Large breed adult chickens are rather hardy stock - they also have many feathers to keep them warm. My girls are large stock, Leghorns and Comets. A house protecting them from the elements is a must (no leaks) but some vent holes near the top of coop roof is also important.
Most large breeds can withstand temps even below 15 - 20 degrees. In Central VA on a typical winter evening, it does not get that low but it can happen.
We wrapped the interior of our coop with black plastic to help insulate and draw in heat from the sun during the day (through the 2 large windows) leaving small vent holes open at the very top.
I have a heat lamp but I worry all night it might catch fire so only use it when temps drop below 18 degrees. It is more to make me feel better about them being outside alone in the cold rather than they would die w/out it.
Young chickens and even some small breeds require supplemental heat at temps below 25-30 degrees or you can risk death.