(a)        Except as provided in paragraph (b) of this Section, the front building wall for all developments on zoning lots having frontage on Fifth Avenue or Park Avenue and for all developments along street lines within 50 feet of their intersection with the street lines of Fifth Avenue or Park Avenue, shall extend along the full length of its street line fronting on such street without setback for a height of 125 feet above curb level or the full height of the building, whichever is less. Above the height of 150 feet above curb level, the front building wall shall be set back 10 feet from the street line. The mandatory front building wall requirements are optional for the next 20 feet along the street line of a narrow street or for the next 75 feet along the street line of a wide street. However, where the front wall of a building with a height less than 125 feet above curb level was constructed with a setback from the street lines, enlargement of such building may be permitted by vertical extension of its existing building wall.

(b)        For any zoning lot having frontage on Fifth Avenue, Duke Ellington Circle, East 109th Street and East 110th Street, the provisions of this paragraph (b) shall apply. The front building wall of any development shall extend along the full length of the Fifth Avenue and Duke Ellington Circle street lines, and along the full length of the East 109th Street and East 110th Street street lines within 50 feet of their intersection with Fifth Avenue and Duke Ellington Circle, and rise without setback to a minimum height of 85 feet above curb level or the full height of the building, whichever is less. Above a height of 150 feet, the front building wall shall be set back at least 10 feet from such street lines. Such mandatory front building wall requirements are optional for the next 50 feet along the East 109th Street and East 110th Street street lines.

(c)        Front wall recesses are permitted within mandatory front building walls for architectural or decorative purposes, provided that the aggregate length at the level of any story does not exceed 50 percent of the length of the front wall where such recesses are permitted. The depth of such recesses shall not exceed six feet. No front wall recesses are permitted within 20 feet of the intersection of two street lines.

Where the aggregate width of balconies located along the front building wall, at the level of any story, exceeds 20 percent of the length of such front building wall, the front building wall may be set back not more than three feet from the street line.

The underlying district height and setback regulations apply along street lines, or portions thereof, not subject to the front building wall requirement.