What is a Lot Line Window?

by | Last updated Apr 27, 2023 | Property Development

A lot line window is a window within an exterior wall of a building that is located on a property line, which is shared with another property. Lot line windows have specific code requirements and limitations that other windows do not. 


I am Jorge Fontan, an architect in New York and owner of NYC based architecture firm Fontan Architecture. In this post, I will review a basic outline of what a Lot Line Window is and why this matters.


Lot Line Windows

As you can see in the diagram below, a lot line window is on a shared property line with another building. Whether or not there is a building on the neighboring property has no impact on the lot line window designation. Lot line windows can be on a side property line or on a rear property line if the building walls containing windows touch the property line.

Lot Line Window

Lot Line Window Diagram


Lot Line Window Requirements in NYC

I am an architect in New York, so I will be referencing New York City regulations. Of course, these may vary from other local regulations.

Existing lot line windows in existing buildings are grandfathered. In the case of adding windows or having windows in a new building or home, there will be restrictions on lot line windows. For example, in some building types it is prohibited to install lot line windows. In cases where you can use lot line windows, they may have to be noncombustible fire rated windows. Fire rated lot line windows can be very expensive and many window manufacturers do not even supply them.


Light and Air Requirements

Residential buildings have very specific requirements about natural light and air that apply to “Habitable Space” as it is defined in the building code. I have another blog post that goes deeper into the Definition of Habitable Space if you are interested. The bottom line is that lot line windows do not count toward natural light and air unless you have a zoning lot merger or easement agreement with your neighbors. In most cases within residential buildings, lot line windows are a luxury that has no impact on the habitable space designation.


Blocking Lot Line Windows

If there is no easement agreement or other type of legal arrangement protecting your windows from your neighbor, they can block your lot line windows or you can block theirs. As long as the new development complies with all applicable building and zoning codes and proper procedures have been followed, it is legal to block lot line windows and there is nothing that can be done to stop it as far as the Department of Buildings is concerned.


Zero Lot Line Windows

The most important point about lot line windows is that nothing protects them fundamentally from being blocked. There is no restriction on blocking lot line windows and that is why they do not count for natural light and air as per Building Code requirements.


Thank You for Reading Our Blog Post on Lot Line Windows.

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


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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.