Different species of butterfly spend the winter in different forms. Small tortoiseshells, commas, peacock, brimstone and some red admirals all overwinter as adults, while several blues and browns survive in chrysalis form.
Those butterflies that overwinter as adults in the UK need shelter, either natural or man-made. The key is that it must be cool and dry, otherwise they are at risk of fungal infections that can kill them. Garden sheds, outhouses or garages are all good spots. Wooden fences, made with untreated wood, are also useful, as are wall crevices. For natural shelter, ivy and woodpiles are ideal. Leaving some areas of long grass and seedheads will give essential sites for grass-feeding butterflies.
Sometimes butterflies overwinter in houses, which can be a bit of a problem:
