This Government site provides a booklet called the Planning - A Guide for Householders
http://www.communities.gov.uk/documents/planni ngandbuilding/pdf/144827
Look at Section D on fences.
It applies in England only - Scotland, Wales and NI may follow this but they do their own thing on Planning these days.
This is an interpretation of the primary legislation that was enacted many moons ago that covers this. Somewhere buried in one of the Town & Country Planning Acts will be the actual wording, but I don't know where. Frankly this 'rule' is well-known and you shouldn't need to quote them more than this booklet.