As wessex, I made a very similar application for a 5.5m extension a few months back which was against a much bigger outhouse than the OPs 1mx1m and not strictly speaking PD. But the outhouse was on the boundary with next door - which had a mirrored outhouse - and my client was on the end of the terrace so it didn't bother anybody the other side. We got the approval. I think it comes down to whether they like the overall proposal or not. If they think something's over the top they'll look for reasons. If not they'll either not look too hard or let it go.