One chicken needs between three and four square feet of space inside the coop. If you're planning to keep at least three chickens, aim for a coop that's nine to 12 square feet. If your chickens are smaller breeds or will spend most of their time outdoors, you can build a smaller coop. However, if they'll be in the coop most of the time, provide at least 10 square feet per chicken. Ensuring your chickens have enough space helps prevent stress and pecking behaviors due to overcrowding.
While you don't absolutely need a traditional coop to keep chickens, you do need to provide shelter and protection. In areas with extreme cold or heat, even free-range chickens require a shelter for temperature regulation. You'll also need secure fencing to keep predators out. Additionally, providing comfortable nest boxes makes it easier to find and collect eggs.