[When is a new agricultural unit created in relation to Permitted Development rights ?]

Class B agricultural permitted development rights state development is not permitted for extension or alteration if A change of use of agricultural buildings has been carried out on the agricultural unit under Class Q or Class S  within the previous ten years. Class A similarly states you cant erect or extend a building on an established agricultural unit where a change of use has been carried out under Class Q within the previous ten years.  If someone has developed using PD rights and then splits the farm into smaller units and sells them do the new owners have PD rights as owners of an agricultural unit?