Walls may overhang their supports only in the case of a single-storey building or the top storey of a building:

Provided that-

i) the base of a solid wall or of the leaf of a cavity-wall shall not overhang its support to a total extent greater than one-third of its thickness, whether the overhang occurs on one or both sides;

ii) a cavity-wall which is built up a solid wall, properly bonded, to a height above its base of not less than its total thickness, shall not overhang its support to a total extent greater than one-quarter of its total thickness;

iii) the support for the overhanging portion of the solidly built-up base of a cavity-wall shall be a corbel, in which each individual course projects beyond the course below by not more than one-third of the height of the course

or, alternatively, shall be a further portion of solid wall, properly bonded, equal in height of a corbel constructed as described in this section.