On a block entirely within an R4 district (without a suffix), optional regulations may be used to develop infill housing in predominately built-up areas. Infill regulations may be used if at least 50% of the area of the block is occupied by zoning lots developed with buildings, and the lot does not exceed 1.5 acres (65,340 sq. ft.) However, 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 or commercial or manufacturing uses. Infill regulations can be found in the definition of predominantly built-up areas in Section 12-10 of the Zoning Resolution.

On sites that qualify for infill housing, the higher floor area ratio (FAR) of 1.35  and lot coverage of 55%, as well as more relaxed parking requirements, permit developments with greater bulk and more dwelling units than are otherwise permitted in R4 districts. Infill regulations typically produce three-story buildings with three dwelling units and two parking spaces—one in a ground-floor garage and the other in the front yard driveway. Infill regulations can also produce small apartment buildings. 

To ensure that infill housing generally conforms to existing neighborhood scale, height and setback regulations for R4 infill housing are the same as for R4 districts. The maximum street wall length for a building on a single zoning lot is 185 feet. Front yards must be at least 18 feet deep to prevent cars parked in the front driveway from jutting onto the sidewalk. Off-street parking is required for two-thirds of the dwelling units.