In article , Jim Webster
writes

I knew there was something about permitted development for domestic but it
wasn't something I have ever had anything to do with. Remember once seeing a
brick built house being built over a timber and tin smaller building while
that was still standing

Section 11 restricts greenbelt house extensions to 40% of the original.
This is to prevent people converting gamekeepers cottages to desirable
mansions.

Once upon a time, an assessment was required to remove grade 1
agricultural land from production. I suspect this is no longer enforced.

regards
--
Tim Lamb