3The enlargement, improvement or other alteration, as often as occasion may require, of any such building as is mentioned in sub-paragraph (a) or sub-paragraph' (b) of paragraph 1 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 seventeen hundred and fifty cubic feet, whichever is the greater, and
(b)in any other case, by more than one-tenth.