Sell flat with planning permission

I have a 1 bed garden flat in SE London. Share of freehold with just my neighbour above, I've had plans drawn up to do a side return and extension that turn it into a really great 2 bed garden flat.

I've decided against doing the works and I'm going to move. Should I get the planning permission approved (buyer then has to do the work within 3 years)?

Or do a pre-application to get a strong indication from the council that they will approve it? (with no limit as to when buyer can do the work)

Does this add value? And which route is best?

Thanks