What is Predominantly Built-up Area?
Predominantly Built-up Area meaning A predominantly built-up area is a blockfront entirely within an R4 or R5 district (without a suffix) in which optional regulations that permit higher floor area ratios and lower parking requirements may be used to produce infill housing. At least 50 percent of the area of the block must be occupied by zoning lots developed with buildings, and the zoning lot that will be developed with infill housing may not exceed 1.5 acres (65,340 sq ft). Infill regulations may not be used to redevelop a lot occupied by a one- or two-family detached or semi-detached house unless the blockfront is predominantly developed with attached or multifamily housing.
reference: Glossary of Planning Terms – NYC Planning