Ramsman wrote:
If I were to fit guttering to our garage (5.3 m x 5.3 m, gabled roof)
to drain to a water butt, would I have to provide a soakaway for the
surplus water? There is no guttering at the moment, and the rain
simply runs off into the ground all along both sides of the roof.

Are you intending to tell the BCO about the guttering?

--
Adam