How to Change Existing Structures into Chicken Coops
Horse stalls were designed to keep horses safe, so with a little tweaking they can keep chickens safe too.
Usually horse stalls have more open areas, such as the top half of the stall door and very top of the stall near the roof. Screen large areas with hardware cloth. Add nesting boxes, roosting bars, and a manure box. Hang your feed and waterers from the ceiling with a chain or S hook.
If some type of automatic water system is already in place, modify it for chickens closer to floor level. Horse stalls are notorious for attracting rats and mice for feed, too. Make sure all previous rodent holes are sealed.