I live in Virginia, in an area where snows last a week or two at most, and December, January and February highs are typically in the mid 40s.
My dogs sleep inside at night, wear good coats when outside on bitter cold days, and have three kennel boxes in two separate areas inside a relatively warm stone garage that is part of the house. Though the garage does not have its own radiators, heat seeps in from the house, and it does not leave too quickly.
A fourth dog house is made of stone, is foam-insulated inside, and also has a deep bed of hay inside. This dog house is large enough that three terriers could easily "nest up" inside the mound of hay, and the door is quite small and off to the side to keep out drafts. This dog house is now covered in ivy, and on its top is where I photograph a lot of red foxes who come to the yard at night (see above). No, the fox do not den inside -- they are there for the food alone.
One kennel area inside the garage has free access to the outside yard (forested with stone paths and small pond) but does not allow unrestricted access to the inside of the garage where the dogs might get into mischief with paint and solvents and other stored items. The insulated kennel box in this location is large, and can easily fit three or four terriers.
The other kennel area inside the garage is a large pen with two doubled-walled dog houses inside individual wire crates. Between the double walls of the dog houses is thick insulated foam, and the box interiors themselves are deeply lined with a bed of clean hay. The door to each of the houses is small to keep in heat and keep out drafts.
This pen works well for when a dog comes into heat, when work men come to the house, or when company is being entertained and a dog leaping at the back door is an unnecessary distraction. When it gets really cold out, and the snow is too deep for wee terriers to traverse, it's also a place where they can tuck into keep warm and wait for better weather.
I have dog heater pads as well, but with the double insulated boxes, these external heat sources appear to be too much. Nonetheless, they are there to plug in if it looks like they are ever needed.
I feed a little more in the winter, topping off their regular kibble with a little olive oil, a hard boiled egg, or a raw chicken wing.
When I come home after work, the dogs get TV time with me in the living room -- a good time to check them over for weight, to practice their down-stays, and to have them tongue-clean the top of my bald head.