(1) Cavity walls of greater thickness than the foundation wall on which they rest shall not be corbelled but may project 25 mm over the outer face of the foundation wall disregarding parging.
(2) Where the foundation wall referred to in Sentence (1) is unit masonry, it is permitted to be corbelled to meet flush with the inner face of a cavity wall provided,
(a) the projection of each course does not exceed half the height or one-third the width of the corbelled unit, and
(b) the total corbel does not exceed one-third of the foundation wall thickness.