Building an Addition in NYC

by | Last updated Jun 3, 2020 | NYC Zoning

Building additions are going on all over New York. Vertical enlargements are especially common in NYC. Here are some basic things to consider if you are looking to do a building addition.


Building an Addition

Zoning Development Rights (Air Rights)

If you have an existing building and you want to know if you can add on top of it you must start by looking at the zoning. People commonly use the term “Air Rights” or the term “Development Rights” to describe what you are allowed to build on a given property.

Do you have the right to enlarge a building?

If you are looking to enlarge a building you need to speak to an architect to determine the feasibility of the project. We often perform a Zoning Analysis for our clients before they embark on this type of project. If you want to learn more about a Zoning Analysis read another article we wrote about Zoning Reports.

The basic idea is to determine what you are allowed to have and subtract what you have now. So if you have a property that is allowed to have 10,000 square feet on it and you currently have a 5,000 square foot building you can therefor add 5,000 square feet.

Vertical Building Addition Example


Building addition Brooklyn

Building Addition in Brooklyn


The image shown is of a property in Brooklyn. The client hired us to determine if they could build an addition on top. So we conducted a zoning analysis. We determined the development rights to be a total of 30,000 square feet. After surveying the existing conditions we determined the existing building was 21,000 square feet.

Development Rights calculation:

Total Development Rights – Existing Building = Addition Size

30,000 sq ft – 21,000 sq ft = 9,000 sq ft

This is a very overly simplified example there are lots of variables to consider. But the basic idea is in this building we could add 9,000 sq ft.


Rules For Building Additions

The new addition must comply with current zoning regulations. In the example used the zoning required this property to have a 30 foot rear yard. The existing building did not have a 30 foot rear year. We call this an existing nonconforming condition. In this case the new addition would not align with the lower floors at the rear yard, because we would be required 30 feet for the addition from the rear property line. The rear setback we made to achieve the 30 foot rear yard requirement created a terrace which would be a nice amenity. We still were able to maximize the 9,000 sq ft. See the diagram below.

Building Addition NYC

Building addition with required 30 feet at rear yard


Considerations For Building Additions


Structural Concerns:

The existing structure will need to be assessed to determine what it can support and what type of structural system will be needed. This may include soil borings, and digging a test pit in the cellar to see the existing foundations. Probes in the walls, floors, and roof may also be required.

There are 2 basic structural methods for a vertical enlargement.

  1. The addition sits right on top of the existing structure. This will usually only be possible in small vertical additions.
  2. The new addition is isolated from the existing structure. In this case we would build new concrete footings in the cellar and put a new steel frame throughout the building to support the addition. This will be more common on large additions.


Filing With DOB:

This work will need to be filed with the Department Of Buildings.

In NYC Some minor additions can potentially be done as an Alt 2. In most cases an addition (especially a vertical one) will be filled as an Alt 1. If the addition in NYC exceeds 110% of the original building. Then you must file an Alt 1 for New Building Requirements. That means 110% in addition to the original 100%. So on a 10,000 sq foot building you would need to add 11,000 sq ft for a total of 21,000 sq ft. In that case you would need to file Alt 1 for New Building requirements.


General Building Considerations:

There will be many requirements and factors to consider when doing a building enlargement especially a vertical one. Here are a few general concerns.

  • Construction Classification
    • Buildings are required certain fire rating based on the Construction Classification.
    • The construction classification requirement varies based on building use and building size.
    • If you enlarge your building will the existing construction classification be suitable for the larger building?
    • Will you need to upgrade the existing structure and construction class?
  • Egress
    • Egress is the number of exits and exit ways in the building including stairs. If you increase the size of the building the egress requirements may also increase.
  • Elevator
    • If you have an elevator in a vertical addition you will need to add to the elevator.
    • Does the addition require an elevator in a building that previously did not have an elevator?
  • Fire alarm
    • If you add floors you may cross the threshold to require a fire alarm.
  • Fire Sprinklers
    • If the building doesn’t have fire sprinklers you may now be required to add them.
    • The sprinklers may need to be in the addition and in the original portion of the building.

Building an Addition to a Building or House

In this article we reviewed some of the basics with regards to Building Additions. This post does not assume to cover every possible issue or condition, but provide a general overview of the topic.


Thank You for reading our blog post on Building an Addition in NYC.

These are some basic considerations when making an addition to your building. I hope this article was helpful. If you want to speak with an architect please contact us directly and see if we can help. Please feel free to leave questions or comments below.


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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.