In any R District, the required depth of the front yard, on a lot where the new construction or alteration of a building is being considered, may be modified without a variance when the Zoning Inspector deter mines:
   (a)   That the average depth of at least two (2) existing front yards on lots within two hundred (200) feet of the lot in question and within the same block front is less than the least front yard depth prescribed in the applicable zoning district.
   (b)   The modified depth shall not be less than the average front yard depth of the lots identified in subsection (a) above.
   (c)   In any case the modified depth shall not be less than twenty (20) feet in any district and need not exceed one hundred (100) feet in any district.