While Jack is dealing with the PWA, may I butt in about the "starting?"
Official "starting" can be quite simple actions such as: clearing ground; setting out pegs and lines where the walls are going: putting in drains etc.
The trouble is, you'll have no "evidence" as to exactly when the start was made. (Yes... I know from your previous threads all about your neighbours, and how they might use this.)
When a Building Regs application goes in, there are stages/inspections which are logged. One of these notifications is "commencement." That's what I always use to prove a start has been made.
Of course, you are not at that stage since you have no Building Regs plans lodged with "Building control" yet. So you can't use that .
My honest opinion is that you should sell as it is now, with Planning already approved, and let the new owner sort out the extension.
The new people might well choose to alter things anyway, rather than being stuck with a "fait accompli".
You have 3 years after all.
I know this isn't what you'd like to hear, but believe me, you don't make a property more valuable/easier to sell by presenting buyers with foundations for something they may not even want.
In a perfect world (i.e. limitless cash) actually building the extension completely is the only way to add value.