an extension of apparatus to a length greater than the length of existing apparatus shall not be treated as a placing of apparatus of greater dimensions than those of the existing apparatus except in a case where the apparatus as so extended serves a purpose (either additional to or instead of that served by the existing apparatus) which was not served by the existing apparatus; and

where the provision of a joint in a pipe or cable is agreed, or is determined to be necessary, the consequential provision of a jointing chamber or of a manhole shall be treated as if it also had been agreed or had been so determined.