Is planning permission required to repair and refurbish a derelict cottage?

If not, how derelict can it be and still be exempt?

Is planning permision required for a sewage treatment system or does that fall within the "curtilage of the house"?

Finally, is it also possible to build an extension, subject to the limits laid down in the 2001 SI?

I am not certain of the answer to this but I believe that you need planning permission for a sewage treatment system as I have seen planning being applied for before for septic tanks etc.
Have you been in touch with the planning department in your local authority to ask them these questions? I would be interested to hear the answer.