A couple of things I considered.
One: I have a number of homes around me that have unused chicken coops. I know because I'm old.
B. They are high in the front and low in the back. Not these new fangled 12 ft high coops we're seeing today.
III. Wouldn't it make sense to bring the "ceiling" lower since chickens will roost as high as they can get, thereby keeping them in the "heat zone" of their own bodies. Put the roost under the ceiling but not so high as to block them from roosting.
When we bought our current home I wanted lower ceilings, since our previous home has 9' ceilings and that was where all the heat was. Our current ceilings are 88" and I am warm and happy.
4. Chickens are domestic birds but no so much so as to need added heat. I've read you may put them at a disadvantage in the event of a black out. I recently discovered their water heater was unplugged and the nipples froze.
E. Gramma sure didn't have a heater to stick in her coop. It was designed to keep them warm enough to survive. This is why the Buckeye was developed. With a pea comb for frost bite protection. Choose breeds according to you climate.
Finally, if you are keeping chickens for pets and just a few do what you will, but if you are keeping large flocks of 15 - 20 or more you might consider these things in building and the housing of your birds. The old ways are not always outdated.
I wish you well,
Rancher