If the extension conforms to all of the requirements here then no planning permission was required anyway (unless, say, the property is in a conservation area) as it's a 'permitted development':
https://www.planningportal.co.uk/info/200130/common_projects/17/extensions
Larger extensions though need to go through the Neighbour Consultation Scheme:
https://www.planningportal.co.uk/info/200130/common_projects/17/extensions/2
If the relevant council failed to use that scheme (when, due to the size of the extension, it was legally required to do so), or if planning permission was actually required and the council failed to notify one or more interested parties, then you can seek judicial review of the way that they handled things (for which you'll certainly need the services of a solicitor or barrister and for which you'll probably need a big bank balance).