http://adlib.everysite.co.uk/resources/000/264/299/annexepps7.pdfSurely the key is ' trade or business'
The following extracted from the above pdf. To be within the rules I would think the total land needs to be a 'Unit' you must be able show it is for the purposes of a trade or business. I could have it totally wrong though.
must be on agricultural land, which
means land in use for agriculture for the purposes of a trade or
business, and excludes any dwelling house or garden;
must be reasonably necessary for the purposes of agriculture within the unit. This condition does
not require that a new building should necessarily accommodate an agricultural use already
existing in the unit. Agricultural developments which are entirely self-contained and have no
direct relationship with the rest of the unit may thus benefit from permitted development rights;
must not give rise to, or alter or extend, a dwelling;
must not be within 25 metres of the metalled part of a trunk or classified road.