Chickens depending upon breed (Mediterraneans and other large combed/wattled breeds are susceptible to frostbite) can stad extreme cold. People in Alaska raise chickens without supplemental heat. Most important is good, draft free ventilation, dry living conditions, and a dependable source of feed and water.
We've found that Barred Rocks do well down to -20F which is, what, -28C?
It is at that point that we find they begin to get uncomfortable. Dry dwelling, nice deep straw, good ventilation and they manage nicely. If your region regularly experiences weather with colder temperatures, you'll want to consider some further measures.
Ours did fine in temps as low as -25, We have an insulated coop and don't heat it, We didn't let them out in the run when we had wind chills under -20, all other days they went out. Chickens handle cold better than hot.
This has been our experience as well. We did have a few hens that got a touch of frostbite on the ends of their combs during their first harsh winter, where we had several weeks of below-zero temps. Our coop is insulated but a bit drafty. They huddle together, and on the really cold nights, we hung a heat lamp in there. I've bought a coop heater for next winter, that should keep the coop at about 40 degrees. I do wonder though if keeping the coop heated (even a little) will make them less able to handle being outside in the colder temps?
But last summer, in a two-week stretch of 90 degree days, we lost one to the heat.
And that was with ice coolers in the coop and access to fresh, cold water.
In really cold weather, mine are outside scratching in well below 0. We have frequently been close to 25 below, plus wind chill. They do need protection from the wind. But the big thing is, when it is cold they need more feed, and a hot feed with corn will help. Many people of feeding the same feed every day, but cold temperature need a high energy feed. However, if you feed them enough, they can take darn cold.