I don’t believe in magic numbers for much of anything about chickens, including how much space they need. We keep them in so many different conditions that no one number can fit everyone. I find the more I crowd them the more behavioral problems I have, the less flexibility I have in dealing with any problems that do show up, and the harder I have to work. You might follow the link in my signature for my thoughts about it, but I don’t give magic numbers, mainly things to consider.
I strongly agree on you keeping that old coop. I suggest you fix it so both coops can open into the same run, maybe even building an internal fence in the run so you can split it if you want or leave a gate open and use it as one run. That will be real handy when you integrate them.
Many building materials come in 4’ and 8’ dimensions. If you use those in your planning you can usually build a larger coop at no more cost with less cutting and waste. 6’ and 12’ dimensions aren’t all that bad either but avoid silly things like 5’ or 11’.
For that many hens I’d certainly want a walk-in coop. Eight feet wide is great and I really like that much room but six feet is wide enough that you can put stuff in there and still have some room to work. One advantage to 6’ width is that you can put a roof with overhang on it, just having it slope one way (so the water drains away from the coop and run) and use still standard length materials. The wider you have to span the heavier the wood needs to be to support snow and ice load. You may have seen some roof collapses in the news lately. Still, I really like eight feet. If you have a double-sloped roof instead of a single sloped roof the supporting wood does not have to be that heavy, but that is more complex construction.
You are on the wet side of the mountains so moisture is likely to be an issue for you. A lot of people have them but I really don’t like a raised floor. If a chicken can get under there it needs to be high enough so you can retrieve a chicken that might not want to be retrieved or they can build a nest under there. If chickens can’t get under there, it can a good place for mice, rats, snakes, or something else to set up housekeeping. You have to give serious thought to how you protect that floor from rot. If it stays damp it can rot. There are things you can do and many people are happy with their raised floors, but my personal preference is for a dirt floor.
With or without a raised floor, you should not build your coop or run in a low spot that water drains to or even stands. A wet coop or run is a dangerous coop and run because of potential for disease. So think about that before you position your coop. I made mine by closing in the end of a shed but it was on a very slight rise. I did a berm and swale on the upslope side to keep rainwater from running in and filled the bottom of it with a few inches of dirt to raise it up even higher. Mine stays pretty dry but if a little rain or snow blows in through my ventilation it dries out pretty fast. If you don’t have a real option as to where you put it, I strongly recommend hauling in dirt or sand and building it up so water drains away from it, not to it.
On the coast you are not going to get the really vicious cold temperatures the northern interior of the country regularly sees, at least not often. With your possible moisture issues ventilation is going to be very important especially in winter. I suggest you leave the top few inches at the top of the walls under your overhang open and put your roosts low enough that any breeze that comes in passes over their heads. That will allow air exchange while keeping a breeze off them. If you can manage that, they can handle really cold temperatures.
Back to the size thing. The bigger you build the coop and run, the more flexibility it gives you in handling things that come up. You have to remain flexible. No matter how much you plan things never work out exactly as you expect.
Good luck!
