On one job we had a women nice as pie to our faces, asking how it was coming along and saying it was looking good only to find out she had written twenty seven letters of complaint!!!
The wonderful thing now is that you can view all their letters online if it is a planning application. To be honest, from my experience letters of complaint from neighbours carry very little weight with planners so don't worry. Just keep up a dialogue with Planning, they are normally quite helpful. They need all the fees they can get these days!
But if the neighbours complain to the Council as there is no planning application they have a duty to keep such complaints confidential. And I do find (and know from many a previous experience) that complainants will often involve Councillors and sometimes MPs and whilst a dialogue with the Planners is normally desirable don't assume a friendly politeness means a planning permission is on the cards.
First thing is to establish with householder extensions and outbuildings etc, is to see if it can be built without planning permission - always the best option if possible and hence the link in my previous post.