Byelaws under sub-paragraphs (a), (b) and (c) of subsection (1) of the last preceding section may be made with respect to—
structural alterations or extensions of buildings, and buildings so, far as affected by alterations or extensions;
buildings or parts of buildings in cases where any material change, within the meaning of this section, takes place in the purposes for which a building or, as the case may be, a part of a building is used,
and, so far as they relate to the matters mentioned in this subsection, may be made to apply to buildings erected before the date on which the byelaws came into force, but, save as aforesaid, shall not apply to buildings erected before that date.
For the purposes of this section, there shall be deemed to be a material change in the purposes for which a building, or a part of a building, is used if—
a building, or a part of a building, being a building or part which was not originally constructed for occupation as a house, or which though so constructed has been appropriated to other purposes, becomes used as a house; or
a building, or a part of a building, being a building or part which was originally constructed for occupation as a house by one family only, becomes occupied by two or more families; or
where byelaws contain special provisions with respect to buildings used for any particular purpose, a building or a part of a building, being a building or part not previously used for that purpose, becomes so used.