(1)Every person giving any notice of his intention to erect a building under section 401, shall specify the purpose for which such building is intended to be used :Provided that for any building, not more than one class of use, consistent with the occupancy of the use group within the meaning of clause (2) of section 398, shall be considered, except in respect of the case where, under this Act or any other law in force for the time being, mixed occupancies of specified nature may be permissible.
