I assume your meat birds are going to be the hybrids that are ready to butcher by 8 weeks. Those grow so fast and don’t do much else other than poop and eat, so they don’t mix really well with your layers. I suggest rather than thinking coop, you consider something more like a shelter, just something that will provide predator protection and weather protection but not much else. Another option is to make your coop big enough that you can section off a section just for the meaties, then open that to your layers when the meaties are gone.
There are so many different coop designs and we have such different priorities it’s hard to be specific. So much of it is just personal preference. But some of the features I like about mine:
I really like a walk-in coop. I’ve found snakes, a dead chicken, eggs on the floor, and a possum in there that would not have been obvious if I were gathering eggs from external nests.
You need access to every spot in the coop. You never know when you will need to catch a hen that doesn’t want to be caught, make a repair, collect an egg from a strange place, clean up a mess, or whatever.
I like the flexibility and room to work extra space gives me. Don’t make it too crowded. Think in terms of “How can I give my chickens and me enough room?” instead of “How many chickens can I shoehorn into this space?”
I use a droppings board under my roosts. That really helps reduce the poop load the coop has to carry. It’s a lot easier to clean a droppings board than change out the bedding, pus the pure poop is great for composting.
I built a swale above mine to divert rainwater runoff and put about 3” of clay dirt inside the coop to assure it will stay dry.
Like I said, some of this is personal preference. With 12 hens I would not crowd it and I would give a lot of important to my comfort when working in or on the coop. Instead of the chickens, think more about yourself.
