JB PARCELSetback Calculator
Setbacks decide the shape of what you can build long before coverage or floor area do. Enter the lot and the required setbacks to see the envelope that is left, and how much of the lot it is.
How this calculator works
Take the side setbacks off both edges of the width and the front and rear off the depth. What is left is the building envelope: the part of the lot a building may occupy. A 25 foot front and 20 foot rear on a 150 foot deep lot leaves 105 feet, and 10 foot sides on an 80 foot width leave 60.
Narrow lots lose proportionally more. The same 10 foot side setbacks take a quarter of an 80 foot lot and half of a 40 foot lot, which is why side setbacks are the number that decides whether an infill lot is worth anything.
Corner lots are the common surprise. The side facing the second street is usually held to a street-side setback much larger than an interior side setback, so a corner lot can have a smaller envelope than the interior lot next to it despite the same dimensions.
The envelope is a maximum footprint, not a permission. Coverage caps the footprint as a share of the lot, floor area ratio caps the total floor area, and parking, stormwater, and easements take ground that is inside the envelope. Work out the envelope first, then check each limit against it, because the smallest one is what you are actually allowed to build.
Worked example: an 80 by 150 foot lot
A 12,000 square foot lot with a 25 foot front setback, a 20 foot rear, and 10 feet on each side.
| Envelope | 60 ft wide by 105 ft deep |
|---|---|
| Buildable area | 6,300 sf |
| Share of the lot | 52.5% |
If this district also caps coverage at 35 percent, the real footprint limit is 4,200 square feet, well inside the envelope, and coverage is what binds. Make it a corner lot with a 20 foot street-side setback and the envelope narrows to 50 feet, losing 1,050 square feet to one line on the plat.
Where the numbers come from
Setback distances, how they are measured, what may project into them, and how corner lots are treated all come from the zoning ordinance for the district a parcel sits in. Nothing here is the legal limit for a specific lot: read the ordinance and confirm with the planning department, and check the plat for easements, which take ground the ordinance does not mention.
Questions investors ask
How do I calculate buildable area?
Subtract the side setbacks from the lot width and the front and rear setbacks from the depth, then multiply what is left. That is the envelope. Coverage, floor area ratio, height, and parking are checked against it separately, and whichever limit is smallest is the one that governs.
What is the difference between a setback and lot coverage?
A setback says where a building may not be. Coverage says how much of the lot the footprint may cover, wherever it sits. A lot can have a generous envelope and a tight coverage cap, and then the cap is the real limit, which is what the lot coverage calculator works out.
Do setbacks apply to sheds, decks, and driveways?
Usually, and often with different numbers. Many ordinances allow accessory structures closer to a rear or side line than the house, and treat decks, eaves, steps, and paving under their own rules. The ordinance defines what may project into a setback and by how much.
What is a street-side setback on a corner lot?
The setback on the side of a corner lot that faces the second street. It is usually larger than an interior side setback and smaller than the front, and it is why two lots with identical dimensions can have very different envelopes.
Can I build closer to the line than the setback allows?
Only with relief from the jurisdiction, usually a variance, and it is granted on hardship grounds rather than on preference. Existing buildings closer than today's setback are often legal nonconforming, which allows them to remain but limits what can be done to them.
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Screening a real property? Pull its appraised value, assessed value, acreage, land use, and last recorded sale off the JB PARCEL parcel map and drop the real numbers into this calculator instead of guessing. Coverage runs across every county we carry.