Our night temperatures lately have been in the teens (-10C), the days barely above freezing. Our coop has no heat or artificial light, but our chickens are doing just fine. After all, they are wearing little down vests underneath those big feathers.
Our chickens don't have a heated coop, even in the dead of winter. If it's really REALLY cold (say, -10F or -24C), with a gale force wind blowing, the breeds with large wattles and combs could get frostbite damage on those tender parts. But our chickens have enough sense to stay out of the wind if it's that bad - no one's ever been frostbit. Our coop isn't insulated either. It's made out of salvaged plywood and 2x4's, with a small, non-opening window (with chicken wire on the inside to keep the birds from pecking at it) on the southern side for light. The roosts are 2x4's ripped into 2x2's - small enough to wrap their toes around, wide enough so they can hunker down over their toes to keep them warm, and sturdy enough to support the lot of them.
As long as their coop is dry inside and they are out of direct drafts, they do just fine. In fact, with both their respiration and their poo giving off moisture, having adequate ventilation is more important. Between wall and roof, the space between the rafters has been left open - with chicken wire screening to keep out predators. The floor is 2x6 planks, set 1.5" apart, a foot above the ground. Wood skirting covers from bottom of the planks to the ground, but the gap between planks is open to the air between wall above and skirting below. More chicken wire covers from wall down over gap and skirting around the three walls outside the pen fence, and is buried six inches deep and then L'ed out another six inches for predator-proofing. The chickens easily step over the gaps in the floor, their poop drops through the gaps (or we push it with the back side of a rake when necessary). It dries out there, doesn't smell, and they can't get into it. The skirting beneath the wall on the pen side has wire attached that L's out over the ground, held down by big rocks. That skirting is easily removed by backing out a few screws, allowing access to rake out underneath three to four times a year (more about our coop design here).
There's no feed or water inside the coop (helps prevent rodents). If we get snow, I shovel out the area around their feed bin and water dishes. The feed bin has a peaked roof over it, hinged on one end so I can lift it up to dump in their laying crumbles. For water dishes, I look for straight-sided stainless steel pots in thrift stores. Aluminum pans bend too easily - the steel ones can stand up to banging them on the frozen ground to break the ice out in the morning. Sometimes we'll have to leave one upside down to let the sun warm it enough to melt it enough to get the ice out, so we alternate between two pans. Aries usually opens up the coop in the mornings, carrying a bucket of water from the house. If it's really cold, I might have to carry more water down later in the day - dumping out the earlier pan now skimmed with ice.
The pen also has a couple of low-roofed long sheds, back-to-back, facing north and south. In the summer, the girls like the north-facing one - a nice place to lounge in the shade. In winter, the south-facing one can be a place to enjoy the weak sun, out of the wind. Here Lacy and LaRue, our two Sex-Links, indulge in a mid-winter dust bath, oblivious to the snow hanging over their heads. When it snows, I shovel paths from house to coop to dog run, so they have plenty of places to get out and about. There's a south-facing shed in the dog run that's also a favorite hangout.
In the winter, the chickens are part of my integrated pest control system. They're given free run over our fenced lot, with things I don't want them getting into fenced or covered with wire laid down. When there's no snow, I'll scatter some cracked corn in a different place every day, especially under the fruit trees, letting the chickens scratch about, ridding the yard of many borers and other garden pests trying to winter over. Underneath the wild bird feeders is also a favorite chicken checkpoint, at least until the guineas come up to bully everyone else away. And everyone comes running when I step outside with the kitchen scraps bucket. The chickens do just fine in winter. They're such a happy bunch.