Butterfly Conservation reminds us that the UK has several species of butterfly that overwinter as adults. The Small Tortoiseshell, Peacock, Brimstone and Comma butterflies remain dormant, usually from late summer through until early spring.
Brimstone and Comma usually find shelter outdoors in evergreen foliage or log piles, but the Small Tortoiseshell and Peacock prefer enclosed places: shed or garage and even houses.
If you do find a sleepy butterfly indoors, BC has advice about how to increase their chance of survival.