For buildings or other structures that are non-complying with the applicable district height and setback regulations, the maximum height of such altered, enlarged, relocated or reconstructed building or other structure, shall not exceed the height permitted pursuant to either paragraph (a) or (b) of this Section, as applicable. An alteration, enlargement, relocation or reconstruction pursuant to this Section may continue an existing non-compliance, increase the degree of an existing non-compliance, or create a new non-compliance with height and setback regulations, and may continue or increase a non-compliance with other bulk regulations associated with such non-complying height, subject to the limitations of this Section. All permitted obstruction allowances shall be measured with respect to the modified envelopes of this Section.

(a)        For pre-existing buildings or other structures that do not exceed the overall permitted height

Where the height of a pre-existing building or other structure does not exceed the overall height permitted by the applicable district regulations, as measured from the top of the lowest usable floor to the highest point of such pre-existing building, the height of such altered, enlarged, relocated or reconstructed building or other structure shall not exceed:

(1)        the applicable sky exposure plane, for buildings governed by sky exposure planes as measured from the reference plane; or

(2)        a horizontal plane equivalent to the maximum building height permitted by the applicable district for all other buildings as measured from the reference plane.

(b)        For pre-existing buildings or other structures that exceed the overall permitted height

Where the height of a pre-existing building or other structure exceeds the overall height permitted by the applicable district regulations, as measured from the top of the lowest usable floor to the highest point of such pre-existing building:

(1)        the height of such altered, enlarged, relocated or reconstructed building or other structure as measured from the reference plane shall not exceed a horizontal plane equivalent to the pre-existing height of such building, as measured from the top of the lowest usable floor, to the highest point of such pre-existing building, provided also that such height shall not exceed the overall height permitted by the applicable district regulations by 10 percent, or 10 feet, whichever is less; and

(2)        for single or two-family residences in R1-2A, R2A, R2X, R3, R4, R4-1, R4A, or R5A Districts, where the degree of the alteration or reconstruction exceeds 75 percent of the floor area, the height of a perimeter wall of such altered, enlarged, relocated or reconstructed building or other structure as measured from the reference plane shall not exceed the higher of the maximum perimeter wall height for the district, or the pre-existing height of such perimeter wall, as measured from the top of the lowest usable floor to the highest point in such pre-existing building before setback.

However, the height allowances of this Section shall not apply to single or two-family residences that are not non-complying with floor area requirements, or where the provisions of paragraph (b) Section 64-622 (Special open area regulations for non-complying buildings) are utilized.