One would not apply for OPP for an extension - its DPP every time. The drawings are public documents once the Planning Application has been made and you may copy them. The granting of PP goes with the house on its sale.
The second part of the process is getting Building Regs approval. You need to find out whether a Building Application has been made. This typically involves taking the planning drawings and annotating them with structural and other details such that Building Control (another department at the Local Council and nothing to do with the Planning people) can assess that new structure will be safe, hygienic and adequately insulated. These drawings are not public documents and you cannot view them or copy them at the council (unless you are the applicant). It might cost you typically �600 to have such drawings made by a plan drawer (no point having an architect do this - just costly) if you cannot acquire them from the vendor and the Building Control application is about �200ish. An alternative route to gaining Building Control approval is to issue a Building Notice to building control. This involves the same application cost but does not involve putting any plans to them. They merely come around and inspect that the Regs have been complied with during construction. Many smaller extensions are done this way, but an architect who is project managing the job will never follow this route - they will want to follow the full process (and charge accordingly).
In summary, the PP is yours as a right, and any attempt to charge you, the vendor, over the odds for any prepared Building Regs drawings could always be overcome by you. I'd seek to negotiate the inclusion of these drawings in the sale package for a small notational amount - say �100.