(1) Empirically constructed external brick bearing-walls, when used to enclose stairways or habitable or working-space above the general roof-level shall be not less than 215 millimeters in thickness.

(2) If the walls referred to in subsection (1) are not hight than 3,6 metres, and the area enclosed between them does not exceed 10 per centum of the area of the roof, they may be constructed as though they did not add to the number of storeys of the building:

Provided that the local authority may, in its discretion, require the thickness of the walls in the storeys immediately under such first-mentioned walls, or under structural members supporting them, to be determined as though the walls above the general roof-level constituted an additional storey.