Hello there!
Our house came with an un-used shed in the back corner of our yard. I never wanted to even open it, convinced some sort of rat or snake would lunge out at me! Finally decided to make some good use of it and build a coop for fresh eggs and for the ambiance they provide to the yard. ;)
Here's a pic of the shed to start. We used pieces of wood that we found behind the shed, had leftover, and asked friends if we could poke around in their scraps. All we bought for this project was a roll of 1/2" mesh, hinges for the egg door, and for the run, some scraps from a hardware store scrap bin - so cheap!
First we cut some holes in the sides of the shed for ventilation. Turns out, what I had been afraid of in that shed WAS happening - here's a Texas rat snake that was hanging out in the rafters, meaning there had (prior to the snake, I'm assuming) also been some rats in that shed too! The snake moved on and just reminded us how predator-tight our coop had to be!
We live in central Texas so summer ventilation is much more of an issue for us than winter weather. The windows were covered with 1/2" hardware cloth with a staple gun and then 1x2s screwed on top of that, for best predator protection.

The back wall of the roost, the wood was in pretty bad shape and I didn't think it could handle having a window cut in it, so we just drilled a bunch of holes and covered it all with some fine mesh that we found behind the shed. We'll put an old piece of wood under the roost area to be able to take it out and hose it off more regularly than a whole coop clean-out.