Provide one foot of perch space for each hen. Always secure your hens at night or they might become the dinner of local raccoons or opossums. An unused garden shed or corner of a garage can easily be converted into a chicken house. A space about eight feet by twelve feet will be very adequate for a few dozen birds. They will also need an outside enclosure about twice this size for exercise.
Chickens like to scratch in the soil and catch bugs, so a pen with grass is preferred.
Chickens can fly, so either make the sides of the pen at least eight feet high or make a cover with chicken wire. A covered pen will protect your flock from hawks. Make sure there is power to the chicken house. During the winter it is a good idea to keep a light burning so that the hens get about 12 hours of light daily. Egg production will suffer otherwise.