Unless you have a listed building, your extension will most likely come under "permitted development." No permission needed, but see the Planning Portal links above for limits on size and position etc.
You will need to serve a Party Wall notice to the neighbours though...
http://partywallnotices.co.uk/
You will definitely need Building Regulations inspections. Go for a "Building Notice". A simple system that needs no technical drawings. It's all carried out with consultations between the Building Control Dept. and the builder.