Under construction now is my greenhouse/coop. Total footprint is 16x48. It will have one adjoining wall with a small woodstove on it, I'm putting the water and food on that wall in the coop, hoping to keep it all from freezing.
It will have screen instead of insulation for the summer.
You will have to be really careful if you are using a straight up greenhouse for coop. It can get awfully hot in there even in the depths of winter, making them vulnerable to sudden deep freezes. You will have to modify it so that you have extra good ventilation even in the winter.
In the summer, you will have to lessen the solar gain. Put a tarp over the top, train vines up and over, use outside curtains on the front windows. If you don't, and don't have great ventilation, you will indeed have precooked chickens.