A setback is the distance a building must stay from each property line. Subtract the front and rear setbacks from the lot length and both side setbacks from the width, then multiply, and you have the buildable area.
This setback calculator does both steps. It also has a prevailing setback mode that averages the front yards of nearby lots.
Enter the front setback of each nearby lot you are comparing against, such as the other lots on your side of the block. Leave unused boxes empty.
Work out the buildable width and length, then multiply them.
That rectangle is sometimes called the building envelope. Your house footprint has to fit inside it.
Take a lot that is 60 feet wide and 100 feet long, with 20-foot front and rear setbacks and 10-foot side setbacks.
| Step | Math | Result |
|---|---|---|
| Buildable width | 60 - 10 - 10 | 40 ft |
| Buildable length | 100 - 20 - 20 | 60 ft |
| Buildable area | 40 × 60 | 2,400 sq ft |
The setbacks are only one limit. Your code may also cap how much of the lot you can cover. Run the same lot through the lot coverage calculator to see whether coverage or setbacks is the tighter rule.
A prevailing setback is a front yard rule based on the houses around you instead of one fixed number. The city averages the front yards of nearby lots on the same block, and your new building has to line up with that average.
Los Angeles is one example. Its code uses a prevailing setback rule in many residential zones. The exact method changes from city to city: which lots count, how many, and whether the deepest and shallowest are dropped. Read your own code before relying on a simple average.
The biggest factor is your zoning district. Residential, commercial and industrial zones each have their own table of setbacks.
Corner lots, lots next to a different zone and lots on a waterfront or slope often get extra rules. Accessory buildings like sheds and garages can have smaller setbacks than the main house.
Wikipedia has a short overview of setbacks in land use if you want the background.
Setbacks are listed in your city or county zoning code, usually in a development standards table for each district. To find the right table, look up your property on your city's zoning map first.
Once you know your buildable area, check how much floor space you can put on it with the floor area ratio calculator. For commercial sites, the parking ratio calculator shows how many spaces you will need.
From the property line, not the curb or the edge of the sidewalk. The front property line is often several feet behind the curb, so check a survey or plat map before you measure.
Many codes let small features like eaves, chimneys and uncovered steps stick a short distance into the setback. Covered porches and decks are handled differently from city to city. Look for a section called allowed projections or encroachments.
A corner lot has two street sides, so many codes require a street side setback that is larger than a normal side setback. Enter that larger number as your side setback for a conservative estimate.
Only with approval. Most cities let you apply for a variance, which usually means showing a hardship caused by the shape or size of the lot. Ask your planning department about the process.
For more useful tools to assist you in property planning and development, check out our other calculators: