Any development or enlargement that meets the location and floor area criteria of paragraph (a) of Section 23-65 and includes a tower shall be constructed as a tower-on-a-base, in accordance with the regulations set forth in this Section. The height of all buildings or other structures shall be measured from the base plane.

(a)        Tower regulations

(1)        At any level above a building base (referred to hereinafter as a "base"), any portion or portions of a building (referred to hereinafter as a "tower") shall occupy in the aggregate:

(i)        not more than 40 percent of the lot area of a zoning lot or, for a zoning lot of less than 20,000 square feet, the percentage set forth in the table in Section 23-65 (Tower Regulations); and

(ii)        not less than 30 percent of the lot area of a zoning lot.

However, the highest four stories of the tower or 40 feet, whichever is less, may cover less than 30 percent of the lot area of a zoning lot if the gross area of each story does not exceed 80 percent of the gross area of that story directly below it.

(2)        Any tower located above a base shall not be subject to the provisions of Section 23-64 (Basic Height and Setback Requirements).

(3)        At least 55 percent of the total floor area permitted on the zoning lot shall be located in stories located either partially or entirely below a height of 150 feet.

When the lot coverage of the tower portion is less than 40 percent, the required 55 percent of the total floor area distribution, within a height of 150 feet, shall be increased in accordance with the following requirement:

(4)        At all levels, such tower shall be set back from the street wall of a base at least 15 feet along a narrow street and at least 10 feet along a wide street, except that such dimensions shall include the depth of any permitted recesses in the street wall.

(5)        No tower or portion thereof shall be located on a narrow street at a distance that is more than 100 feet from the intersection with a wide street.

Unenclosed balconies, subject to the provisions of Section 23-13 (Balconies), are permitted to project into or over open areas not occupied by towers.

For the purposes of determining the permitted tower coverage and the required minimum distance between buildings or portions thereof, that portion of a zoning lot located within 125 feet from the wide street frontage along the short dimension of a block shall be treated as if it were a separate zoning lot.

(b)        Building base regulations

(1)        Street wall location

(i)        On a wide street, and on a narrow street within 125 feet of its intersection with a wide street, the street wall of the base shall occupy the entire street frontage of a zoning lot not occupied by existing buildings. At any height, at least 70 percent of the width of such street wall shall be located within eight feet of the street line, and the remaining 30 percent of such street wall may be recessed beyond eight feet of the street line to provide outer courts or balconies.

However, no such recesses shall be permitted within 20 feet of an adjacent building fronting on the same street line or within 30 feet of the intersection of two street lines.

(ii)        On a narrow street beyond 125 feet from its intersection with a wide street, no street wall of a base is required nor shall any street wall provided beyond 125 feet count toward the computation of any permitted recesses on such wall.

(iii)        Where the street wall of an adjacent building fronting on the same street line is located within 10 feet of the street line, the street wall of the base shall be either located at the street line or aligned with the street wall of the adjacent building for a distance of not less than 20 feet measured horizontally from the side wall of such existing building.

(2)        Height of street wall

All street walls of a base shall rise vertically without setback to a height of not less than 60 feet nor more than 85 feet except:

(i)        On a wide street, if the height of the street wall of an adjacent building fronting on the same street line exceeds 60 feet and if such street wall is located within 10 feet of the street line, the street wall of the base shall match the height of the street wall of the adjacent building to a maximum height of 100 feet by either of three alternatives:

(a)        the street wall of the base shall be extended vertically to the height of the adjacent building for a distance of not less than 20 feet measured horizontally from the side wall of such adjacent building;

(b)        at least 50 percent of the width of the street wall of the base shall be extended vertically to the height of the adjacent building; or

(c)        a dormer shall be provided pursuant to paragraph (b)(3) of this Section. Such dormer shall match the height of the adjacent building.

Such street wall of the base fronting on a wide street may be extended along a narrow street within 70 feet of its intersection with the wide street.

(ii)        On a narrow street beyond 100 feet of its intersection with a wide street, the street wall of a base shall rise vertically to a height of at least 60 feet when the adjacent building is either less than 60 feet or greater than 85 feet, or match the height of the adjacent building when the height of such building is between 60 feet and 85 feet.

For the purposes of this paragraph, (b)(2), inclusive, the height of an adjacent building shall be the height of a street wall, before setback, if applicable, of that portion of an existing building nearest the development or enlargement, fronting on the same street line, and located on the same or an adjoining zoning lot.

(3)        Dormer

For the purposes of this Section, a dormer shall be a vertical extension of the street wall of a base allowed as a permitted obstruction within a required front setback area. A dormer may be located anywhere on a wide street, and on a narrow street within 70 feet of its intersection with a wide street.

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 highest story of the base. For each foot of height above the base, the aggregate width of all dormers at that height shall be decreased by one percent of the street wall width of the highest story of the base. Such dormer shall count as floor area but not as tower lot coverage.

(4)        Open areas

All open areas at ground level, located between the street line and the street wall of a base shall be landscaped except in front of entrances and exits to the building.

(c)        Modification of tower coverage and floor area distribution requirements

The tower lot coverage and floor area distribution requirements set forth in paragraph (a)(3) of this Section shall be modified for buildings that provide articulation of a base in accordance with the following provisions:

(1)        Recesses

Recesses shall occupy, in the aggregate, between 30 and 50 percent of the width of each eligible story of the base, and measure at least two feet in depth. In addition, the width of any individual recess provided within eight feet of the street line shall not exceed 25 percent of the width of the street wall of the base, unless such recess is provided in combination with an additional recess located beyond eight feet of the street line.

Furthermore, all recesses shall comply with the provisions of paragraph (b)(1) of this Section or paragraph (a)(1) of Section 35-64 (Special Tower Regulations for Mixed Buildings), as applicable. For each street frontage of a building with recesses provided in accordance with this paragraph, (c)(1), the percent of lot coverage of the tower portion of the building may be decreased by 0.5 percent, and the minimum percent of total building floor area distribution below a level of 150 feet may be reduced by 0.25 percent.

(2)        Dormers

For each street frontage with dormers, provided in accordance with paragraph (b)(3) of this Section, that measure, at their lowest level, at least 50 percent of the width of the street wall of the highest story of the base, and measure, at their highest level, at least 25 percent of the width of the highest story of the base, and rise at least 25 feet above the base, the percent of lot coverage of the tower portion of the building may be decreased by 0.5 percent, and the minimum percent of total building floor area distribution below a level of 150 feet may be reduced by 0.25 percent.

(3)        Matching provision

For each street frontage that provides an extension of the street wall of a base that matches the height of an adjacent building in accordance with paragraph (b)(2)(i)(b) of this Section, the percent of lot coverage of the tower portion of the building may be decreased by 0.5 percent, and the minimum percent of total building floor area distribution below a level of 150 feet may be reduced by 0.25 percent.

However, the total percent of lot coverage of the tower portion of the building shall not be decreased by more than 2.0 percent, nor shall the minimum percent of total building floor area distribution below a level of 150 feet be reduced by more than 1.0 percent.