As long as your coop is dry, draft free and well ventilated your birds should be fine without supplemental heat.
This is a highly discussed and debated subject. It is best to research the threads labelled for your state. I don't know where you live. Almost every state has a thread on BYC. It's best to go to your peeps and see what works for them. It also is a personal preference as to what kind of chicken owner you are, what kind of breeds you get, do you plan to "show" birds so you can't have marred or scarred from frost bit combs....etc. etc. etc. Many many BYCers and many portions of the lower 48 do not need additional heat in their coops for the usual common breeds that are selected. Again there is a bazillion threads on here to read through and see what others have experienced. Number one priority for wintering chickens is Ventilation and reduction of humidity. Then watching your birds activity to see if added heat is necessary for them. Most of the time it is not. If they are down from their roost chickening about and waiting to go outside of the coop...you don't need heat probably. If they are up on the roost warming toes, acting nervous in the cold...offering a bit of heat is not a bad of idea. (usually way below zero for me) When you add heat though...humidity will climb and when things are above 32 F and thawing...diseases can become problematic. Birds do not do well back and forth from warm to cold and back to warm.
