Overlay district

An overlay district is a mapped zoning layer that modifies the rules of the underlying district wherever the two coincide, rather than replacing the underlying district. New York's most common example is the commercial overlay: strips mapped along shopping streets within residence districts, allowing local retail and service uses on lots that remain residentially zoned underneath, with the overlay's own rules governing the commercial floor area and its parking.

The analytical consequence is that a lot's zoning is a stack, not a single code: underlying district, possible overlay, possible special purpose district, each layer able to modify use and bulk outcomes. Reading only the underlying district code on a map answers less than it appears to — the overlays are where much of a mixed street's actual zoning lives.