Meaning of high-rise residential building

3.—(1) In these Regulations “high-rise residential building” means a building containing two or more sets of domestic premises that—

(a)is at least 18 metres above ground level; or
(b)has at least seven storeys.
(2) For the purposes of paragraph (1)—

(a)the height of a building is to be measured to the height to the top storey in accordance with Appendix D to Approved Document B;
(b)when determining the number of storeys a building has—
(i)any storey which is below ground level is to be ignored,
(ii)any mezzanine floor is a storey if its internal floor area is at least 50% of the internal floor area of the largest storey in the building which is not below ground level, and
(iii)a storey is treated as below ground level if any part of the finished surface of the ceiling of the storey is below the ground level immediately adjacent to that part of the building.