Planning question - 'stepping back' new extension

[mummmypirate]

We're planning a side return extension. Is it right that to be acceptable to planning, rear extensions must follow the lines of the existing house e.g. if the rear is L shaped the addition must be stepped back from the existing rear wall and not flush and squared-off?

No. Look at the guidance for your area as produced by your planning authority. There is a difference between permitted development rights and wanting a bigger extension than permitted development.