It is possible to keep guinea pigs outside, either the whole year round or half a year. The problem in the second case is that the guinea pigs have to acclimatise when moving from inside to outside. Thus they should only move out when the temperature is mild, neither frosty nor too hot. The perfect time to move the guinea pigs outside is therefore around May.
A guinea pig should never live alone, especially not outside. If the guinea pigs overwinter outside, the group should at least contain four members. Although guinea pigs don’t like to huddle together, their presence is enough to keep warm with their output body heat.
Another important thing when moving guinea pigs outside is to familiarize them with grass and greenery prior to their moving. Otherwise they might get ill.
A daily health check is recommended when keeping guinea pigs outside. Old or ill guinea pigs should never overwinter outside and instead should be kept well guarded against the cold. The same applies for pregnant guinea pigs.
If it is really hot outside, the guinea pigs need to be guarded as well. Solar exposure should be avoided by placing some bushes that provide shade. Good ventilation inside the enclosure and especially inside the houses has to be ensured.
Every guinea pig living outside needs at least one den, where it can hide and is secured. The den needs to be big enough for all guinea pigs to find a place, but should not be too big, because then it would lose too much heat in the winter. For keeping the guinea pigs warm it should be well lagged with heat-insulating materials like styrofoam. On the ground there should be a thick layer of hay or straw for the same purposes. The den should be cleaned at least twice a week to avoid bacteria and illnesses.
To protect the guinea pigs against birds of prey and other predators, the enclosure must be covered with a wire mesh preferably.