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Ancillary Dwelling Unit (ADU) Types 

Newly Created ADUs

  • Above Grade ADU within a 1-Family Primary Dwelling
    An Above Grade ADU is an additional, self-contained dwelling unit created entirely above the grade plane within a one-family home. These units can be located in an attic, on a new floor addition, or as part of a horizontal enlargement. They must be fully separated from the primary dwelling with their own entrance and meet all general requirements under Building Code (BC) Appendix U, including light, ventilation, fire separation, and utility standards.

    The ADU and primary one-family home remain a R-3 occupancy, and a 1-hour fire separation is required between the two units. This typology provides a flexible option for homeowners looking to create a long-term rental unit or space for extended family often within their existing home’s footprint. Attic ADUs specifically require automatic sprinklers, while other above-grade ADUs within a 1-family building are exempt.

  • ADU In a Basement or Cellar
    Basement and cellar ADUs offer a way to legalize or add housing below grade, but they come with strict requirements to ensure safety and code compliance. These units are only allowed in existing one- or two-family homes, and cellar ADUs must have ceilings at least two feet above the grade plane. Units must provide proper light and air, emergency egress, water sensors, fire separation, and separate utility shutoffs.

    Basement/cellar ADUs located in flood-risk areas—including Special Flood Hazard Areas, coastal zones, and future rainfall zones—are strictly prohibited. Legalization of existing subgrade units is only allowed in designated program areas under Local Law 126, with a phased 10-year compliance schedule. All subgrade ADUs must be sprinklered, meet radon/vapor mitigation standards, and comply fully with BC Appendix U before obtaining a Certificate of Occupancy.

  • ADU Separated by Fire Wall from a 2-Family Primary Dwelling
    In a two-family home, a third unit may be added as an ADU if it is physically separated by a fire wall. This approach allows the property to remain classified as R-3 occupancy and avoids triggering Multi-Dwelling Law requirements, which would otherwise apply to three-family buildings. The fire wall must be made of concrete or masonry per BC 706 and fully separate the ADU’s structural and building systems from the main dwelling, from the foundation to the roof.

    These ADUs are considered separate buildings, even if attached, and must meet all general and specific provisions under BC U203. They must be fully code-compliant with independent exits, fire protection, and vertical circulation. This typology is often used for side additions or rear yard extensions that are structurally independent but has an integrated massing on a single lot.

  • ADU Detached from the Primary Dwelling
    Detached ADUs are stand-alone dwellings located on the same lot as a one- or two-family primary residence. These include new backyard cottages and garage conversions. Detached ADUs must comply with zoning limitations on setbacks, maximum height, and footprint, typically no more than one-third of the required rear yard area.

    These units must always be sprinklered and are classified as R-3 occupancy. When created as new buildings, they must be at least 10 feet from the main residence and 5 feet from property lines. If converting a garage, existing non-compliances (like setbacks) may be retained. All detached ADUs must also provide a minimum 5-foot-wide clear access path from the street, unobstructed by fences or structures, to meet Fire Code and Building Code Appendix U access requirements.

  • Use of Manufactured Homes as ADUs
    Manufactured homes certified by New York State or the US Department of Housing and Urban Development (HUD) may be used as an ADU, provided they comply with BC Appendix U. These units must bear a certification emblem and are treated similarly to other detached ADUs in terms of site placement, access, fire safety, and utilities. The factory-fabricated home must also be installed per DOB-approved site connection requirements.

    Plan examiners review these ADUs in accordance with TPPN 6/88, which outlines the scope of DOB’s review for manufactured housing. While the internal construction is state or federal-certified, the site-related components such as foundations, utility hookups, and code-compliant access are subject to local approval.

Pre-Approved ADUs

At the behest of the Mayor’s Office of Policy and Planning (MOPP), the NYC Department of Buildings (DOB), in conjunction with the NYC Department of Housing Preservation (HPD) and the Economic Development Corporation (EDC), is establishing an online Pre-Approved ADU Plan Library, hosted on HPD’s ADU homeowner assistance website ADU for You.

A Pre-Approved ADU Plan is a site-agnostic architectural design that meets NYC Construction Codes. To use a Pre-Approved ADU design, homeowners must contact the associated Registered Design Professionals (RDPs), who will then review the site and file a standard job application with the DOB. Applications using a Pre-Approved ADU must include site-specific elements such as zoning analysis and utility connections.

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