The New York City zoning laws control density in part by defining the protocols and processes for development on any given property, including the air over the property. The NYC Department of City Planning explains that property owners can calculate these air rights, or unused development rights, with formulas from the zoning codes.
Not only does this air space have potential development value to the owner of the property, but the owner could also lease or sell it.
The city zoning laws permit a certain amount of floor area per lot using the floor area ratio. The FAR comes from the number the zoning law has set for the lot multiplied by the size of the lot. With the size of the lot, the size of the building and the number of square feet of floor area the zoning law allows, an owner should be able to see whether the allowed floor space is greater than the current building has.
Not all areas in a building may count as floor space that must contribute to the calculation, and some zoning designations may limit the amount of floor area that is usable.
Shared lot lines
According to City Realty, two properties that share a minimum of 10 feet of the line between their lots have the ability to purchase air rights to the adjacent property. A property owner may sell air rights to the developer next door, but in some districts, the zoning laws allow people to transfer air rights in creative ways that are not always adjacent.
Uncovering the air rights to the property and putting them to good use could become a significant undertaking, but the value may be high enough to make it well worth the trouble.