What is an Impervious Surface? Zoning Definition

What Impervious Surfaces Are

Impervious surfaces are materials that block water from penetrating into the soil. Concrete, asphalt, roofing, and compacted gravel are all impervious or near-impervious. When rain falls on these surfaces, it runs off rather than being absorbed, increasing stormwater volume and potentially causing flooding, erosion, and water pollution. Many zoning codes limit the total percentage of a lot that can be covered by impervious surfaces to manage these environmental impacts.

Why It Matters for Property Buyers

Impervious surface limits are broader than lot coverage limits. While lot coverage typically only counts building footprints, impervious surface rules also count driveways, patios, walkways, pools, and any other hard surfaces. This means you could be within your lot coverage limit but still exceed impervious surface limits. If you are planning to add a large driveway, patio, or pool to a property, check the impervious surface maximum for the zone -- it could be the limiting factor.

Practical Solutions and Considerations

• Permeable pavers, porous concrete, and gravel systems may not count as impervious surface in some jurisdictions, allowing you to create usable hardscaped areas without exceeding limits.
• Green roofs and rain gardens can sometimes offset impervious surface coverage through stormwater credits.
• Impervious surface limits are especially strict in watersheds, near waterways, and in areas prone to flooding.
• Exceeding limits can trigger requirements for on-site stormwater management systems (such as detention basins or rain gardens), adding cost and complexity to your project.
• When calculating impervious coverage, measure all hard surfaces including seemingly minor ones like stepping stones and shed foundations.