Navigating building permits in New York City can feel overwhelming. The NYC Department of Buildings (DOB) maintains a complex filing system, and understanding which type of alteration application your project requires is an essential first step in any renovation approval process.
In NYC, an Alteration Type 1 (Alt 1) filing is required when a project results in a new or amended Certificate of Occupancy. An Alteration Type 2 (Alt 2) filing applies to alterations that do not require a new Certificate of Occupancy. Under the current DOB NOW system, an Alt 1 filing is referred to as an Alt-CO.
I am Jorge Fontan, AIA, a New York City architect and owner of Fontan Architecture, a Manhattan-based architecture firm. Our office works on apartment and townhouse renovations, conversions, enlargements, and new construction. In this article, we explain the difference between Alt 1 and Alt 2 filings and clarify when a new or amended Certificate of Occupancy is required.
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NYC Certificate of Occupancy
A Certificate of Occupancy (C of O) is an official document issued by the NYC Department of Buildings that establishes a building’s legal use and occupancy classification. It defines how a property may legally be used — for example, residential, commercial, or mixed-use.
Any change that affects a building’s legal use, occupancy classification, or means of egress may require a new or amended Certificate of Occupancy. This distinction is central to understanding the difference between an Alt 1 and an Alt 2 filing.
Buildings constructed prior to 1938 may not have an original Certificate of Occupancy on record. However, if such a building undergoes an alteration that changes its legal use or occupancy classification, a new Certificate of Occupancy may still be required.
Alteration Type 1 (Alt 1)
An Alteration Type 1 (Alt 1) filing is required when a project results in a new or amended Certificate of Occupancy.
In general, an Alt 1 is necessary when a renovation changes a building’s legal use, occupancy classification, or permitted occupant load.
Projects That Typically Require an Alt 1 Filing
- Converting a commercial building to residential use
- Converting residential space to commercial use
- Increasing the number of dwelling units (for example, converting a two-family home into a three-family building)
- Changing the legal occupancy classification of a space
- Increasing the permitted occupant load
- Adding a vertical enlargement, such as constructing an additional story
- Constructing an addition that alters legal use or occupancy
- Converting SRO units into a single-family residence
- Converting retail space to a restaurant with an occupant load exceeding 75 persons
- Constructing a roof deck where it affects use, occupancy, or egress requirements
Because Alt 1 applications involve modifications to the Certificate of Occupancy, they must be prepared and filed by a Registered Architect or Professional Engineer.
Selecting the proper filing strategy is an important early step in the design process. At our office, we evaluate these issues at the outset of each project to ensure the appropriate application type is selected and to anticipate how the filing will affect approvals, code compliance, and overall project timeline.
Alteration Type 1 to Meet New Building Requirements (“Big Alt”)
In certain cases, a project may be filed as an Alteration Type 1 but required to comply with current new building regulations. This situation is commonly referred to as a “Big Alt.”
A “Big Alt” occurs when an enlargement increases a building’s floor area to more than 110% of the existing floor area. When this threshold is exceeded, the project must comply with the current New York City Building Code as though it were new construction, even though it is technically filed as an alteration rather than a New Building (NB) application.
For example, if an existing building contains 10,000 square feet and the proposed enlargement increases the total floor area by 11,000 square feet or more, the application must be filed as an Alteration Type 1 to meet new building requirements.
Importantly:
- The application remains an Alt 1 filing, not a New Building permit.
- The project is reviewed under the same code standards that apply to new buildings.
- Existing floor area that is demolished and rebuilt is counted as new floor area when calculating compliance thresholds.
This distinction can significantly impact structural design, accessibility compliance, energy code requirements, and overall construction cost.
Alteration Type 2 (Alt 2)
An Alteration Type 2 (Alt 2) filing applies to renovations that do not change a building’s Certificate of Occupancy. These projects do not involve a change in legal use, occupancy classification, or means of egress.
In many residential renovation projects in New York City, the work is filed as an Alt 2.
Examples of Alt 2 Projects
- Interior apartment renovations
- Combining apartments (in most cases, where legal use and occupancy remain unchanged)
- Renovating a retail store while maintaining retail use
- Renovating a restaurant that remains a restaurant with no change in occupancy load
- Removing or constructing non-structural interior walls
- Adding a bedroom within an existing apartment (where legal occupancy is not altered)
- Adding a bathroom to an apartment or townhouse
- Townhouse interior renovations that do not affect legal use or occupancy
- Alterations where a Letter of No Objection is issued instead of a new Certificate of Occupancy
Unlike an Alt 1, an Alt 2 filing does not result in a new or amended Certificate of Occupancy. However, these applications still require plan examination, permits, inspections, and final sign-offs. Even though the Certificate of Occupancy is unchanged, these filings must still be prepared by a Registered Architect or Professional Engineer.
Letter of No Objection (LNO)
A Letter of No Objection (LNO) is issued by the NYC Department of Buildings when a property does not have a Certificate of Occupancy on record — typically in buildings constructed prior to 1938 — and the proposed work does not change the building’s existing legal use or occupancy classification.
Rather than issuing a new Certificate of Occupancy, the DOB may issue an LNO confirming that the proposed alteration is consistent with the building’s existing legal use.
If approved, the project is typically filed as an Alteration Type 2 rather than an Alt 1.
For example, converting a retail store to a restaurant in a pre-1938 building may qualify for an LNO provided the occupancy load and use group remain within allowable limits (such as a restaurant under 75-person occupancy, where applicable).
An LNO does not replace a Certificate of Occupancy, but it allows certain alterations to proceed without requiring a new C of O.
Alt 1 vs Alt 2: Key Differences at a Glance
- Certificate of Occupancy:
- Alt 1 results in a new or amended C of O.
- Alt 2 does not change the C of O.
- Legal Use or Occupancy Change:
- Alt 1 involves a change in legal use, occupancy classification, or occupant load.
- Alt 2 maintains the existing legal use and occupancy.
- Typical Projects:
- Alt 1 includes conversions, enlargements, and use changes.
- Alt 2 includes interior renovations and alterations within existing legal parameters.
- Regulatory Impact:
- Alt 1 filings are generally more complex and may trigger additional code compliance requirements.
- Alt 2 filings are typically more straightforward but still require formal review and sign-off.
NYC Filing Considerations
NYC Department of Buildings filing protocols are nuanced, and the appropriate application type depends on the specific scope and legal conditions of each project. Determining the correct filing strategy requires careful review of the building’s existing Certificate of Occupancy, zoning designation, and proposed scope of work.
Selecting the proper filing pathway early in the process can significantly affect approvals, compliance requirements, and overall project timeline.
If you are planning a renovation, conversion, or enlargement in New York City, we would be happy to discuss your project. At Fontan Architecture, we evaluate feasibility at the outset and guide our clients through the entire process — from design and DOB approvals through construction and final sign-offs — ensuring the project is thoughtfully designed, carefully executed, and fully compliant.
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This post was written by Jorge Fontan AIA a Registered Architect and owner of New York City architecture firm Fontan Architecture. Jorge Fontan has earned 3 degrees in the study of architecture including two degrees from the City University of New York and a Masters Degree in Advanced Architectural Design from Columbia University. Jorge has a background in construction and has been practicing architecture for 20 years where he has designed renovations and new developments of various building types.
