How to use the setback calculator
The first question on almost any site is how much of it you can actually build on. Setbacks, lot coverage and floor area ratio each answer part of that, and they interact. This calculator works out the building envelope left inside the setbacks, compares it with the coverage limit, and tells you how much total floor area the zoning allows, so you can test a site before drawing anything.
- Choose feet or metres and enter the lot width, along the street, and the lot depth.
- Enter the setbacks from the zoning code: front, rear and both sides. On a corner lot, use the street-side setback for one side.
- Add the optional limits: maximum lot coverage, FAR and a maximum number of storeys. Leave any of them at 0 to ignore it.
The results show the envelope dimensions, the buildable area and its share of the lot, the maximum footprint after coverage, which rule governs, the maximum floor area and the storeys needed to reach it.
Setback and buildable area formulas
Envelope width = Lot width − Side 1 − Side 2
Envelope depth = Lot depth − Front − Rear
Buildable area = Envelope width × Envelope depth
Max footprint = min(Buildable area, Lot area × Coverage)
Max floor area = min(Lot area × FAR, Max footprint × Storeys)
The envelope is the plan area a building may occupy at ground level. Upper floors may be further limited by height planes, daylight angles or step-backs, which this calculator does not model.
| Term | What it limits | Typical form in a code |
|---|---|---|
| Front setback | Distance from the street line | 20–30 ft or 6–9 m in suburban zones |
| Side setback | Distance from neighbouring lots | 5–10 ft or 1.5–3 m each side |
| Rear setback | Distance from the back line | 20–25 ft, or a share of lot depth |
| Lot coverage | Footprint as a % of lot area | 30–50% for houses |
| FAR | Total floor area ÷ lot area | 0.4–1.0 residential, higher downtown |
The typical figures above are only examples of the ranges you will see. Always use the numbers from the zoning ordinance for your parcel.
Worked examples
A suburban house lot
A 50 × 100 ft lot has 5 ft side setbacks, a 20 ft front and a 25 ft rear setback. The envelope is 40 × 55 ft, or 2,200 ft², which is 44% of the 5,000 ft² lot. A 40% coverage limit allows only 2,000 ft², so coverage governs. With an FAR of 0.5, the maximum floor area is 2,500 ft², about 1.25 storeys over the full footprint.
A metric townhouse plot
A 20 × 30 m plot with 6 m front and rear setbacks and 1.5 m sides has a 17 × 18 m envelope, or 306 m², 51% of the 600 m² plot. Coverage of 50% caps the footprint at 300 m². An FAR of 1.2 allows 720 m², which needs 2.4 storeys, so a three-storey limit does not bind.
A corner lot
A 60 × 120 ft corner lot has a 25 ft front, a 20 ft rear, a 5 ft interior side and a 15 ft street side. The envelope is 40 × 75 ft, or 3,000 ft², which is 41.7% of the 7,200 ft² lot. With no coverage limit the whole envelope is available.
Who this calculator helps
- Architects running quick feasibility checks and site capacity studies.
- Developers and investors screening lots before buying.
- Real estate agents answering what a buyer could build or add.
- Homeowners planning an extension, garage or accessory dwelling unit.
- Planning and architecture students learning how zoning shapes buildings.
Tips for setback checks
- Take lot dimensions from the survey or title plan, not from online maps, which can be off by several feet.
- Check for easements, utility lines and landscape buffers, which can shrink the envelope further.
- Test total floor area with the floor area ratio calculator, and count the parking the building triggers with the parking space calculator.
- Once the footprint is known, get a first budget from the construction cost calculator.
- Confirm the setbacks with the planning department early; reading the wrong zone or overlay is a common and costly mistake.
Have a massing sketch for the envelope? Turn it into a client-ready image with Sketch to Render.
