All mine are outdoors, I use Chartwells hutches with a thermal cover (hutch hugger) and their main cover. Even in the depth of winter, they are pretty comfortable but mine do have a threshold. Their bedroom area in the winter is filled with boxes and stuffed with hay and fleeces over the top plus snuggle safes and finally a heated blanket. Additionally, you can purchase a dog bed warmer pad, it can hook up to your outdoor electric supply, check if it has room for a transformer type plug, the cable is armoured to help with hungry and interested mouths...
I keep old carpet too and line the hutches in the winter with the carpet then use puppy pads that have the plastic film lining then their equine bedding and hay. I also use the bottle covers to stop them from freezing over.
So yeah, it is really possible to keep outdoor piggies outdoor, but I do have an outdoor weather station, trained cctv and I keep checks on them during the day for everything....my babies are precious.
In the really super bad winter I can bring them all indoors for emergencies, but with all of the above you should be okay. Just bear in mind, I'm in a really rural area and the weather can hammer us from the north and the pigs are south facing so they are better protected. But temperature wise - its lower in the countryside than cities.