This is my brooder in the coop, pretty much set up for your temperatures. When It is below freezing when I brood out here I wrap the whole thing in plastic. That chimney to the left where the heat lamp goes still provides good ventilation. When it is below freezing I use two heat lamps for redundancy, otherwise just one. In the heat of summer I use a lower wattage heat lamp. Some people use heat plates or heating pads. I think I call what Canoe is talking about a "hover". Lots of different ways to do this.
To me the biggest issue with brooding outside is the temperature swings. I've seen it go from the mid 20's F to over 70 F within 36 hours. The brooder needs to have one spot warm enough in the coldest temperatures and a spot cool enough in the warmest temperatures. I achieve that with ventilation and distance. This brooder is 3' x 6'. I've seen frost in the far end but the end the chicks were on was toasty.
If you can show us what your coop will look like we might be able to give you specific suggestions. It would help to know how you would provide the heat, heat lamp, pad, plate, whatever. There are little tricks for each that can make them easier or safer.
To me the biggest issue with brooding outside is the temperature swings. I've seen it go from the mid 20's F to over 70 F within 36 hours. The brooder needs to have one spot warm enough in the coldest temperatures and a spot cool enough in the warmest temperatures. I achieve that with ventilation and distance. This brooder is 3' x 6'. I've seen frost in the far end but the end the chicks were on was toasty.
If you can show us what your coop will look like we might be able to give you specific suggestions. It would help to know how you would provide the heat, heat lamp, pad, plate, whatever. There are little tricks for each that can make them easier or safer.
, and just over 250sq/ft of run space. We plan on free-ranging as much as possible on property but want them to have enough enclosed space as well. The wall between the storage area and coop will be a half wall topped with hardware cloth. I know this will mean a dusty/dirty storage area, but it'll be nice to be able to be undercover in the winter without having to go into the coop. We're also putting egg boxes on that wall so we can be inside the storage area to collect eggs. 
