Alt 1 & Alt 2 Explained, NYC DOB Alterations

by | Last updated Jul 2, 2023 | Building Conversions, NYC DOB, Property Development

What’s the Difference Between an Alt 1 and Alt 2?

In NYC, an Alteration Type 1 or Alt 1 is a building alteration that requires a new Certificate Of Occupancy. An Alteration Type 2 or Alt 2 is a building alteration that does not require a new certificate of occupancy. Under the new DOB Now system, an Alt 1 is now called an Alt CO. 

This post will review the old DOB system nomenclature from the HUB and the BIS. Some older jobs are still under these terminologies. The concept is the same. If you want to see another post reviewing the new system, you can go to What is an Alt CO?

I am Jorge Fontan, an architect in New York and owner of Manhattan-based architecture firm Fontan Architecture. In our office, we work on various building projects, including small renovations through major alterations, building conversions, and new buildings. We are constantly wrapped up in DOB matters. In this post, we will outline the difference between an Alt 1 and an Alt 2 while explaining when you do and do not need a new or amended Certificate of Occupancy. 


DOB Terminologies

There are different types of fillings with the NYC Department Of Buildings to acquire NYC Building Permits. One very common question we get from clients is, “Do I need an Alt 1, or do I need an Alt 2?” Let’s see the difference between an Alt 1 and an Alt 2. First, the DOB loves abbreviations, so let’s review some.

  • Alt 1 – Alteration Type 1
    • Alt CO – Alteration Certificate of Occupancy (the new Alt 1)
  • Alt 2 – Alteration Type 2
  • C of O, or CO – Certificate of Occupancy
  • DOB – New York City Department of Buildings
  • LNO – Letter of No Objection


Alteration Type 1 or Alteration Type 2

The basic difference between an Alteration Type 1 and an Alteration Type 2 is the effect on the Certificate of Occupancy for the building. If the work you propose will require getting a new or amended Certificate of Occupancy, then you must file an Alt 1. Alternatively, if the work does not require a new or amended Certificate of Occupancy, you can file an Alt 2.

If you are building a New Building, you will File an NB Application.



NYC Certificate of Occupancy

The Certificate of Occupancy document lists the building’s legal use or occupancy. The Department of Buildings (NYC DOB) issues the Certificate of Occupancy.

Buildings built before 1938 must only have a Certificate of Occupancy if they undergo an alteration requiring a new C of O. We have another post if you want to read more about Certificates of Occupancy.


Alteration Type 1

The entire point of an Alt 1 in NYC is to change the Certificate of Occupancy or, if the building is pre-1938, to get your first C of O. The following changes require an Alt 1 and a new C of O: Change in use, egress, or occupancy.


Alteration Type 1 Architect

You will need an architect when filing an Alt 1. Make sure to consult with your architect about what type of alteration is right for your project. At our office we always make sure to game plan these issues in advance.


Alteration Type 1 to Meet New Building Requirements

When planning a building addition that increases the square footage by more than 110%, you will follow new building regulations but will not file for a new building permit.

If the building is 10,000 Square feet and you wish to add 11,000 square feet or more, it must be filed as an “Alteration Type 1 to meet new building regulations”. Your building application is reviewed with the same requirements as a new building. The existing floor area that is demolished and rebuilt also counts as a new floor area. 


Alteration Type 1, OT “No Work”

An alt 1 with “No work” is when you change the Certificate of Occupancy or get a new Certificate of Occupancy, but no actual construction work is proposed.


Alteration Type 2 (Alt 2)

An NYC DOB Alteration Type 2 is when you are doing a renovation and not changing the Certificate of Occupancy. This type of application means no change to the use, egress, or occupancy. An example would be an interior or exterior renovation that leaves how the building is used the same. An Alt 2 is now called an Alteration or Alt. Here are some examples:


NYC DOB Alteration Type 2 Projects


NYC Architect Alteration Type 2

You will need an architect when filing an alt 2. Consult with your architect about the type of alteration for your project.


Letter of No Objection

If a building does not have a Certificate of Occupancy or the change is in the same use and occupancy group, you can apply for a “Letter of No Objection” (LNO). If this is approved, you can file an alt 2. One example would be converting a store to a restaurant in a building with no Certificate of Occupancy (if under 75-person occupancy).


Alteration Type 1 or Alteration Type 2 NYC Filing

DOB protocols are complicated, and every situation should be assessed case by case. In this article, we reviewed some basics regarding the difference between the Alt 1 and Alt 2 filing in the NYC DOB. This post does not assume to cover every possible issue or condition but provides a general overview of the topic.


Thank You for reading our Blog Post on Alt 1 and Alt 2 applications.

I hope this was helpful. If you want to speak with an architect about a potential project, contact us at Fontan Architecture directly.


Contact Fontan Architecture

Jorge Fontan

Jorge Fontan

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.