Here is the coop I built several years ago. It measured 8'x12' and I got all the materials from an old storage building that I took apart and reused.
The bricks didn't last long, a strong wind picked it up and shifted it so I ended up just sitting it on the dirt.
I got the window at the hardware store. someone returned it so it was only $20.
There are screens over the window and at the back for venting.
I covered the large opening inside with chicken wire to keep them out of the storage area.