6.—(1) In relation to development within category 5, 7 or 12(b) specified in Part 2, 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.N.I.
(2) In relation to development within category 5 or 12(b) where the area of gross floor space 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.
(3) In relation to development within category 7 where the area of gross floor space is not an exact multiple of 500 square metres, the area remaining after division of the total number of square metres of gross floor space by the figure of 500 shall be treated as being 500 square metres.