I don't believe in magic numbers for how many square feet per chicken you need. If you follow the link in my signature you can see some of the reasons why. We are are all so unique in our goals, climate, flock make-up, management techniques, and so many other things that no magic number works for all. How much room someone needs per bird might be a bit different for someone keeping 4 hens in a small backyard in Anchorage, Alaska as compared to someone in Miami, Florida that has a flock of a coupe of roosters, 20 to 30 hens, and a bunch of chicks growing up with the flock. I've seen people on this forum recommend everything from 1 square feet per bird to 15, often with no mention of run size, climate, management techniques, or anything else. Whose magic do you believe?
I don't do Seramas or any other bantams so I have no experience with them. I have observed that the more I crowd my full-sized fowl chickens the more behavior problems I have, the harder I have to work, and the less flexibility I have to deal with issues that come up. I don't know the minimum space you can shoehorn 10 Seramas in without them eating each other or otherwise injuring each other. I don't know how much time you want to devote to poop management, the tighter you pack then in a space the more the poop will build up and will need to be dealt with. Or if you lock them up at night for predator protection will you be down there everyday of the year at the crack of dawn to let them out before you have behavioral problems. Your run space is part of the room requirement if they have access to it when they need it. If they are awake and cannot get to it then it does you no good. I like to have the flexibility to put a broody buster on a predator proof area if I need it. Sometimes when chickens are sleeping in the nest you can solve that problem by putting up an additional roost so the weak can get away from the bullies on the roost. Or room to fence off a separate area if I'm integrating. In my opinion, a lot of the reasons to have enough space are for your comfort and convenience, not just the chickens. Other than cost, there is a reason people generally do not complain about having too much room. Their stress levels are usually lower, at least some of the time.
I don't know the minimum size I'd recommend for ten Seramas. If you are building from scratch and purchasing materials I'll mention that most building materials most economical sizes are generally in 4' or 8' dimensions. If you take this into consideration you can often save on cutting and waste for little if any extra cost. Part of that consideration may be that cut-offs can sometimes be used to build nests or other things. Also I like an overhang so I can leave the top of the walls open (covered with hardware cloth for predator protection) for ventilation which is very important in your climate. A 6' width isn't horrible. There are always tradeoffs
Another aspect is that you need to be able to reach everywhere inside the coop for cleaning, maintenance, or to retrieve and egg or chicken. That's everywhere from the floor to the roosts. I find a 4' x 8' is pretty much the limit for being able to comfortably be able to reach inside from the outside, and that is going to require a few access doors. Doors require hardware which can be expensive and require more building time. I'm sure some people are happy with coops larger than 4x8 with outside access but I'd personally want anything bigger to be walk-in. There is a lot of personal preference in what I'm saying, I recognize that, but I will mention that my grow-out coop is an elevated 4x8 with limited access.
Yet another aspect, in your climate your birds should have access to the outside practically every day of the year. In addition to room for the birds you need room for waterers and feeders. In a 20 square feet coop where are you going to put the feeders and waterers so they don't poop in them from the roosts? They need enough clear space on the coop floor to land without bumping into something on the way down and to fly to the roosts unless it is so crowded they cannot spread their wings so they hop up and down. That may mean you need to crowd it even more with ramps. If you can build your run so you are comfortable it is predator-proof and you commit to feeding and watering in the run instead of the coop, you can permanently leave the pop door to the run open. Your actual coop space requirements drop but you lose some flexibility. For example, what do you do if you find out the hard way your run isn't a predator proof as you thought?
I don't know if you will get anything out of this that will help you. I'm not trying to tell you what you have to do, it's meant more to give you things to think about. Personally I would not be comfortable with 10 Seramas in a 20 square feet coop, but in the right circumstances you can probably make it work. Good luck!
