Is Planning Permission required to create a separate planning unit? - Public forum - Planning Advisory Service (PAS)

Is planning permission required to fence off land and create a separate planning unit? I would assume the actual fencing off is permitted development while still owning the land. But does the newly created planning unit become unlawful once it is transferred to a new owner? The "material use of the land" has not been decided yet, although would be much the same and it's prior use. Thank you for any helpful comments.