There are a lot of variables in how much room you need per chicken. If you follow the link in my signature you can see some of the things I think are important. It’s not coop in isolation and run in isolation, but how much total room is available when they need it. Some variables are flock make-up (sexes and age differences), climate (how long they may be stuck in the coop only), management techniques, individual chicken temperament and personality, flock dynamics, but there are several other things to consider.
In general the tighter I pack them the more behavioral problems I have to deal with, the harder I have to work, and the less flexibility I have to deal with issues. The behavioral issues most people think about range from feather picking to outright cannibalism, but it includes other day to day activities. If you don’t have adequate roost space for your flock (this does not mean a certain inches per bird but considers your unique flock’s needs) this can lead to chickens sleeping and pooping in the nests. If you are trying to integrate, especially younger chickens, this can lead to bullying or even death.
An example of working harder. Chickens poop a lot. The more chickens you have crammed into a smaller space the more the poop builds up. The tighter the chicken density, the harder you have to work to manage that poop.
Some examples of flexibility. If you discover that your chickens are getting picked off by a predator in your run, is your coop big enough to hold them while you solve the predator problem? Don’t just think of days you can take off from work and perfect weather, but maybe you need to take a kid to the doctor or you need to catch a plane to go to a family funeral. These things don’t always happen at your convenience. If a hen goes broody and you don’t want her to hatch chicks, do you have enough room to use a broody buster?
You may notice that a lot of these things have little to do with the chickens but a lot more to do with your comfort and convenience. I consider my time and stress level important.
You will see a lot of numbers bandied about on this forum. People starting out need guidelines, I understand that. A lot of the guidelines are intended to keep people out of trouble even if their management or conditions are less than ideal. One problem with guidelines is that I’ve seen people on here say anything from 1 to 15 square feet per chicken in the coop will work, often with no mention of a run or management practices. Which guideline do you go by? In certain circumstances you can get 1 square feet to work, in others most of that 15 may be necessary.
I think you are right, that 4 square feet assumes they will spend extended periods in the coop. But there are conditions that go with that. It’s generally talking about a flock that are all the same sex (hens) and the same level of maturity. You have to have enough room to feed and water inside without them pooping in that from the roosts. They are already integrated. The size of the flock has an impact, I go through that in my article, the more chickens you have the less space per chicken you need from a behavioral aspect. You still have to manage the poop.
One concern I have with yours is that you say you will be adding chickens. I don’t know what ages or sexes you are considering adding, those and your techniques for integrating can make a difference, but you generally need a lot more room when you are integrating than after they are all mature and have worked out the pecking order.
Will what you are talking about work for 10 chickens? Maybe, depending on your management techniques, climate, and all the other variables. Management will be very important. Personally I’d never build something that small because you have no flexibility in how you have to manage them and no ability to adjust to issues. I often go to visit my grandkids and have to get someone to take care of my chickens. I want to make that as easy as I can on them because I want to see my grandkids, not stay home to take care of my chickens because I can’t find someone to take care of them.
But it is your choice. If you want to lock yourself into management that rigid you certainly can. With the right management techniques you can probably make it work. Good luck!
