A dog house with a sturdy door and good ventilation works well at night when it is not brutally cold. Half inch metal hardware cloth is a great invention. When mine were in a house outside I used that, and two strands of equine electric fence, at 6 inches and 15 inches above the ground. It worked well in mild weather. Hot summer nights, because we had attached a porch covered top bottom and sides with hardware cloth (metal, half inch), I could give them big drinking pans they would sometimes jump into to cool down.
In my experience, even cold-hardy ducks may not be up for extreme cold. We have a few in the flock of runners that just don't hold up well. I want them to thrive, not just survive (too corny sounding, but that is my feeling).
Some of the bigger ducks with thick feather jackets do pretty well as long as they have cuddle buddies and are in well insulated shelter out of the wind, with dry and relatively clean bedding. I used very deep bedding - a foot and a half of shavings, with some straw on top - I also installed a sliding drop ceiling of plexiglass panels so I could reduce the volume of air to try to keep warm while still having decent air circulation. It was not working for a number of the ducks, though.
Meanwhile, I had cleared an area in the walkout basement as a storm shelter. We can get some ripping storms here, and there are trees around - which is great - so flying branches are a concern. And, like this winter, we can get some extended sub-zero weather patterns. So we moved them into the storm shelter for the winter nights. Eventually, because of the weather (our first winter together we had 3 feet of snow on the ground by the end of January) I saw so many advantages to having them in the storm shelter longer term, that I put in some studs and a base and attached plastic poultry fence as "walls" and that is the night pen. I can hear them, the risk of predator incursion is extremely low, it stays above freezing even though it's unheated, they have natural light from a large window, it is working out.
Very few people will have the same situation as I do. So I just want to say that I would rather think ahead to worse case. Arctic cold for weeks on end, multiple feet of snow, high winds, 100˚F sunny days, how will you keep their body temperatures in the healthy range?
I do not have to shovel a path 50 feet just to get to the duck house anymore. So thinking ahead about access should it snow (I know Georgia has seen more snow in the last few years than I ever saw growing up there) would be good. And I recall the wicked ice storms. Access to water, to electricity if you need it, within sight and sound so you can help out in case of trouble, all things to think about.
Also, some little slope around the swim area - even 2% helps water move out of their pen. I have a shallow channel that carries water to a garden area.