They can stay outside, but it requires extra work.
I keep my girl's on a sheltered patio (it has a roof and walls on three sides). I have insulated the hutch with foam pads and bubble wrap, three fleece blankets, a windproof/waterproof hutch cover, and a tarp that has a heat-reflective lining on the underside. On top of hutch I have also placed a quilt, which can be pulled down over the front.
Inside the hutch, I line the floors with cardboard, then add a layer of wood shavings, and then a layer of hay. The bedroom is packed with hay from floor to ceiling, and they have two snuggle safe heat pads to sleep with. I usually warm the heat pads up morning and evening, but with temperatures being so low at the minute, I am doing it on an 8-2-8-2 rotation, so the pads get reheated every six hours. I change out the damp bedding once a day at least, and try to keep on top of any condensation.
Every hutch set up will inevitably be different. The most important things are to keep it dry inside, free from drafts and protected from the elements. Your girls will need plenty of hay to burrow in, a source of heat, and insulation.