To learn what uses are permitted for properties in the city of Philadelphia, one would start by checking the base zoning district for a given property and cross-reference that zoning designation with the relevant section in the Philadelphia Code. A developer would also need to do a smidge more diligence to learn if the property is subject to one, or more, of the 33 additional overlay zoning districts that apply in combination with Philadelphia's base zoning districts. Overlay zoning districts impose additional requirements, restrictions and conditions, which can complicate development throughout the city.

Overlay zoning districts provide city council an additional tool to tailor zoning regulations to different areas throughout the city when creating an entirely new zoning designation might be unwieldy. The ANC (Airport Noise Compatibility) overlay zoning district, for example, allows the city to craft zoning and land use policy for areas adjacent to the Philadelphia International Airport without painting with such a broad brush that industrial, commercial or even residential zoning districts throughout the rest of the city would be affected by land use decisions relevant only to the airport. Another example exists in the DRC (Delaware River Conservation) overlay zoning district, which prohibits the use of certain advertising signage and new structures along certain nature trails along the Delaware River. The city's purpose for creating this district was to enhance quality of life by protecting the parks and trails on the waterfront.