Are you sure you need planning?

http://www.planningportal.gov.uk/permiss...xtensions/

Single story extensions of a certain size fall under permitted development. Generally, planning can be very inconsistent. I've had all sorts of problems with planners.

The issue with most councils at present is that their planning departments are woefully understaffed. There are a few brilliant officers in Lewisham planning who hopefully are thill there, and some pretty rubbish ones who hopefully still aren't.

Good luck.