One time when this came up, I was in a particularly sensitive Conservation Area. I made the usual application, and sent off the forms and the fee.
Some time later, they returned both with a note to say PP wasn't needed because of PD Rights. It may be handled differently though in other Districts.
Jack mentioned withdrawal of PD Rights. This certainly happens for various reasons. In my opinion, it's a cop-out by the Planning Dept. Junior Officers tend to be quite nervous about committing themselves. I had a case recently where the Chief Officer was on holiday. The guy I was dealing with Ok-ed my proposal, so I prepared to get the project going. The senior Officer then returned from holiday, gave him an earful and turned the application down for various reasons.
As with any organisation, they're after the money. You can easily satisfy yourself as to whether your proposal is under PD....... so can they!
If I'm happy that a proposal complies, then I tend to just go ahead. In fact, I don't usually trouble them with it.... it only worries them.
It is your decision entirely though, Fiery. I don't know your details. There may be some aspect of your project that is debatable. Only you can know.
Costing much the same as a PP application is a "Certificate of Lawfulness". It's self-explanatory. It carries the same weight as a regular PP for legal conveyancing purposes.