In M1-6D Districts, the height and setback provisions of this Section shall apply to all buildings.

(a)        Rooftop and permitted obstruction regulations

(1)        Permitted obstructions

The provisions of Section 33-42 shall apply to all buildings.

In addition, a maximum base height or sky exposure plane may be penetrated, as follows:

(i)        Structural columns

Structural columns may penetrate a maximum height limit or sky exposure plane, provided that such columns are one story or less in height, have a street wall no greater than 30 inches in width, and are spaced not less than 15 feet on center.

(ii)        Dormers

On any street frontage, dormers may be provided in accordance with the provisions of paragraph (c) of Section 23-623 (Permitted obstructions in certain districts).

(2)        Screening requirements for mechanical equipment

For all developments and enlargements, and conversions of non-residential buildings to residences, all mechanical equipment located on any roof of a building or other structure shall be subject to the provisions of Section 37-22 (Special Rooftop Screening and Enclosure Regulations).

(b)        Height and setback

(1)        Street wall location

The street wall shall be located on the street line and extend along the entire street frontage of the zoning lot up to at least the minimum base height specified in paragraph (b)(2) of this Section. On the ground floor, recesses shall be permitted where required to provide access to the building, provided such recesses do not exceed three feet in depth as measured from the street line.

Above the level of the second story, up to 30 percent of the aggregate width of street walls may be recessed beyond the street line. However, no recesses shall be permitted within 20 feet of an adjacent building and within 30 feet of the intersection of two street lines.

(2)        Base height

(i)        Along wide streets

On wide streets, and on narrow streets within 50 feet of their intersection with a wide street, the street wall of a building shall rise without setback to a minimum base height of 125 feet and may rise to a maximum base height of 155 feet.

(ii)        Along narrow streets

On narrow streets, beyond 50 feet of their intersection with a wide street, the street wall of a building shall rise without setback to a minimum base height of 85 feet and may rise to a maximum base height of 135 feet. However, for buildings on zoning lots meeting the criteria set forth in paragraph (a) of Section 23-664 (Modified height and setback regulations for certain Inclusionary Housing buildings or affordable independent residences for seniors), the maximum base height may be increased to 155 feet.

As an alternative, the minimum and maximum base heights applicable to a wide street may apply along a narrow street to a distance of 100 feet from a wide street.

(3)        Required setbacks and maximum building heights

(i)        Along wide streets

For buildings, or portions thereof, located on wide streets and on narrow streets within 100 feet of a wide street, the portion of such building above the maximum base height set forth in paragraph (b)(2)(i) of this Section shall be set back from the street wall of the building at least 10 feet along a wide street and at least 15 feet along a narrow street, except such dimensions may include the depth of any permitted recesses in the street wall. The maximum height of such buildings shall be 290 feet. In addition, the gross area of each of the highest two stories of such building shall not exceed 80 percent of the gross area of the story directly below such highest two stories.

(ii)        Along narrow streets

For all buildings, or portions thereof, located on narrow streets beyond 100 feet of a wide street, no portion of such building or other structure shall penetrate a sky exposure plane which begins at the maximum base height set forth in paragraph (b)(2)(ii) of this Section and rises over the zoning lot with a slope of four feet of vertical distance for every foot of horizontal distance.

Any portion of such building or other structure that is located beyond 15 feet of the street line may penetrate such sky exposure plane, provided such portion does not exceed a height of 210 feet. However, for buildings on zoning lots meeting the criteria set forth in paragraph (a) of Section 23-664, such maximum height may be increased, provided that the maximum number of stories does not exceed 23, the maximum height of a building with a non-qualifying ground floor does not exceed a height of 230 feet, and the maximum height of a building with a qualifying ground floor does not exceed a height of 235 feet.

In addition, the gross area of each of the top two stories of a building may not be greater than 80 percent of the gross area of the story directly below such top two stories.

(4)        Maximum length of building wall

The maximum length of any story located entirely above a height of 150 feet shall not exceed 150 feet. Such length shall be measured in plan view by inscribing within a rectangle the outermost walls at the level of each story entirely above a level of 150 feet.