Setbacks are the empty zones zoning requires you to leave around the edges of your lot. You cannot build in these areas regardless of how much FAR you have.

Rules vary by district and lot type. In a typical R6A, buildings must maintain a 30-foot rear yard, match the block's prevailing front yard depth, and meet side yard rules based on whether the building is detached, semi-detached, or zero-lot-line. The Department of Buildings enforces these at permitting through the Zoning Resolution's bulk regulations.

Setbacks shrink your buildable footprint, which forces developers to either build taller to recover the lost square footage or accept a smaller project altogether. Many owners assume FAR is the only constraint on what can be built but setbacks can make full FAR utilization impossible without a variance, particularly on narrow or oddly shaped lots.

If FAR tells you how much you can build and lot coverage tells you how wide you can go, setbacks tell you where you're not allowed to start in the first place.

Example: A 5,000 SF lot that is 25 feet wide with a 30-foot rear yard requirement loses 750 SF of buildable area immediately before FAR or lot coverage even enters the picture. On a narrow lot, trying to recover that square footage by adding more floors can quickly become difficult or financially unfeasible.