What do I consider necessary in a coop? Not a lot.
Predator protection.
Protection from the elements. That varies a lot depending on where you are.
Roosts.
A way for the chickens to get outside.
That’s probably it, though some people might include nests.
Some of the additional features I built into mine.
A built-in brooder where the top serves as a droppings board.
Removable roosts.
Nests that allow me to lock a chicken in there if I wish, plus adjacent space where I can feed and water the hen locked in there.
A window to provide light.
Electricity to the coop with a water spigot just outside. This includes lights and electrical outlets.
It’s much bigger than the bare absolute minimum.
Lots of permanent ventilation up high plus vents that can be opened down low. The window has hardware cloth so I can open it and still have predator protection.
An additional roost away from the main roosts and a little lower which makes integration a lot easier. The younger or weaker can safely roost away from the bullies on the roosts.
Something I did that is useless to me. Access to the nests from outside. I never use them. I much prefer to go inside to see what is going on inside the coop. I’ve found possums, snakes, and dead chickens I would have otherwise missed.