Height and depth
3. In the measurement of height or depth from ground which is not level the height or depth shall be taken to be the mean height or depth, except that–
(a)for the purpose of types 3, 4, 5, 18 or 19 of Schedule 3; and
(b)for any other purpose where the difference in level is more than 2.5 metres,
the height or depth shall be taken to be the greatest height or depth.