Impervious Surface Ratio (ISR)
The impervious surface ratio (ISR) is a zoning metric that measures the percentage of a lot covered by surfaces that do not absorb rainwater: buildings, pavement, concrete, asphalt, and other hard surfaces. Florida local governments regulate ISR through zoning ordinances and comprehensive plans to manage stormwater runoff, prevent flooding, and protect water quality.
How ISR Is Calculated
ISR = Total Impervious Area ÷ Total Lot Area
Example: A 10,000 sq ft lot with a 0.60 ISR limit allows a maximum of 6,000 sq ft of impervious surface. The remaining 4,000 sq ft must be permeable (landscaping, natural ground, or approved permeable materials).
What Counts as Impervious
- Building footprints (all floors counted only once, at ground level)
- Asphalt and concrete driveways and parking lots
- Sidewalks and patios
- Swimming pools and pool decks
- Standard pavers
Some jurisdictions exclude or partially credit permeable pavers, green roofs, and porous concrete from ISR calculations, incentivizing sustainable design.
ISR vs. Lot Coverage
- Lot coverage: Measures only the building footprint as a percentage of the lot
- ISR: Measures all impervious surfaces (buildings + hardscape)
- ISR is always equal to or greater than lot coverage
- Both are regulated separately; the more restrictive limit controls site design
Impact on Development
ISR limits directly affect site planning decisions: building size and placement, parking lot layout and capacity, landscape buffer dimensions, and stormwater management facility sizing. Projects exceeding the ISR limit must seek a variance or redesign the site plan.
Related Terms
- Zoning — Land use regulation framework
- Variance — Exception to zoning standards
- Setback — Required distance from property lines
- Floor Area Ratio — Building intensity measurement
Barnes Walker Land Use Services
Barnes Walker attorneys advise developers on ISR compliance, variance applications, and site plan approvals in Manatee and Sarasota counties. Request a legal inquiry for assistance.
Reviewed by the attorneys at Barnes Walker, Goethe, Shea & Robinson, PLLC