The street wall of a building may be vertically extended above a height of 85 feet above curb level without setback in accordance with either of the following provisions:

(a)        A dormer may be allowed as a permitted obstruction within the required initial setback distance above a height of 85 feet above curb level. The street wall of a dormer shall rise vertically as an extension of the street wall of the building. A dormer may be located anywhere on a wide or narrow street frontage.

On any street frontage the aggregate width of all dormers at the required initial setback level shall not exceed 60 percent of the width of the street wall of the story immediately below the initial setback level. For each foot of height above the required initial setback level, the aggregate width of all dormers at that height shall be decreased by one percent of the width of the street wall of the story immediately below the initial setback level. Such dormers shall count as floor area but not as tower lot coverage.

(b)        On a wide street and on a narrow street within 50 feet of its intersection with a wide street, the street wall of a building may be vertically extended without setback within the required initial setback distance above a height of 85 feet above curb level, up to a maximum height of 125 feet, provided that the aggregate width of such street walls shall not exceed 50 percent of the width of the street wall of the story immediately below the initial setback level and provided the street wall of the building contains special architectural expression lines at a height of 85 feet above curb level.