There is a chrysalis attached to the cabin in my garden, which this very useful page identifies as most likely a large white. That's the butterfly waiting to emerge, not the cabin! I'm really hoping to catch it in the act and am checking a couple of times a day, though it's been there for a couple of weeks already. Reading up on the matter we might expect 10-15 days to pupate during warm weather (they stay inside for months over winter) so hopefully any day now.

If you look closely at the photo you can see a silk thread tying it down to the surface to its left.