When designing and/or building a chicken coop, making sure that it meets some basic requirements will go a long way toward ensuring your hens' safety and comfort.
Make sure your coop is properly sized. At least 4 square feet per bird is ideal if your chickens can free range during the day. If they are going to be cooped up most or all of the time, go for 10 square feet per bird for a small homestead chicken flock.
Ensure your chicken coop is protected from predators and from rodents. A solid floor, hardware cloth dug in 12 inches deep all around the coop, and hardware cloth over any open areas is key. Also make sure there are no gaps or openings that rodents could get into.
The coop should be easy to clean. Think about mucking out all the used bedding at least several times a year, possibly as often as every few weeks if you have a small coop in a suburban area. Include an easy to clean area - some use a removable, cleanable tray - under the roosting area where most poop accumulates.
Include roosting poles in your chicken coop. Roosting poles are key for laying hens. They should be about two inches wide with rounded edges. You need at least 5 inches of space per bird on the poles. Angle the poles against a wall so that the sides are at an angle, like a ladder leaning against a wall. Multiple poles should be at least 10 inches apart.
Ensure plenty of ventilation but keep them warm, too. You have to strike a balance between allowing air to flow through the coop and the hens getting too chilly. Most people overinsulate and don't have enough air flow through the coop. Their feathers and other hens' bodies keep them fairly toasty when they have roosting poles. You mainly want to ensure that there isn't a lot of wind whistling through the coop. Make sure there is at least one window that you can open. Preferably there are two windows in two different sides of the coop.
Make sure there is room for a feeder and waterer. Ideally, you can hang feeders and waterers six to eight inches from the ground. This prevents the chickens from roosting on them and fouling them with droppings, and from kicking them over, sleeping in the feeder, and so on. Easy access here will also help with your daily chicken chores going smoothly.
Build good nest boxes. You'll need one nest box for every four to five hens. They need to be raised off the ground at least a few inches. And they should be in a dark, comfortable corner that isn't where you have to walk past it every day so that the hens feel safe to lay their eggs there.