Impervious surface: what counts and why it limits land

What an impervious surface is

An impervious surface is a surface that water cannot pass through, so rain runs off it instead of soaking into the soil. North Carolina state stormwater rules often use the term built-upon area for a close idea. Local ordinances may use impervious surface, impervious area, or lot coverage. Each term has its own definition in its own rule.

What counts as impervious surface

What counts as impervious surface depends on the ordinance that applies. Roofs and pavement count almost everywhere. Other surfaces depend on the definition.

• Roofs on houses, garages, and outbuildings.
• Pavement on driveways, parking lots, streets, and sidewalks.
• Patios and concrete pads.
• Pools, under many ordinances.
• Compacted gravel, under some ordinances and not others.

Slatted decks and pervious pavement may count differently, or not at all. State law and local rules have changed what counts over time. The current ordinance text and the reviewing staff decide.

Why rules limit impervious surface

Rules limit impervious surface because hard surfaces send more runoff, faster, to streams and lakes. That runoff carries sediment, oil, and nutrients, and it floods downstream land. Limits come from sources such as zoning ordinances, water supply watershed rules, and state stormwater programs. Water supply watershed rules protect drinking water reservoirs, and local governments apply them through overlay districts.

How impervious limits shape a lot or site

An impervious limit caps the total hard surface on a lot or site. On a home lot, the house, driveway, patio, and any pool share one allowance. A lot can meet every setback and still run out of allowance.

On a larger site, buildings, parking, roads, and equipment yards all count. A data-center campus or a subdivision uses a large amount of hard surface. A tight limit can force a smaller building, less parking, a taller design, or engineered controls. Some rules allow more coverage when the project builds and maintains stormwater controls. See how many houses per acre and data center site selection criteria.

How stormwater control works

Stormwater control works by catching runoff and then slowing, storing, or treating it before it leaves the site. Common controls include wet ponds, dry detention basins, bioretention areas, and permeable pavement. These controls take land of their own, often at the low end of the site. A civil engineer designs them, and the local government or the state reviews the design. An owner or an association maintains them after construction.

What the public record shows

The public record shows the zoning district, watershed overlay maps, and the text of the ordinance that sets the limit. Aerial imagery shows existing hard surfaces, and some local governments publish impervious area layers.

The record cannot show how staff will count a given surface, how much allowance earlier permits used, or what a final stormwater design needs. Planning staff, stormwater staff, and a civil engineer confirm those. See what we check on zoning and moratoriums and on flood, wetlands, and soils.

Questions

Does gravel count as impervious surface?

It depends on the ordinance and on state rules as they apply to the site. Ask local planning or stormwater staff how they count gravel before you plan around it.

Is lot coverage the same as impervious surface?

Not always. Many ordinances count only building footprints as lot coverage, while an impervious limit counts every hard surface. Read the definitions in the ordinance that applies.

Can a project exceed an impervious limit?

Sometimes, through engineered stormwater controls, a variance, or a different permit path. Planning staff and a civil engineer tell you which options exist for the site.