Can you erect a PD extension onto something that already has planning permi - Public forum - Planning Advisory Service (PAS)

My view is that you cannot, but if i`m wrong please tell me. Plansmith has advised a client that if he reduces his existing 6 m deep rear extension (which was given pp years ago) back to 3m in depth he can then build above it with a first floor PD extension......i don`t think he can ......?