For the purpose of this Part every building or compartment shall be regarded according to its use or intended use as falling within one of its purpose groups set out in the Fifth Schedule to the By-Laws and, where a building is divided into compartments, used or intended to be used for different purposes, the purpose group of each compartment shall be determined separately;

Provided that where the whole or part of the building or compartment, as the case may be, is used or intended to be used for more then one purpose, only the main purpose of use of that building or compartments shall be taken into account in determining into which purpose group it falls.