I strongly suspect the builder was human. :-)
It looks remarkably similar to a commercially available roosting pocket similar to this one here.
Ian Chester said:It was found in our garden in the conifer hedge Paul. No reason to post anything other than to establish what bird made it. It would appear tits are getting everywhere...
?
Perhaps put there by a neighbour / previous householder? As Wendy has said, I wouldn't rule out the possibility of it being used as a nest if it's been there for a while.