That temperature is fine for most breeds of chickens, with the following conditions:
--they are out of the wind
--they had a chance to adjust to the temperatures during the fall and early winter
--you provide enough food and water
When I say "out of the wind," I mean not enough wind to ruffle...
Two possible modifications, given that it is inside the garage, and already partly framed:
You could cover the frame with hardware cloth (contain chickens, protect from predators), and not bother with solid walls. This will be fine for the chickens, will make their space feel more open, and...