Coop space per bird
As a starting point, standard-size breeds call for a minimum of 3-4 square feet of covered coop floor per bird when they also get outdoor run access, and something nearer 8-10 square feet per bird if they'll spend most of their time shut inside (as in cold regions with little winter time outdoors). Bantams get by on roughly half those figures. Always plan around the largest flock you anticipate rather than the one you're starting with, because most keepers bring in more birds over time and enlarging a coop after the fact is tougher than simply building a bit bigger from the start.
Run space per bird
Your outdoor run calls for a good deal more room than the coop itself: budget a minimum of 8-10 square feet per bird for the run, rising to 15+ square feet per bird when the flock won't get to free-range somewhere else during daylight hours. Birds crammed into an undersized run are much likelier to develop pecking-order aggression and feather picking; where room is scarce, introducing vertical features such as perches and platforms lets birds make fuller use of the space.
Nesting boxes: how many you actually need
A single nesting box for every 3-4 hens does the job; hens are content to share and frequently gravitate to the same one or two favored boxes even when others sit empty. Typical dimensions for standard breeds are about 12x12x12 inches, set below the roosting bars so the birds don't bed down in the boxes (which soils the eggs), with a few inches of bedding inside.
Roosting bar space
Give each standard-size bird 8-10 inches of linear roosting bar (roughly 6 inches for bantams), and choose a flat-topped bar such as a 2x4 turned on its flat face rather than a rounded dowel, since a flat surface lets the birds tuck their bodies over their feet to keep them warm when it's cold. Position the roosts above the nesting boxes, because chickens naturally seek out the highest spot available to sleep.
Ventilation and predator-proofing
Year-round, a coop requires ventilation up near the roofline (higher than the roosts, so sleeping birds aren't hit by direct drafts) to keep ammonia from building up and to head off respiratory disease, and this holds true even in cold climates; buttoning a coop up too tightly for winter is a widespread and serious error. On the predator side, fit hardware cloth (not chicken wire, which bigger predators can rip open) over every opening, sink or apron the run's perimeter fencing at least 12 inches deep to defeat diggers, and add a latch predators can't work open, since raccoons especially manage to defeat simple ones.
Frequently asked questions
How many chickens can a 4x8 coop hold?
A 4x8 coop, which gives you 32 sq ft, houses roughly 8-10 standard-size birds comfortably at the 3-4 sq ft per bird figure, provided they also get outdoor run access; cut that down to about 4-5 birds if they'll be shut inside the coop most of the time.
How many nesting boxes do I need for 12 hens?
Figure on 3-4 nesting boxes for a 12-hen flock, following the roughly 1-box-per-3-4-hens guideline; going beyond that usually just leaves boxes empty, since hens tend to gravitate toward a couple of favored spots.
Can I use chicken wire for the run?
Ordinary chicken wire will hold chickens in but can't be relied on to shut predators out; hardware cloth (welded wire mesh, usually with 1/2 inch openings) is the safer benchmark for any run or coop opening within reach of raccoons, dogs, or other predators.
Do chickens need a heated coop in winter?
The majority of cold-hardy breeds get by without added heat and handle temperatures well under freezing as long as the coop stays dry, free of drafts at the birds' level, and adequately ventilated above the roosts; heat lamps bring a genuine fire hazard and are generally advised against in poultry-keeping guidance.