Land records · Parcel maps · October 7, 2026
What Is Impervious Surface? Limits and Fees
What is impervious surface? Roofs, pavement, and other hard surfaces that block rain from soaking in. How zoning limits and stormwater fees use it.
What is impervious surface? Impervious surface is any hard surface that prevents rainwater from soaking into the ground, such as roofs, driveways, patios, sidewalks, parking lots, and compacted gravel in some definitions. Local governments regulate impervious surface because it increases stormwater runoff, flooding, and water pollution. Zoning codes may cap the percentage of a lot that can be impervious, and many cities charge stormwater fees based on the amount of impervious area on each property. This page is general land-records background, not legal advice.
Common impervious surfaces
| Surface | Usually counted |
|---|---|
| Roofs | Yes |
| Concrete and asphalt | Yes |
| Pavers on compacted base | Often |
| Gravel driveways | Sometimes, depending on code |
| Decks with gaps over soil | Often not |
| Permeable pavers | Sometimes partly credited |
| Pools | Varies |
Impervious surface limits
Zoning codes may set maximum impervious coverage, such as 40 percent of the lot. This is different from lot coverage, which often counts only buildings. Some codes have both limits. Shoreline and watershed areas often have stricter caps.
Stormwater fees
Stormwater utilities often calculate fees using impervious area measured from aerial imagery. Homes may be billed a flat rate based on typical impervious area, while commercial properties pay by measured square footage. Credits may be available for rain gardens or detention.
Why it matters
More impervious area means more runoff, which may require a stormwater management plan for larger projects. Adding a patio or driveway expansion could push a lot over limits.
Measuring impervious surface
Calculate area of all hard surfaces from a survey or site plan. Some GIS viewers include impervious layers derived from imagery.
Ways to reduce impervious surface
- Permeable pavers or pavement.
- Gravel or grass pavers where allowed.
- Rain gardens and dry wells.
- Removing unused paving.
Example
A lot of 10,000 square feet has a 2,000-square-foot house, a 600-square-foot driveway, and a 400-square-foot patio, totaling 3,000 square feet, or 30 percent impervious. If the code limits impervious coverage to 35 percent, the owner can add up to 500 more square feet.
Checking your limits
Look up zoning and read the district standards. See how to find zoning of a property. Overlays may set stricter limits.
Drainage and neighbors
Even within limits, new hard surfaces should drain away from neighbors and into appropriate paths, such as drainage easements.
Impervious surface in watershed and shoreline areas
Areas that drain to drinking water reservoirs, lakes, or coastal waters often have tighter impervious limits, sometimes 10 to 25 percent, because even modest increases in hard surfaces can harm water quality. These limits may come from an overlay district rather than base zoning, so check both. Projects over the limit may need mitigation, such as planting buffers or installing infiltration practices.
Permits for new hard surfaces
Some jurisdictions require a permit for new patios or driveway expansions above a size threshold, mainly to track impervious area.
Bottom line
Impervious surface includes roofs, pavement, and other hard surfaces that block infiltration. Zoning codes may cap it, and stormwater fees often depend on it. Measure your existing coverage before adding patios or driveways, and check local definitions. Explore parcels via the Platbookmapper map.
What is impervious surface FAQ
What counts as impervious surface?
Roofs, pavement, and other hard surfaces.
Is gravel impervious?
Sometimes, depending on local definitions.
What is an impervious surface limit?
A maximum share of a lot that can be hard surface.
How are stormwater fees calculated?
Often by impervious area.
Is lot coverage the same as impervious coverage?
Not always; lot coverage often counts only buildings.