How the Multiple Dwelling Law and Housing Maintenance Code Shape COs

In New York City, Certificates of Occupancy (COs) dictate how a building can be legally used. Two key laws—the Multiple Dwelling Law (MDL) and the Housing Maintenance Code (HMC)—govern how residential buildings must be constructed, maintained, and occupied. Property owners who fail to comply with these laws risk CO violations, fines, or even vacate orders issued by the Department of Buildings (DOB).

The MDL establishes legal classifications for multiple dwellings, while the HMC sets maintenance and safety standards that buildings must follow even after receiving a CO. Together, these laws ensure that buildings remain safe, habitable, and legally occupied throughout their lifespan.

The Multiple Dwelling Law (MDL) and Certificates of Occupancy

The MDL (§ 301) requires that no multiple dwelling in NYC be occupied without a valid Certificate of Occupancy. This law governs buildings with three or more independent residential units, including tenement buildings, loft conversions, and high-rise apartment complexes.

Key MDL Provisions That Affect COs:

• Legal Occupancy (§ 4, MDL) – A multiple dwelling must meet specific fire safety, egress, and zoning requirements before a CO is issued. If an apartment building was constructed before 1938, it may not have a CO unless significant renovations were made.
• Illegal Conversions (§ 302, MDL) – Changing a building’s use without DOB approval—such as adding apartments beyond what the CO allows—violates MDL and can trigger a DOB vacate order.
• Loft Conversions (§ 281, MDL) – The Loft Law allows former manufacturing spaces to be converted into residential units, but these buildings must meet modern safety and habitability standards before obtaining a CO.
• Egress & Fireproofing (§ 30-40, MDL) – Buildings must have two means of egress, self-closing fire-rated doors, and enclosed stairwells to be legally occupied. Non-compliance prevents a CO from being issued or updated.

The MDL ensures that a building’s legal use matches its CO, preventing unsafe residential conversions and ensuring tenant protection through strict zoning and fire safety regulations.

The Housing Maintenance Code (HMC) and CO Compliance

While the MDL establishes how buildings can be occupied, the HMC ensures they remain safe and habitable after a CO is issued. This law sets maintenance standards for plumbing, heating, ventilation, and essential building services.

Key HMC Provisions That Affect COs:

• Essential Services (§ 27-2005, HMC) – Buildings must maintain heat, hot water, electricity, and structural integrity. Landlords who fail to provide these services can receive DOB violations, which may prevent CO renewal.
• Illegal Alterations (§ 27-2087, HMC) – Combining or subdividing apartments without DOB approval violates HMC and may invalidate a CO.
• Minimum Room Sizes & Light/Air Requirements (§ 27-2071, HMC) – Residential units must meet minimum square footage, natural light, and ventilation standards to comply with CO occupancy limits.
• Vacate Orders (§ 27-2140, HMC) – Severe maintenance violations, such as collapsing ceilings, major plumbing failures, or unsafe structural conditions, can result in a DOB vacate order and potential CO revocation.
• Short-Term Rental Restrictions (§ 27-2004, HMC) – Illegal Airbnb-style rentals in multiple dwellings violate both HMC and CO regulations, leading to DOB enforcement actions.

The HMC ensures that residential buildings remain livable, structurally sound, and compliant with city safety standards after a CO is issued.

CO Violations and Compliance Challenges

Both MDL and HMC violations can result in CO revocation, fines, or DOB-issued stop-work orders. Some of the most common issues that trigger CO-related penalties include:

• Illegal Unit Conversions – Adding extra apartments or exceeding legal occupancy limits without updating the CO.
• Failure to Maintain Essential Services – Not providing heat, water, or structural upkeep as required by HMC.
• Unapproved Building Alterations – Removing egress paths or modifying layouts without a DOB permit.
• Short-Term Rental Violations – Operating illegal Airbnb rentals in MDL-governed buildings.

To avoid CO violations, property owners must obtain DOB permits for structural changes, maintain building safety standards, and ensure that occupancy use remains compliant with MDL and HMC regulations.

Conclusion

The MDL governs legal occupancy and zoning compliance, while the HMC ensures that buildings remain habitable and well-maintained. Together, these laws shape how Certificates of Occupancy are issued and enforced in NYC. Property owners must update COs when making occupancy changes and comply with fireproofing, egress, and maintenance regulations to avoid penalties, vacate orders, and DOB enforcement actions. Understanding these laws is essential for keeping buildings legally occupied and ensuring long-term compliance with NYC housing regulations.