How to use the setback buildable envelope calculator
- Choose feet or meters and enter the lot width and depth.
- Enter the front, rear, left and right setbacks.
- Enter the lot coverage limit as a percentage of the lot, or leave it blank for no limit.
- Read the buildable rectangle, its area and the footprint allowed by the coverage limit.
Formula
Buildable width = lot width - left setback - right setback
Buildable depth = lot depth - front setback - rear setback
Usable area = buildable width x buildable depth
Footprint limit = lot area x coverage limit
The allowed footprint is the smaller of the usable area and the coverage limit.
A 60 by 120 foot lot
The lot area is 7,200 square feet. With a 25 foot front setback, a 20 foot rear setback and 8 feet on each side, the buildable rectangle is 44 by 75 feet, a usable area of 3,300 square feet.
A coverage limit of 35% allows 0.35 x 7,200 = 2,520 square feet, which is less than the rectangle, so the coverage limit controls and the footprint is 2,520 square feet. If the limit were 60%, the rectangle would be the tighter constraint.
Setbacks and coverage
A setback is the distance a building must keep from a lot line. Together the four setbacks carve a rectangle out of the lot, and everything inside it is available for the main structure. Lot coverage is a separate rule that caps how much of the lot all roofed buildings may take up, and it often allows less than the rectangle does on a large lot.
This tool gives the geometry only. It does not know your local rules, which can add height limits, allowances for porches and eaves, special rules for corner lots, or different setbacks for sheds and garages.
Using the numbers
Take the setbacks from your local zoning document or ask the planning office, since they differ by place and zone. For floor area limits the Floor Area Ratio Calculator is the next step, and the Lot Coverage Calculator checks a plan against the limit. The Land Subdivision Lot Yield Calculator helps when splitting a parcel, and the Deck Calculator sizes a deck for the space you have.
Assumptions and limits
- The lot is a rectangle and the setbacks are measured straight in from each lot line.
- The figures shown are examples. Use the setbacks from your local rules.
- Easements, slopes and special overlays are not included.
- A setback that uses up the whole lot gives no buildable area, and the tool says so.
Frequently asked questions
Can I use meters?
Yes. Pick meters and enter every length in meters. The areas are shown in square meters.
What if the coverage limit is blank?
Then only the setbacks limit the footprint, and the usable area is the answer.
Is the whole rectangle allowed to be built?
Only if the coverage limit and other rules allow it. The smaller of the two numbers is the footprint.
Does this work for a corner lot?
It can, if you enter the setbacks that apply to each side, which are often larger on the street side.