3The enlargement, improvement or other alteration, as often as the person having the right to carry out such operations may desire, of any such building as is mentioned in paragraph 1(a) or (b) of this Schedule, or any building substituted for such a building by the carrying out of any such operations as are mentioned in that paragraph, so long as the cubic content of the original building is not increased or exceeded—
(a)in the case of a dwellinghouse, by more than one-tenth or 1,750 cubic feet, whichever is the greater; and
(b)in any other case, by more than one-tenth.