In an ideal world of course all piggies would be inside for the winter but I would guess that generally speaking most guinea pig owners keep them outside but as Sokel said you have to be 100% dedicated to their welfare.
If they are to remain outside try thinking about different ways to keep them warm, rather than just thinking extra hay.
Perhaps first you could look at where the hutch is situated, could it be moved to a much more sheltered spot? Is it truly out of the prevailing wind and rain? Keep an eye every night on the temperature charts on tv so that you will know when it really does turn icy cold.
Think about getting it raised abit more off the ground and covering it with insulting material and waterfproof outer sheeting.
It's also worth looking at the sleeping compartment - is the door letting in cold air? Is the partition too big? Is your bedding thick enough? Look at every aspect and make changes where possible.
Cardboard is a good insulator so save every bit you can and line the bedding area, perhaps put a cardboard box inside it too stuffed with hay. If you can invest in some vet bed you could put this on layers of newspaper/cardboard and chopped straw. If you have an oversized piece you can turn it up against the door to stop any draughts coming in. If the partition lets lots of cold air in you could put a tunnel from the bed area to the rest of the hutch and seal off the remainder with some timber. The aim is to give them one area where they are cosy and can keep warm overnight.
A snugglesafe hot water bottle is a really good investment too as they keep their heat for several hours, you could pop that in last thing at night. And of course make sure they have lots and lots of hay and extra food.
I would persevere with the water bottle, you can put an old sock over it to keep it from freezing but if you have your hutch front well insulated it shouldn't anyway.
So there are a few suggestions to help you on your way, I hope it helps. It is "do able" but just requires a bit of lateral thinking and some effort on your part.
I'm not saying they should be kept outside but if that's really the only option please do everything you can to ensure they are ok, be responsible.
Without exception all the piggies I have ever rescued have come from homes where they lived outside 12 months of the year so clearly it happens but I think it is also a deciding factor too that people just don't want one more winter cleaning out the piggies, if their heart isn't in it it's a good excuse for them to be rehomed.
Hope some of these suggestions have helped
