When to use: Site planning and zoning compliance. Impervious cover = (building footprint + paving) / lot area, driving stormwater and zoning limits. Floor Area Ratio (FAR) = total gross floor area / lot area, capturing multi-story building intensity. Lot coverage is just the building footprint over the lot. Always confirm against the local zoning ordinance for the applicable maximums.
This calculator computes three zoning compliance metrics — impervious cover percentage, floor area ratio (FAR), and lot coverage — that govern site development intensity under municipal ordinances and stormwater regulations across the United States.
Impervious cover percentage = (building footprint + all paved surfaces) / lot area × 100. This metric directly controls stormwater runoff generation: impervious surfaces prevent rainfall infiltration and increase peak discharge and runoff volume to downstream drainage systems. Most municipalities cap impervious cover at 50–80% depending on zoning district and watershed sensitivity.
Floor Area Ratio (FAR) = total gross floor area (all stories) / lot area. FAR limits building intensity independent of the number of stories — a 1.0 FAR on a 10,000 sf lot permits 10,000 sf of total floor area, which could be one 10,000 sf story or two 5,000 sf stories. Lot coverage = building footprint / lot area × 100, which differs from impervious cover because it excludes paving and only counts the building envelope.
Impervious cover limits are established in local zoning ordinances and may be further constrained by watershed-based regulations under municipal MS4 stormwater permits (EPA Phase I and Phase II NPDES). The EPA Low Impact Development guidance recommends limiting impervious cover to 10–25% in sensitive watersheds to minimize hydrologic impact. In Texas Hill Country, for example, many municipalities cap impervious cover at 15–20% to protect the Edwards Aquifer recharge zone.
FAR and lot coverage limits are set in the municipal zoning code (UDC or LDC) and may vary by zoning district. ASCE 7 does not directly govern FAR, but IBC Chapter 5 ties allowable building area to occupancy, construction type, and sprinkler systems, which interact with FAR limits on dense urban sites.
When approaching impervious cover limits, engineers have several design strategies: green roofs (not counted as impervious in some codes), permeable pavement (counted as 0–50% impervious depending on jurisdiction), bio-retention cells that replace traditional impervious pavement, and structured parking (vertical garage) to reduce surface lot area.
For FAR-constrained projects, underground parking, mechanical parking systems, and above-grade structured parking all reduce the surface footprint without reducing rentable floor area. In urban mixed-use zones with FAR above 2.0–3.0, podium construction (parking below grade or at grade beneath residential or office towers) is standard.
Enter lot area, building footprint, gross floor area (GFA) for all stories, and total paved area (parking lots, drives, sidewalks). Then set the maximum impervious cover and FAR from your local zoning code. Results show whether your proposed development is within both limits. The impervious cover result turns green when compliant and red when it exceeds the maximum, making it easy to iterate on design options.
Most municipal codes do not count green roofs (vegetated roof systems) as impervious cover, provided the growing medium depth meets a minimum (typically 6+ inches) and the system is designed for stormwater retention. Some jurisdictions provide a partial credit (e.g., 50% impervious) for extensive green roofs with shallow growing media. Always verify with the local stormwater authority.
Yes, in most jurisdictions. Permeable asphalt, pervious concrete, and open-cell pavers are typically credited as 0–50% impervious depending on the local code and the underlying soil's infiltration rate. Hydrologic Soil Group D soils (clays) may receive little or no credit because the underlying soil cannot absorb infiltrated water fast enough.
FAR limits vary widely: suburban office parks often set FAR at 0.3–0.5; urban commercial zones 1.0–3.0; downtown core zones 5.0–20+ in dense cities. New York City's highest-density commercial zones allow FAR up to 15. FAR above 1.0 generally requires structured or underground parking to meet parking ratios within the lot.
Lot coverage counts only the building footprint (the area within the exterior walls projected to grade) as a percentage of lot area. Impervious cover adds all other hard surfaces — driveways, parking lots, sidewalks, patios. A site can comply with lot coverage limits while still exceeding impervious cover limits if extensive paving is used.
Any new development or redevelopment that increases impervious cover typically triggers a stormwater permit and zoning review. Many jurisdictions set a threshold (e.g., adding more than 2,500 sf of impervious area) above which a full drainage study and impervious cover calculation are required. Redevelopment projects must often demonstrate no net increase in peak runoff.