In R6 or R7-1 Districts, in C1 or C2 Districts mapped within R6 or R7-1 Districts, or in C4-2 or C4-3 Districts, where the number of accessory off-street parking spaces required for additional dwelling units created by conversions of any kind exceeds the number of spaces which may be waived as of right under the provisions of Sections 25-262 (For conversions), 36-363 (For conversions in C1 or C2 Districts governed by surrounding Residence District bulk regulations) or 36-364 (For conversions in C4 Districts), the Board of Standards and Appeals may waive all or part of the required spaces, provided that the Board finds that there is neither a practical possibility of providing such spaces:

(a)        on the same zoning lot because of insufficient open space and the prohibitive cost of structural changes necessary to provide the required spaces within the building; nor

(b)        on a site located within 1,200 feet of the nearest boundary of the zoning lot because all sites within such radius are occupied by substantial improvements.