Means air space permanently open -

(i) if it is a street,
(ii) if its freedom from encroachment is protected by any low or contract ensuring that the ground below it is either a street or is permanently and irrevocably appropriated as an open space.

In Determining the open air space required for construction of a building, any space occupied by an existing structure may, if it is ultimately to become a permanently open air space, be treated as if it were already such a place.