -- 5 gallon buckets, cardboard boxes, plastic bins, etc. They can just sit on the floor if you wish. I have yet to spend money building a nest box and I currently have about 10. You only need one for about every 4 chickens (as I said, i used to have lots more chickens.) Some people use a community nest box -- maybe 3' wide. Some chickens do fine with them and some prefer more privacy.
Nests are not something you need right away, anyway. If you raise your chicks in your coop (which I strongly recommend -- no chicks in my house!) you won't even need a roost for a while -- but you will want electricity for a heat lamp or ceramic heater. If you time it right you may not need heat for more than 2 or 3 weeks and may be able to get away with extension cords (I didn't say that....
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BTW, Silkies don't necessarily do well in a flock of large fowl chickens, which is what you are talking about. if you get one (I wouldn't get just one,) you might want to be prepared for making a separate living space for them. Their crest is actually a skull defect so is prone to injury, and others often want to peck on the crest, or worse.
And here is an excellent thread on ventilation, written by a Canadian and really more suited to cold climates, or enclosed coops anyway:
https://www.backyardchickens.com/a/...-go-out-there-and-cut-more-holes-in-your-coop
Good luck!
