Unused FAR is the difference between the allowed floor area that can be built and the built floor area. This can be a good measure of a site's development potential.

If a lot is in an R10 zone (10 FAR), and the lot size is 2,000, the buildable square footage would be 20,000. If it has an 8,000 square foot building already there, the remaining FAR would be 12,000 square feet or 6 FAR (12,000/20,000).

In NYC, FAR is determined by the zoning district in which the property is located. Two same-sized lots can have vastly different buildable square footage. Due to this, if one lot were 2,000 sqft and is in an R8A, the FAR would be 6.02, so the buildable square footage would be 12,040. If an Identical lot were in an R10 with 10 residential FAR, the buildable square footage would be 20,000. If they both had a 5,000 square foot building on them, the first property would have 7,040 unused square feet, and the second would have more than double that at 15,000.

For owners, knowing how much unused FAR they have on their property could be very important in determining if there is much development potential on the site. Developers look for underdeveloped sites that they can develop on, if a building has maxed out the FAR, or is on a lot zoned with a lower FAR, there might not be much development potential.

Example: If a property has a 7,500 square foot lot, and is zoned in an R10 (10 FAR), the total buildable square footage is 75,000. If there is a building on the lot that is 30,000 square feet, the remaining buildable square footage is 45,000, and the remaining FAR is 6 (45,000 / 75,000)