It is often advisable to speak to your neighbours about this kind of thing before you submit for planning permission because doing so can make planning permission easier to achieve. It will also make getting the party wall agreement completed easier. In any case, it should be served a full 2 months before any work is to begin.
Note: that even if they do not sign the agreement, your neighbours cannot simply prevent you from carrying out building works. If they refuse the agreement a surveyor must instead carry out a ‘schedule of condition, which will outline how summit and your chosen contractor can carry out the work in complete safety. It must also show how any loss or damage to your neighbour through the building works will be mitigated.