Notwithstanding anything in section 12 (2) (a) of the principal Act (carrying out of works for the maintenance, improvement or other alteration of a building not to constitute development if it is wholly internal or does not materially affect the building's external appearance) the carrying out of works for the alteration of any building by providing additional space therein below ground shall, if begun after the commencement of this section, be treated for the purposes of the principal Act as involving development.