Setback & Buildable Area Calculator Formula

Understand the math behind the setback & buildable area calculator. Each variable explained with a worked example.

Use the Setback & Buildable Area Calculator

Formulas Used

Buildable Area (footprint)

buildable_sq_ft = buildable_area

Total Lot Area

total_lot = total_lot_area

Buildable % of Lot

buildable_pct = total_lot_area > 0 ? (buildable_area / total_lot_area) * 100 : 0

Buildable Width

build_width = buildable_width > 0 ? buildable_width : 0

Buildable Depth

build_depth = buildable_depth > 0 ? buildable_depth : 0

Max Total Floor Area (all stories)

max_total_floor = max_floor_area

Variables

VariableDescriptionDefault
lot_widthLot Width (feet)(ft)75
lot_depthLot Depth (feet)(ft)120
front_setbackFront Setback (feet)(ft)25
rear_setbackRear Setback (feet)(ft)20
side_setbackSide Setback Each Side (feet)(ft)8
num_storiesPlanned Stories2
total_lot_areaDerived value= lot_width * lot_depthcalculated
buildable_widthDerived value= lot_width - 2 * side_setbackcalculated
buildable_depthDerived value= lot_depth - front_setback - rear_setbackcalculated
buildable_areaDerived value= buildable_width > 0 ? (buildable_depth > 0 ? buildable_width * buildable_depth : 0) : 0calculated
max_floor_areaDerived value= buildable_area * num_storiescalculated

How It Works

Setback & Buildable Area

Setbacks are the minimum required distances between a building and the property lines. They create the "buildable envelope" that constrains where a structure can be placed on a lot.

Setback Types

  • Front setback: Distance from the front property line (street-facing), typically 15-30 ft
  • Rear setback: Distance from the rear property line, typically 15-25 ft
  • Side setback: Distance from each side property line, typically 5-15 ft
  • Corner lots: May have two front setbacks

    • Calculating the Buildable Envelope

    Buildable Width = Lot Width - (Side Setback x 2) Buildable Depth = Lot Depth - Front Setback - Rear Setback Buildable Area = Buildable Width x Buildable Depth

    Additional Constraints

    Setbacks define the maximum footprint, but the actual building may be smaller due to:

  • Lot coverage limits
  • Floor area ratio (FAR) limits
  • Parking requirements
  • Building height restrictions

    • Worked Example

    75 x 120 ft lot with 25 ft front setback, 20 ft rear setback, 8 ft side setbacks, planning a 2-story home.

    lot_width = 75lot_depth = 120front_setback = 25rear_setback = 20side_setback = 8num_stories = 2
    1. 01Total lot area: 75 x 120 = 9,000 sq ft
    2. 02Buildable width: 75 - (8 x 2) = 59 ft
    3. 03Buildable depth: 120 - 25 - 20 = 75 ft
    4. 04Buildable area: 59 x 75 = 4,425 sq ft
    5. 05Buildable as % of lot: 4,425 / 9,000 = 49.2%
    6. 06Max floor area (2 stories): 4,425 x 2 = 8,850 sq ft

    Ready to run the numbers?

    Open Setback & Buildable Area Calculator