Sch 3 para 10
(1) Any reference in this Schedule to the cubic content of a building shall be construed as a reference to that content as ascertained by external measurement. (2) For the purposes of paragraph 1 the cubic content of a building is substantially increased or exceeded— (a) in the case of a dwellinghouse, if it is increased or exceeded by more than one-tenth or 1,750 cubic feet, whichever is the greater; and (b) in any other case, if it is increased or exceeded by more than one-tenth.