There are a lot of designs out there, and a lot of really cute looking coops. It is overwhelming, to say the least. Luckily, my wife spotted a coop that looked perfect for the rainy Pacific NW, and there were detailed plans available. When I am overwhelmed, I will gladly pay for some well documented, well thought-out plans.
I'd been following a forum post where they were talking about using a horse stall freshener like kitty litter in the roost. I thought it would be nice to have a removable tray to facilitate cleaning. Turns out an inexpensive washing machine drip tray was close to the inner dimension of the planned roost, and only required adding a few inches here and removing a few there.
Somewhere else, I had read about water nipples. The standard feeders have two problems. The first is that the birds like to roost on the top of them. The second is that they like to poop when they roost. That leads to lots of water changes. Most people put water nipples on the bottom of five gallon buckets and then hang them, but with a small coop, there wasn't the space or access to do that. Enter PVC. I integrated 2" PCV piping so that it could be easily filled. I used the remaining PCV to make a feeder attached to the door. Now food and water move with the coop and don't take up any floor space.