How To Find Your Property’s Impervious Surface Limit

This page is the practical version: where the number lives, how to measure what you already have, how to tell whether a planned patio or driveway fits, and what to do if it does not. For what the rules are and how AWS handles compliance work, see our impervious surface compliance page.

Where Your Limit Comes From

Two separate rules can apply to the same project, and people often confuse them.

The first is local lot coverage. Your zoning code assigns each district a maximum impervious or lot coverage percentage. This is the number that governs most residential projects, and it varies by municipality and by district within a municipality.

The second is state stormwater review, which only engages on larger projects. Pennsylvania and New Jersey each set a threshold where a project stops being a local permit question and becomes an engineered stormwater submission. Most homeowners never reach it. Most commercial sites do.

You need the first number before you do anything. The second only matters once the project gets big.

How To Find Your Impervious Surface Limit

Four steps, in order. The first two take a phone call.

1. Identify your zoning districtYour municipality’s zoning map shows which district your parcel sits in. Most towns publish it online, and your property tax record or deed will list the parcel number you need to look it up.
2. Look up the coverage limit for that districtThe zoning ordinance lists a maximum impervious coverage or lot coverage percentage per district. Read the definition section too, because how a town defines impervious coverage determines what you have to count.
3. Confirm it with the zoning or code enforcement officeCall and confirm the limit for your specific parcel, and ask what documentation a permit application will require. This call also surfaces any overlay district, floodplain rule, or watershed restriction that would not show up in the base zoning table.
4. Measure what you already haveAdd up every existing hard surface before you add the project. This is where most people are surprised, because the total is usually higher than they expect.

What Counts Toward The Limit

The specifics come from your local definition, but these are counted almost everywhere:

• Roofs on the house, garage, and any covered structure, measured at the footprint including overhangs in some towns
• Driveways and parking areas in asphalt, concrete, or compacted material
• Patios, walkways, and sidewalks in concrete, stone, brick, or standard pavers
• Accessory structures such as sheds, gazebos, and detached garages
• Pools and pool decks, though treatment of the water surface itself varies
• Decks, depending on whether your town counts a surface with gaps over pervious ground

Two items cause most of the arguments. Decks are treated inconsistently: some municipalities count a raised deck fully, some partially, some not at all if water passes through to soil beneath. Gravel is frequently counted as impervious because compacted stone sheds water nearly as well as asphalt. Check both in your local definitions rather than assuming.

Measuring What You Already Have

For a rough number, your county parcel viewer or a satellite image with a measuring tool will get you close enough to know whether a project is comfortable, borderline, or clearly over. For a permit submission, most municipalities want a plot plan prepared by a licensed surveyor showing existing and proposed impervious area.

The arithmetic itself is simple. Here is a worked example on a 10,000 square foot lot in a district with a 30% limit.

A 16 by 20 foot patio adds 320 square feet and fits, with 140 to spare. A 20 by 30 foot patio adds 600 and puts the property 140 square feet over. That is the entire calculation, and it is worth running before you fall in love with a design.

Our companion post on how impervious surface limits affect your outdoor living walks through the design trade-offs when a project lands in that borderline zone.

What Triggers State Review In Pennsylvania And New Jersey

Local lot coverage governs small projects. These thresholds are where a project becomes an engineered stormwater submission instead. Your local ordinance can always be stricter than the state floor.

The quarter-acre New Jersey figure is the one that catches people. A large commercial pad or a sizeable residential addition can reach 10,890 square feet of new impervious surface faster than expected, and crossing it changes the project from a permit application into an engineered design.

Why The Limits Exist

Paving changes the math of a rainstorm. On natural ground most rainfall soaks in or evaporates and only a small share runs off. As impervious cover rises, that balance inverts.

Runoff more than quintuples between natural ground and a fully paved surface. That is the whole justification for the rules: when enough lots in a watershed shed their rain instead of absorbing it, the combined volume floods low spots, erodes stream banks, and overwhelms storm drains.

The volume on a single property is larger than most people picture. One inch of rain on 1,000 square feet of hard surface produces about 623 gallons of runoff, because an inch of water over a square foot is 0.623 gallons. A 1,600 square foot roof sheds roughly 1,000 gallons in a one-inch rain, and that water goes somewhere whether or not anyone planned for it.

When Impervious Limits Are Not Your Problem

Not every project needs this analysis. A few cases where you can stop worrying:

• Replacing a surface in kind. Resurfacing an existing driveway or rebuilding a patio on the same footprint usually adds no impervious area, so it does not move your total. Confirm your town measures net new coverage rather than treating it as new construction.
• Interior work. Renovations that do not change the building footprint have no impervious consequence at all.
• Small projects well under the limit. If your rough measurement shows you are at 18% in a district that allows 35%, a walkway is not going to change that. Run the numbers once, then build.
• Projects below the local permit threshold. Many municipalities exempt small accessory structures and modest hardscape from review entirely. Ask before paying anyone to document something nobody will read.

The inverse warning matters as much. Permeable pavers are not a guaranteed exemption. Some municipalities give full credit, some partial, and some none at all. Specifying permeable materials to stay under a limit only works if your ordinance recognizes the credit, so confirm that before it becomes the plan.

If Your Project Goes Over The Limit

Going over does not kill a project. It changes the path, and there are two routes.

Reduce the impervious area. Shrink the footprint, or substitute materials your town treats as permeable, where the ordinance allows the credit. See our paver options for surfaces that may qualify.

Manage the runoff the new surface creates. An approved system that captures and infiltrates the additional water often lets you build the full design. Which system depends on your soil and the volume involved:

• Dry wells that collect roof and surface runoff and percolate it into the soil
• Infiltration trenches and deep percolation systems where more capacity is needed
• Downspout extensions and yard drainage to route water to a controlled outlet
• Detention and retention basins on larger commercial sites, covered in our guide to retention versus detention basins

The expensive version of this is discovering the problem after the concrete is poured. Removing finished work or retrofitting drainage around it costs multiples of what designing for it would have.

Frequently Asked Questions