11. In relation to development within category 2–
(a)the area of gross floor space to be created by the development shall be ascertained by external measurement of the floor space, whether or not it is to be bounded wholly or partly by external walls of a building,
(b)where the area of gross floor space to be created by the development exceeds 75 square metres and is not an exact multiple of 75 square metres, the area remaining after division of the total number of square metres of gross floor space by the figure of 75 shall be treated as being 75 square metres.