Last updated: February 16, 2026
Setbacks Decide Where the House Goes
You find a 60 × 100-ft lot that checks every box —price, neighborhood, school district. Then you pull the zoning code and discover a 25-ft front setback, a 10-ft rear setback, and 5 ft on each side. Suddenly the footprint you pictured doesn’t fit. That gap between what the lot looks like and what you can actually build on is the buildable envelope: the rectangle left over after every setback is subtracted. This tool takes your frontage, depth, and setback distances, computes the envelope dimensions and area, and shows what percentage of the parcel is actually available for a structure.
The result is a planning-grade estimate. Use it to screen lots before you hire an architect, compare two parcels side by side, or sanity-check a builder’s site plan. For a permit-ready number, you still need the official setback lines from your local planning department and a licensed surveyor’s confirmation.
What the Envelope Numbers Tell You
| Output | How it’s calculated | Why it matters |
|---|---|---|
| Envelope width | Frontage − left side setback − right side setback | Sets maximum building width |
| Envelope depth | Lot depth − front setback − rear setback | Sets maximum building depth |
| Envelope area | Width × depth | Total buildable footprint |
| Coverage % | Envelope area ÷ lot area × 100 | Shows how much of the lot you can use |
| Frontage compliance | Actual frontage ≥ minimum required | Flags under-width lots before you buy |
If you also enter a proposed building footprint, the tool checks whether both width and depth fit inside the envelope and whether the building coverage stays within the percentage cap your zone allows.
A Lot That Looked Easy Until It Wasn’t
Lot: 55 ft frontage × 120 ft deep. R-1 zone requires 50 ft minimum frontage, 25 ft front setback, 20 ft rear setback, 5 ft each side.
- Frontage check: 55 ft ≥ 50 ft → passes
- Envelope width: 55 − 5 − 5 = 45 ft
- Envelope depth: 120 − 25 − 20 = 75 ft
- Envelope area: 45 × 75 = 3,375 sq ft
- Coverage: 3,375 ÷ 6,600 = 51.1 %
Proposed house: 42 × 60 ft (2,520 sq ft footprint). Width fits (42 ≤ 45) and depth fits (60 ≤ 75), so the footprint clears the envelope. Building coverage is 2,520 ÷ 6,600 = 38.2 %, comfortably under a typical 40 % cap.
Now the surprise: the buyer’s architect wanted a detached garage 24 ft behind the house. From the rear wall of the house at 60 ft deep, only 15 ft of envelope remain. A 24-ft garage doesn’t fit without a rear-setback variance. The tool can’t model accessory structures automatically, but running the envelope showed the constraint before anyone drew plans.
Corner Lots and Other Gotchas
- Corner lots have two front setbacks. Many codes treat the street-facing sides of a corner lot as fronts, which means two 25-ft setbacks instead of one front and one 5-ft side. Enter the larger setback on the street side or the buildable area will look bigger than it really is.
- Mixing feet and meters in the same form. The tool converts everything to one unit internally, but only if you pick the right unit at the top. If the survey is in meters and you enter the numbers with “feet” selected, every setback is wrong by a factor of 3.28. Double- check the unit dropdown before you calculate.
- Ignoring easements inside the envelope. A setback envelope tells you where the zoning code allows a building. An easement across the middle of that envelope removes another slice. If you have recorded easements, run them through the easement area estimator first, then subtract that area from the envelope total.
- Confusing envelope coverage with building coverage. Envelope coverage is the percentage of the lot where you could build. Building coverage is the percentage the structure actually occupies. A zone might allow a 54 % envelope but cap actual building coverage at 35 %. The tool reports both numbers—read the right one for the question you’re answering.
Questions From the Permit Counter
Can I build right up to the envelope edge? Technically yes—the envelope boundary is the setback line. But most contractors leave 6–12 in. of margin so footings don’t accidentally cross the line during construction. A survey stake can shift half a foot; building a wall exactly on the limit is asking for an encroachment letter.
What if my lot isn’t a rectangle? The tool assumes parallel boundaries. For pie-shaped cul-de-sac lots or flag lots, the buildable shape is more complex than a simple rectangle. Use the result here as a rough upper bound, then model the actual outline in the irregular plot area calculator.
Do decks and porches count toward coverage? It depends on your jurisdiction. Many codes exempt uncovered decks under a certain height but count covered porches. The tool lets you enter any footprint dimensions—if your code counts a covered porch, add it to the building width or depth and check the coverage again.
Does the tool handle height restrictions? No. The envelope here is two-dimensional—width and depth only. Height limits, floor-area ratios, and daylight-plane rules are separate zoning constraints that require a three-dimensional analysis your architect can run.
What the Tool Assumes
All geometry is rectangular—parallel lot lines, uniform setback distances on each side. Real lots can taper, curve, or have angled boundaries that change the envelope shape. Unit conversions use standard factors (1 yd = 3 ft, 1 m = 3.28084 ft). The tool does not account for overlay districts, height caps, floor-area ratios, or accessory-structure rules. For anything that goes on a permit application, confirm setback distances with your local planning department and have a surveyor mark the lines on the ground.
Need to convert the buildable area to regional units? Open the land area converter for kanal, marla, bigha, and more.