If you are in a warm climate, then the coop is really just for sleeping, or hanging out. Some people in warm climates don’t even create 4 solid walls, but place the walls they do have in such a way as to shield from prevailing winds, rain and to provide shade. Of course, in this scenario, the birds have 24/7 access to the run area, and enough roosts in a protected area.
However, if you are in an area where climate is very cold or wet, you need to plan for an enclosed/protected/draft-free coop that the birds can stay in if the weather is bad. Generally 4sqft min per bird of coop space should be provided. 20 birds= 80 sqft min. Or an 8’x10’ shed minimum size.
Good luck.