As part of the City of YES affordable housing initiative, the Department of Buildings (DOB) recently promulgated Rule 105-08, which lays out the design, safety, and code requirements for homeowners who want to add ancillary dwelling units, or ADUs: secondary, self-contained homes on the same lot as a primary residence.
Prior to this legislative update, which was approved by New York City Council in June, legal ADUs were highly unlikely to be part of the urban canvas. Concerns about how much space would be needed and available for a proper dwelling without encroaching on neighboring properties, coupled with fears about poor construction and lax safety measures, kept ADUs from being a viable option for expanding available housing.
Faced with an ongoing housing crisis and increased construction costs, the City developed an approach that allows ADUs to be constructed so long as they follow certain regulations. ADUs can now include basement apartments, converted garages, backyard structures, or additions that function as independent living spaces.
What Rule 105-08 Covers
Rule 105-08 adds Appendix U to the New York City Building Code, establishing standards for creating ADUs in one- and two-family homes. The appendix outlines where ADUs can be built, minimum safety features, and coordination with other codes such as the Zoning Resolution and the Housing Maintenance Code.
Here are the main highlights:
Definition of an ADU
In order to be a legal ADU, a self-contained residential unit must have its own kitchen, bathroom, and sleeping area and be located on the same lot as a primary home. It can be attached, detached, or located within the main house (for example, in a basement or cellar).
Location
ADUs are generally permitted in residential zoning districts that allow one- and two-family dwellings. However, not all spaces qualify. For example, cellar ADUs are allowed only under strict conditions, and only once the Housing Maintenance Code is amended to permit such occupancy.
Egress and Life-Safety
Every ADU must meet egress standards under Chapter 10 of the Building Code. Cellar ADUs must have two independent exits to ensure safe evacuation during an emergency.
Additionally, sprinkler systems are mandatory:
- A cellar ADU must have a full NFPA 13D sprinkler system (as modified by Appendix Q of the Building Code).
- If the building has three or more stories and includes a cellar ADU, the entire building must be sprinklered.
Light and Air
To ensure adequate light and ventilation for below-grade spaces, Rule 105-08 specifies that yards or open spaces adjacent to ADU windows can’t sit more than six inches below the windowsill.
Code Coordination
Appendix U works alongside several existing building appendices (including Appendix Q, which governs small dwellings). However, specific sections, like § § U202.8-U202.11, have unique provisions for ADUs that cover flexibility in unit size, access, and building layout.
Permitting Process
Owners must submit applications through the DOB for all ADU projects. The process mirrors standard building permits, but includes additional documentation verifying that the proposed unit meets Appendix U standards. The rule encourages coordination between owners, architects, and code officials early in the design phase.
What Rule 105-08 Means for Homeowners
The new ADU rule creates legal opportunities to generate rental income, and/or provide housing for family members or caregivers.
However, property owners should understand that:
- Existing illegal conversions won’t automatically qualify. Those must be brought up to full code compliance.
- Sprinkler and egress requirements can be costly but are non-negotiable.
- Professional design and permitting are essential. DIY conversions are not allowed under the rule.
What Rule 105-08 Means for Architects
Architects now have a clear framework for designing ADUs in compliance with city code. This includes:
- Applying Appendix U and cross-referencing Appendix Q for small residential construction.
- Designing cellar and basement ADUs with enhanced fire protection and exit systems.
- Verifying that yards, courts, and windows meet light and ventilation criteria.
- Preparing thorough documentation for DOB approval.
This rule will likely create a new wave of small-scale residential projects, giving architects opportunities to innovate in compact design, modular systems, and adaptive reuse of existing spaces.
Next Steps
If you have an existing, non-compliant ADU, it will need to be brought up to the new code standards. If you are considering building an ADU, it’s vital that both you and your architect understand the rules and plan ahead. Now that there are established regulations, we expect the DOB will be proactive in enforcing them.
