Impervious Surface: Definition, Limits and Examples

An impervious surface is a hard surface that keeps rain from soaking into the ground, such as a roof, driveway, patio, sidewalk or parking lot. Zoning and stormwater codes often cap how much of a lot it can cover, and many cities bill a stormwater fee on it. Whether gravel, decks and pavers count depends on the local code.

This guide is written by Pillar, a land research platform. The rules and rates below come from the linked statutes, codes and agency pages, read on November 14, 2026. It is general information, not legal or engineering advice; confirm the rules for a property with the local planning or stormwater office or a licensed professional in that state.

What is an impervious surface?

The U.S. Geological Survey, in a 2007 report prepared with EPA for the Federal Highway Administration, describes impervious surfaces as "manmade objects that prohibit the infiltration of rainwater into the ground and cause increased surface flow." The same report quotes EPA's 2003 draft definition: "A hard surface area that either prevents or retards the entry of water into the soil mantle or causes water to run off the surface in greater quantities or at an increased rate of flow."

A 2024 appendix to EPA Region 1's draft general permit for commercial, industrial and institutional sites with 1 acre or more of impervious cover in the Charles, Mystic and Neponset River watersheds of Massachusetts uses a similar definition for the purposes of that draft permit, "any surface that prevents or significantly impedes the infiltration of water into the underlying soil" (EPA Region 1, Appendix E), and says EPA uses "impervious surface," "impervious area," and "impervious cover" interchangeably. Local codes may say "lot coverage" or "built-upon area" and define them differently.

Impervious surface examples

The two federal definitions above name these:

• Rooftops, buildings and other structures
• Driveways, parking areas, roads and storage areas
• Walkways and patios
• Concrete or asphalt paving and other nonporous material
• Gravel roads (EPA, 2003), and artificial turf and compacted gravel or soil (EPA, 2024)

On the other side, Raleigh, North Carolina, treats natural footpaths without gravel, pool water surfaces and drainfields as 0% impervious. The hard cases are gravel, decks, pools and permeable pavers, and codes split on them.

Is gravel an impervious surface?

It depends on the code, and on how the gravel was laid.

• Often yes. Athens-Clarke County, Georgia, answers yes for its stormwater utility: "Once gravel is compacted, from cars or heavy equipment, for example, surface water runs off of it much like a paved surface." Maryland's Critical Area law counts area "covered with gravel, stone, shell, decking, a paver, permeable pavement, or any manmade material" as lot coverage (Md. Code, Nat. Res. 8-1802(a)(21)).
• Unless it is designed to absorb water. New Hampshire's shoreland law lists "paved, gravel, or crushed stone driveways, parking areas, and walkways" as impervious "unless designed to effectively absorb or infiltrate water" (RSA 483-B:4, VII-b).
• Not in some cases, by statute. North Carolina's G.S. 143-214.7D, added in 2024, says that for state and local stormwater programs, ASTM number 57 stone laid at least four inches thick over geotextile fabric is not built-upon area or impervious surface. Neither is gravel or other landscaping material on areas with foot or bicycle traffic, or "on portions of driveways and parking areas that will not be compacted by the weight of a vehicle." An owner or developer may opt out of these exemptions.

Are pavers impervious?

• Maryland Critical Area: pavers and permeable pavement both count as lot coverage, with a limited allowance for approved pervious materials (see credits below).
• Raleigh, North Carolina: the city's stormwater FAQ says permeable pavement and artificial turf are "considered 100% impervious unless they are designed by a professional engineer and treated like a permanent stormwater control measure" under UDO 9.2.2.B-H and the state's design criteria.
• New Hampshire shoreland: patios count as impervious unless designed to absorb or infiltrate water.

Do decks and pools count as impervious surface?

• Raleigh's FAQ says slatted wood decks that drain to an unpaved surface are 50% impervious, or 30% if the ground under the deck is washed gravel. The water surface of a pool is 0% impervious.
• North Carolina's 2024 statute says a slatted deck, "the water area of a swimming pool," and drain-through artificial turf installed to the maker's specifications over a pervious surface are not built-upon area. It bars local governments from enforcing a definition that does not comply, except as federal law specifically requires. As we read it, that overrides a local rule counting slatted decks. Raleigh's FAQ still showed the 50% deck rule on November 14, 2026, so ask the city's reviewer.
• Maryland Critical Area counts the ground under "a stairway or impermeable deck" but excludes "a permeable deck," a wood mulch pathway, and a fence or wall less than a foot wide built without a footer.

As we read these codes, concrete or pavers around a pool count like any other paving.

Impervious surface limits: real examples

A limit is usually a percentage of lot area. Here are four, with their conditions.

• Raleigh, North Carolina (grandfathered lots). These are lots recorded before May 1, 2001, or lawfully recorded later as part of a subdivision approved before that date. The date alone does not qualify a lot. Under UDO 9.2.2.A.1, a grandfathered lot of one acre or less is exempt from the city's active stormwater control measures when it is used for a detached house, an attached house or another listed single-unit or two-unit dwelling, including accessory uses, and is not part of a larger common plan of development approved after May 1, 2001. UDO 9.2.2.A.4 lists maximum impervious coverage for exempt lots of 20% in R-1, 25% in R-2, 38% in R-4, 51% in R-6, and 65% in R-10 and all other base districts. It is not a flat ban: an exempt lot that goes over becomes subject to the city's active stormwater control measures. The city's homeowner page applies these percentages to residential lots of one acre or less, and sets other rules for larger lots, such as 5% for undeveloped lots over an acre. Lots created after May 2001 may have no limit or a lot-specific one, which the city says may be on a survey or the recorded plat.
• Maryland Chesapeake and Atlantic Coastal Bays Critical Area. Nat. Res. 8-1808.3 controls lot coverage limits in limited development areas and resource conservation areas. Except as otherwise provided in its subsection (d), lot coverage is limited to 15% of a parcel or lot. A lot of half an acre or less that existed by December 1, 1985 (Chesapeake Bay Critical Area) or June 1, 2002 (Atlantic Coastal Bays Critical Area) is limited to 25%; one over half an acre and under an acre that existed by those dates, to 15%. In a subdivision approved after those dates, coverage may not exceed 15% unless a local jurisdiction restricts it further, though an individual lot of one acre or less may exceed 15%. Separate restrictions apply in the buffer: lot coverage "may not exceed the minimum amount necessary for water-dependent uses," except for a modified buffer mapped or established under an approved local program, a variance granted under the Critical Area subtitle, or as provided in a waterfront revitalization or waterfront industrial area under a local program.
• New Hampshire protected shoreland. Under RSA 483-B:9, V(g), no more than 30% of "the area of a lot located within the protected shoreland" may be impervious "unless a stormwater management system designed and certified by a professional engineer is implemented." If impervious area will exceed 20% but stay below 30%, a stormwater management system must be implemented and maintained that is designed to infiltrate increased stormwater from development occurring after that paragraph took effect, under the state environmental department's rules. The commissioner may grant waivers. The protected shoreland is generally land within 250 feet of the reference line of public waters (RSA 483-B:4, XV).
• North Carolina water supply watersheds. Under 15A NCAC 02B .0624, low-density single-family detached residential projects in the critical area of a WS-II watershed are held to one dwelling unit per two acres, an 80,000-square-foot lot excluding road right-of-way, or 6% built-upon area. In the balance of a WS-II watershed the figures are one unit per acre, 40,000 square feet, or 12%. High-density projects can go up to 24% and 30% in those two areas with stormwater control measures that meet the rule. Local governments implement the rule and may allow only low-density development. Low-density projects must also release runoff to vegetated areas as dispersed flow or transport it through vegetated conveyances "to the maximum extent practicable," or use a curb-outlet system that meets the rule. Vegetated conveyances must be "maintained in perpetuity to ensure that they function as designed," so the percentage alone is not an approval.

What is an impervious surface ratio?

It is the same measure written as a decimal. The Hamilton County Rural Zoning Commission in Ohio publishes an ISR user guide for the Hamilton County Zoning Resolution. It divides total impervious area by net lot area. Lot area leaves out public right-of-way, an access easement for a public street, the handle of a panhandle lot, and any area not zoned to allow the proposed use. In the guide's table, a low-intensity permitted non-residential use has a maximum ISR of 0.60 and can apply directly for a zoning certificate. Above 0.60, it first needs a PUD-1 administrative approval before the Rural Zoning Commission, which ties coverage to planned unit development review. Thresholds vary for some uses, and the guide says the adopted Zoning Resolution governs if the two conflict. Note the denominator: this county subtracts right-of-way, while Maryland measures against the "total lot or parcel."

How to calculate impervious surface coverage

Measure each impervious area, add them up, divide by the lot area, and multiply by 100. Codes differ on which surfaces count and on the denominator.

Worked example (an illustration, not a real lot). Take a 0.25-acre lot. One acre is 43,560 square feet, so the lot is 10,890 square feet (our acreage calculator does the conversion). It has:

• House and attached garage roof: 1,900 square feet
• Concrete driveway, 12 by 50 feet: 600 square feet
• Paver patio, 16 by 20 feet: 320 square feet
• Front walk, 3 by 30 feet: 90 square feet
• Shed, 10 by 12 feet: 120 square feet
• Slatted wood deck over lawn, 12 by 16 feet: 192 square feet

Under Raleigh's R-4 limit. Assume a grandfathered R-4 lot with a detached house that meets UDO 9.2.2.A.1, including its common-plan condition, so 38% applies: 10,890 × 0.38 = 4,138 square feet. The hard surfaces total 1,900 + 600 + 320 + 90 + 120 = 3,030 square feet. The deck adds 96 at the FAQ's 50%, for 3,126 square feet (28.7%), or nothing under the state statute, for 3,030 (27.8%). Either way there is about 1,000 square feet of headroom.

Add a 16-by-32-foot pool with a 4-foot concrete deck around it. The pool water (512 square feet) counts as zero. The concrete ring is 24 × 40 − 512 = 448 square feet and counts in full. With the deck at 50%, the total is 3,574 square feet (32.8%), still under 38%. Raleigh's FAQ requires an as-built impervious survey "when the proposed amount of impervious surface added to a property is within 10 percent or 400 square feet (whichever is greater) of the maximum," so ask the reviewer first.

Under Maryland's Critical Area limit. Put the same lot in the Chesapeake Bay Critical Area. Assume it existed on or before December 1, 1985, it is in a limited development area or resource conservation area, and all the work is outside the buffer. The limit is 25%, or 2,722.5 square feet. Maryland counts the pavers. It excludes a permeable deck, which the state's Critical Area Commission glossary describes as a deck with gaps that let water pass freely. Assume this slatted deck qualifies. The lot then starts at 3,030 square feet (27.8%), already over. Under 8-1808.3(f), a local jurisdiction may allow more if new coverage and water quality impacts have been minimized and the owner does on-site mitigation or pays a fee in lieu. The total may not exceed the limit "by more than 25% or 500 square feet, whichever is greater." As we read it, 25% of 2,722.5 is about 681, so the (f)(2) ceiling is about 3,403 square feet, before any separate allowance for approved pervious materials. The 448-square-foot pool deck brings the lot to 3,478 square feet (31.9%), past that ceiling. The glossary lists swimming pools as structures, and the statute counts area occupied by a structure. If the pool's 512 square feet count, the total is 3,990 square feet (36.6%). That is also more than the ceiling plus the 500 square feet allowed for approved pervious materials, about 3,903. Confirm the pool's counted area with the local Critical Area reviewer.

Same lot, same improvements: comfortable in one place, over the limit in another.

How to find the impervious limit for a real lot

1. Find the zoning district on the official map, then look in its dimensional table for "impervious surface," "lot coverage" or "built-upon area." Our R1 zoning guide compares those standards across ten codes.
2. Read the definitions: what counts, how decks, gravel, pools and pavers are treated, and the denominator.
3. Check overlays and state rules. Shoreland, critical area and watershed rules can be stricter than the district.
4. Read the recorded plat for a lot-specific limit, as in Raleigh. See how to read a plat map.
5. Read the covenants, which can add their own rules; see deed restrictions before buying land.
6. Measure what exists. Raleigh requires a signed and sealed survey of existing impervious area if a lot is already at or above its zoning limit.

Credits and exceptions

• Maryland pervious materials. Where the other conditions in 8-1808.3(f) are met, subsection (f)(6) covers "development that uses pervious materials that have been approved by the Commission as part of a local program." For that development, the adjusted limits in (f)(2) and (f)(3) "may be exceeded by up to 500 square feet." Subsection (f) says a local jurisdiction "may allow" the extra coverage, so it is not automatic.
• Raleigh engineered options. The FAQ gives two ways to exceed a lot's maximum. One is a stormwater analysis showing no more than a 0.04-foot rise in flood levels for the 2, 10, 25, 50 and 100-year storms. The other is a volume control device that captures the difference in runoff volume between the proposed development and the maximum impervious limit or the existing condition, whichever is greater, for the 90th percentile storm (1.4 inches in 24 hours). Both need a stormwater tracking permit and an as-built impervious survey. The device also needs a maintenance manual, a deeded easement and a replacement fund payment of 24% of estimated construction cost. UDO 9.2.2.A.4 also gives a one-time 400-square-foot increase to a lot whose existing house predates the ordinance, under the conditions it lists.
• New Hampshire. Coverage over 30% within the protected shoreland is allowed with an engineer-certified system that keeps post-development volume and peak flow for the 10-year, 24-hour storm at or below pre-development levels. The waterfront buffer may also need planting under RSA 483-B:9, V(g)(3).
• Baltimore fee credits. Baltimore's credit page offers single-family owners credits for practices "like planting trees, installing rain gardens, and using rain barrels." A fee credit lowers the bill; it does not change a zoning limit.

Impervious surface fees and the equivalent residential unit

Many stormwater utilities bill on impervious area, using an equivalent residential unit (ERU): the impervious area of a typical home, as each utility defines it. Larger non-residential parcels pay for more units.

• Baltimore, Maryland. The city's fee page sets the ERU at 1,050 square feet, the impervious area of the median-sized house in the city. Non-single-family properties pay $78 per ERU per year. Single-family homes pay $52 a year for no more than 820 square feet, $78 for more than 820 and no more than 1,500 square feet, and $156 for more than 1,500.
• Pittsburgh, Pennsylvania. Pittsburgh Water sets one ERU at about 1,650 square feet. Effective March 8, 2026, residential Tier 1 (400 to under 1,015 square feet) pays $5.75 a month, Tier 2 (1,015 to under 2,710) pays $11.50, and Tier 3 (2,710 and up) pays $22.99. Non-residential properties pay $11.50 per ERU a month.

As an illustration, a commercial parcel with 21,000 square feet of roof and paving is 20 ERUs in Baltimore, or $1,560 a year at $78. The same parcel is about 12.7 ERUs in Pittsburgh, about $146 a month. Each utility's own rounding and credit rules apply.

What impervious limits mean when you buy or develop land

Coverage caps drive lot yield. North Carolina's watershed rule measures project density as total built-upon area divided by total project area. In a WS-II balance-of-watershed area, a low-density single-family detached project measured that way gets 12%. Assume a tract with 20 acres of project area after the rule's exclusions, such as land below the normal high water line. That is 104,544 square feet for every roof, driveway and street combined. If the streets took an assumed 33,000 square feet, 71,544 would remain: about 3,577 square feet per lot at 20 lots, or 2,385 at 30. That holds only if the local ordinance measures by built-upon area rather than lot size or units per acre. A shorter street layout leaves more for houses. The high-density option of up to 30% gives 261,360 square feet, but requires stormwater control measures. Price the land on the option the local ordinance allows.

New impervious area triggers stormwater rules. In New Jersey, N.J.A.C. 7:8-1.2 defines "major development" to include creating one-quarter acre (10,890 square feet) or more of "regulated impervious surface" since February 2, 2004. Other prongs cover disturbing an acre or more, creating a quarter acre of regulated motor vehicle surface since March 2, 2021, and reconstructing a quarter acre of impervious or motor vehicle surface since January 20, 2026. A combination of those three surface types also counts once it totals a quarter acre, and "the same surface shall not be counted twice." Developments in a common plan of development or sale count together. Subchapter 5 sets the design and performance standards for major development, unless a qualifying regional plan applies alternative standards that are at least as protective. Section 7:8-1.6 has transition rules. An application to the Department of Environmental Protection under one of the permit statutes it lists stays under the requirements in effect on July 17, 2023, if it is declared administratively and technically complete (or, for CAFRA individual permits, complete for public hearing or public comment) before July 20, 2027. That exemption is limited to the land area and scope addressed by the qualifying applications or approvals. It ends when the qualifying Department permit expires or otherwise loses effect; if there are several qualifying Department permits, the one that expires last governs. Revisions can void the exemption unless the Department finds each one has only a de minimis impact on water resources. Some public roadway, railroad and film studio projects have their own dates. The section does not decide when local ordinances apply. Section 7:8-1.5 lets other agencies or entities impose more stringent requirements.

Where limits show up in the deal documents:

• The zoning code and overlay maps, for the base limit and any watershed, shoreland or critical area rule.
• The recorded plat, for lot-specific limits like Raleigh's.
• The survey. Ask the surveyor to state existing impervious area by surface type.
• Covenants and HOA documents, for private coverage limits.
• Stormwater fee records. In Athens-Clarke County, the FAQ says, the utility moves the account to the new owner after the closing attorney files Georgia's PT-61 transfer form. Ask the seller for the current bill.
• Prior permits. Maryland's Critical Area statute says a lot "legally developed as of July 1, 2008, may be considered legally nonconforming" for lot coverage. Ask whether existing paving was permitted.

Check before an offer: the limit and its definitions, existing impervious area, how much your plan adds, whether that crosses a stormwater threshold, and what the required facilities cost. Floodplain rules can stack on top; see flood zone X and land development.

Pillar's site plan tool sketches rough lot counts, unit yields and coverage on a parcel's actual geometry, a quick test of whether a coverage cap breaks the deal. Confirm the cap in the code before relying on it.

Quick answers

Is gravel impervious? Often, once compacted. North Carolina excludes #57 stone at least four inches thick over geotextile fabric.

Are pavers impervious? It depends on the code. Pavers and permeable pavement both count in Maryland's Critical Area. Raleigh counts permeable pavement unless it is engineered as a stormwater control measure.

Does a pool count as impervious surface? In North Carolina the water area of a pool does not count, but concrete or pavers around it do. Other codes differ.

What is a typical impervious surface limit? There is no national figure. The examples above run from 5% on large undeveloped lots in Raleigh and 6% in a North Carolina WS-II critical area to 65% in Raleigh's R-10 district.

What is an impervious surface fee? A stormwater utility charge based on a property's impervious area, often billed in equivalent residential units.