Chickens need two to four square feet of space each within their coop, with separate nesting areas and perches to roost on. They also need outdoor space, which can be a screened run, a pen, or fenced yard.
They will need protection from predators both day and night. Unless supervised, they will need overhead protection from hawks and other predatory birds, which can be as simple as bird netting that will delay or confuse the predcator long enough for the chickens to reach the safety of their coop. Neighborhood dogs and coyotes are a threat both day and night. Permanent enclosures shoud have burried fence to prevent digging underneath and both coops and small runs can have metal hardware cloth floors to prevent access from below. At night, they will need to be secured from coyote, racoons, weasels, owls and other predators. Double latches on all doors and hatches will confuse clever racoons.
Whether you build or buy a coop, or modify an existing shed, make sure that you have full and easy access to the inside of the coop to clean it periodically. Since your chickens will be outdoors all winter, they will need fresh water brought to them several times a day unless a heated waterer is used to keep it from freezing.