I will be the first to admit that my original piggies (Rosie and Bramble) lived in a hutch outside (we were in the UK at the time).
After a year we ended up building a Summer house for all of our animals (we had rabbits and guinea pigs by this point) and could see that being outside, year round in the British climate was less than ideal.
But they were ok, and this seemed fine 15 years ago. Infact people would marvel at the fact that we couldn't actually use our Summer House, as it was full of animal cages.
So 18 months ago when I decided to embark once again on my guinea pig journey, I knew they would live outside.
Because guinea pigs live outside.
A good quality wooden hutch was bought, a suitable hutch cover was researched, and I spoke to another local owner who had piggies outside year round.
And then I bought some weeny baby piggies home far earlier than planned when it was too cold to put them outside, so the outdoor hutch was hauled into the living room temporarily.
18 months later and the outdoor hutch has been sold, and the guinea pigs are now permanently inside in a C&C.
All of the up to date advice available suggests that it is not ideal for a guinea pig to be outside in the climate I currently live in (Switzerland).
For me having them indoors has been a wonderful experience, meaning they are part of day to day family life, handled regularly, and protected from predators (martens are an issue where I live) and extremes of climate (in Winter we often see temperatures of minus 20 degrees centigrade).
I am lucky because I have the space to have them inside and none of the family objects to this (in fact they wouldn't have it any other way now).
I know there are several local people who think it is fine to have their piggies outside year round, and their piggies do indeed seem fine.
But for me I am happier with them indoors.
Everyone is different - you need to do what works and is best for you.