Stormwater Management Basics for Homeowners
Why Does My Project Need Stormwater Management?
If your municipality has told you that your new garage, addition, patio, driveway, pool, or other home improvement requires stormwater management, you may be wondering why. This is especially common when your property has never had an obvious drainage problem.
The basic issue is impervious area. Lawns and landscaped areas allow some rainfall to soak into the ground. Roofs, pavement, patios, and other impervious surfaces reduce that opportunity and can cause more rainfall to become runoff.
Stormwater regulations are intended to manage that change. Depending on where you live, how much impervious area you are adding, what has previously been constructed on the property, and other site conditions, your project may be exempt, may qualify for a simplified process, or may require a complete stormwater management plan.
The important part is that there is no single answer that applies everywhere.
Every Municipality Is Different
Pennsylvania does not have one uniform set of municipal stormwater requirements for every homeowner project. Local stormwater ordinances establish their own thresholds, exemptions, definitions, design standards, submission requirements, and review procedures.
Two nearly identical garages on similar properties in neighboring townships can therefore have very different requirements.
Even the seemingly simple question of what counts as impervious area can vary. A swimming pool may count in one municipality and not in another. Decks may be considered pervious under certain circumstances in one ordinance and treated differently elsewhere. Gravel may be treated as impervious. Patios, sheds, driveway expansions, and other improvements may also count.
That is why the first step in a residential stormwater project should not be designing a stormwater facility. It should be determining what the municipality actually requires for that particular property and project.
What Is Impervious Area?
Impervious area generally means a surface that prevents or significantly reduces rainfall from soaking into the ground. Building roofs, asphalt, and concrete are common examples.
Other surfaces are less straightforward.
A deck with gaps between the boards may allow rainfall to reach the ground below it. A gravel driveway may appear to be pervious but may nevertheless be classified as impervious under the applicable ordinance. Swimming pools are another common example where municipal definitions and exemptions vary.
For that reason, it is best not to assume that something does or does not count simply because it seems pervious or impervious. The applicable ordinance needs to be reviewed.
Why Does a Certain Number of Square Feet Matter?
Most municipal stormwater ordinances establish thresholds based on new impervious area, cumulative impervious area, earth disturbance, or some combination of them.
Below one threshold, a project might be exempt from some or all stormwater requirements. Cross the threshold, and a stormwater plan may suddenly be required. Larger projects may trigger additional calculations, testing, reports, agreements, inspections, or other requirements.
This can understandably seem arbitrary when adding a relatively small amount of additional pavement or roof area changes the regulatory requirements for the entire project. But once a project crosses a threshold established by the ordinance, a different set of requirements may apply.
Sometimes the New Project Is Not the Only Area That Has to Be Managed
This is one of the more surprising parts of residential stormwater regulation.
Suppose a homeowner previously constructed a patio, shed, pool, driveway expansion, or other improvement that qualified for an exemption at the time. Years later, the homeowner proposes another improvement.
Depending on the municipal ordinance, the new project may cause the property to cross a cumulative impervious-area threshold.
When that happens, the stormwater requirement may not necessarily be limited to the newest improvement. An ordinance can require stormwater management based on the cumulative development on the property, potentially bringing previously exempt impervious area into the calculation.
In other words, an improvement that legitimately required no stormwater facility when it was constructed can still matter when the next project is proposed.
This is one reason Elsa Associates reviews the history of improvements to a property rather than looking only at the footprint of the project currently being proposed. For an example, see our Manheim Township case study.
Impervious Area and Earth Disturbance Are Different
These terms are often confused, but they measure different things.
Impervious area generally refers to the finished surfaces that affect runoff, such as roofs, pavement, and patios.
Earth disturbance refers to the area physically disturbed during construction. It can include excavation, grading, utility trenches, construction access, stockpiles, and areas that will eventually be restored to lawn.
A 1,000-square-foot building therefore does not necessarily result in only 1,000 square feet of earth disturbance. The disturbed area can be substantially larger.
That distinction matters because Pennsylvania’s erosion and sediment control requirements are based on earth disturbance, while many municipal stormwater thresholds focus heavily on impervious area.
What Happens at 5,000 Square Feet of Earth Disturbance?
Pennsylvania’s Chapter 102 regulations require appropriate erosion and sediment control best management practices (BMPs) for earth disturbance activities generally, regardless of whether a formal written erosion and sediment control (E&S) plan is required.
Once a project reaches 5,000 square feet of total earth disturbance, a written Erosion and Sediment Control Plan is generally required. The E&S Plan addresses how erosion and sediment will be controlled during construction through measures such as limiting disturbance, stabilization, silt fence, sediment traps, and other appropriate BMPs.
For many residential projects below one acre, the practical question then becomes whether the municipality requires that E&S Plan to be submitted to the applicable County Conservation District for review.
That varies. Some municipalities require Conservation District review when the project exceeds 5,000 square feet of disturbance, while others have different procedures. The Pennsylvania Department of Environmental Protection (DEP) specifically notes that a project involving less than one acre may nevertheless have to submit its E&S Plan to the Conservation District because of municipal requirements.
So once again, the local requirements matter.
And What Happens at One Acre?
This is not usually an issue for a typical residential garage, patio, addition, or pool, but there is another significant regulatory threshold at one acre of earth disturbance.
Construction activities involving one acre or more of earth disturbance generally require NPDES permit coverage for stormwater discharges associated with construction activities under Pennsylvania’s Chapter 102 program. The one-acre threshold can also apply when a smaller project is part of a larger common plan of development or sale that collectively involves one acre or more of disturbance.
At that point, the project moves beyond the typical municipal residential stormwater submission and into a more substantial state permitting process involving the Pennsylvania Department of Environmental Protection and, in many areas, the applicable County Conservation District.
Most ordinary homeowner projects never approach this threshold, but it helps illustrate why impervious area, earth disturbance, municipal stormwater requirements, erosion and sediment control, and NPDES permitting are related but separate concepts.
What Is the Stormwater Facility Actually Doing?
Depending on the ordinance and the project, a stormwater system may be designed to accomplish several different things.
It may capture runoff from new impervious surfaces. It may allow runoff to infiltrate back into the ground. It may reduce the total volume of runoff leaving the property. It may temporarily store water and release it more slowly so that the peak rate of runoff does not increase.
Not every project requires all of these things in the same way.
That is why simply saying that a property “needs a stormwater system” does not tell you very much. The design depends on which stormwater requirements the project actually has to satisfy.
What Does a Residential Stormwater System Look Like?
For many homeowner projects, the finished stormwater facility is less dramatic than people expect.
A residential system might include an underground stone infiltration bed, infiltration trench, rain garden, surface basin, or another type of stormwater Best Management Practice, commonly called a BMP. Downspouts, yard drains, pipes, or other conveyance systems may direct runoff to the BMP.
Underground infiltration facilities are particularly common on constrained residential properties because the lawn can often be restored over the facility after construction.
The appropriate solution depends on the property, soils, grading, available space, setbacks, utilities, proposed improvements, and municipal requirements.
Why Can’t I Just Run a Pipe to the Street?
Moving water away from a house and complying with a stormwater ordinance are not necessarily the same thing.
A pipe may successfully carry roof runoff away from a foundation, but discharging that water directly to the street, another property, or a downstream drainage system may not satisfy requirements for infiltration, runoff-volume reduction, or peak-rate control.
It can also simply transfer a drainage problem somewhere else.
A stormwater design therefore needs to consider not only where the water goes, but also what the ordinance requires to happen to it before it leaves the property.
Do I Need an Infiltration Test?
Sometimes.
Some municipalities or types of stormwater facilities require field infiltration testing before an infiltration BMP can be designed. Other municipalities provide simplified procedures or assumed infiltration rates for certain small residential projects.
Site conditions matter as well. Soils, groundwater, bedrock, slopes, existing fill, septic systems, wells, utilities, and nearby structures can all affect whether infiltration is appropriate and where a facility can be located.
There is no universal answer.
Why Does the Stormwater Facility Have to Be So Far From Everything?
Stormwater ordinances and design standards commonly establish setbacks from buildings, property lines, wells, septic systems, slopes, and other features.
Those setbacks can become one of the biggest design constraints on a residential lot.
A property may have plenty of open lawn and still have relatively little area that is available for a stormwater BMP after the house, proposed improvement, property lines, utilities, septic system, and required setbacks are considered.
This is another reason it can be useful to think about stormwater before the location and size of a proposed garage, addition, pool, or other improvement are completely fixed.
Why Is There an Operation and Maintenance Agreement?
Installing the BMP is only part of the obligation.
Many municipal ordinances require the property owner to execute an Operation and Maintenance Agreement, commonly called an O&M Agreement, for privately owned stormwater facilities.
The basic purpose is straightforward: the stormwater facility is supposed to continue functioning after construction is complete, so someone needs to remain legally responsible for maintaining it.
Depending on the municipality, the agreement may identify the BMPs, establish inspection and maintenance responsibilities, give the municipality certain inspection or access rights, and describe what can happen if the facility is not maintained.
Many Pennsylvania stormwater ordinances require these agreements to be recorded with the property. Some municipal ordinances expressly provide that the O&M agreement or BMP obligations constitute restrictive covenants or real-property obligations that run with the land.
That means the responsibility is not necessarily finished when the original homeowner sells the house.
Does the Stormwater Facility Really Stay With the Property?
Often, yes.
Where the applicable ordinance and recorded documents establish the BMP and its maintenance obligations as permanent property obligations, a future owner takes the property subject to those requirements.
This is intentional. A stormwater facility designed to manage runoff from a house, driveway, garage, or other permanent improvement cannot simply stop functioning because ownership changes.
At the state level, for projects that require a Chapter 102 permit, Pennsylvania regulations require recorded notice of post-construction stormwater management (PCSM) BMPs and provide for long-term O&M responsibility as a covenant running with the land and binding subsequent owners. Municipal stormwater ordinances commonly use similar recorded O&M mechanisms for private BMPs.
The exact documents and obligations depend on the municipality and project.
What Maintenance Does a Stormwater Facility Need?
That depends on the BMP.
Maintenance can include keeping inlets and pipes clear, removing accumulated sediment or debris, maintaining vegetation, repairing erosion, preventing inappropriate structures or fill from being placed over the facility, and addressing conditions that prevent the BMP from functioning as designed.
An underground system may be largely invisible after the lawn is restored, but invisible does not mean maintenance-free.
The approved O&M plan or agreement should identify the required inspection and maintenance activities.
Can I Build Something Over the Stormwater Facility Later?
Do not assume that you can.
A BMP occupies part of the property for a specific purpose. Building an addition, patio, pool, shed, driveway, retaining wall, or other improvement over or near it can interfere with its storage volume, infiltration area, pipes, access, or required setbacks.
If future improvements are reasonably foreseeable, it is worth considering them when the original stormwater system is being designed.
Changing or relocating an existing BMP later may require municipal approval and potentially a revised stormwater plan and calculations.
Why Does the Township Want a Bond or Financial Security?
Some projects require the applicant to post financial security, often casually referred to as a bond, for required stormwater improvements.
The purpose is not to charge the homeowner an additional construction cost. The security is intended to make sure the improvements shown on the approved plan are actually constructed.
Depending on the applicable ordinance and approval process, the security might take the form of a bond, letter of credit, cash escrow, or another acceptable financial guarantee. Municipal ordinances commonly provide for financial guarantees covering required stormwater facilities and other improvements, with release occurring after the required work has been satisfactorily completed.
In practical terms, if a required stormwater facility is approved but never installed correctly, the municipality does not want to be left without a mechanism to ensure completion.
Not every small residential project requires financial security, and the requirements vary by municipality and type of approval.
Why Might I Need an As-Built Plan or Completion Certification?
Some municipalities require documentation after construction showing that the stormwater facility was installed in accordance with the approved design.
Depending on the municipality, this might involve an engineer’s certification, an as-built or record drawing, photographs, survey information, contractor documentation, or some combination of these.
This matters particularly for underground BMPs. Once an infiltration bed is covered with soil and lawn, neither the homeowner nor the municipality can readily see its dimensions, stone depth, piping, or other components.
Post-construction documentation provides a record of what was built and can be a condition of final approval or release of financial security.
Why Can a Small Home Project Become a Fairly Involved Process?
Because several different requirements can overlap. A seemingly simple garage or addition can potentially involve zoning and building setbacks, municipal stormwater thresholds, cumulative impervious-area calculations, grading and drainage, erosion and sediment control, infiltration testing, stormwater calculations, BMP setbacks, an O&M agreement, easements or recorded documents, financial security, construction inspections, as-built plans or completion certification, and, for substantially larger projects, County Conservation District and DEP permitting requirements.
Not every project requires all of these things. In fact, many do not. The important part is determining which requirements actually apply before designing more than the project requires.
Common Homeowner Questions
Does a pool count as impervious area?
Sometimes. Municipalities treat pools differently. The applicable ordinance needs to be reviewed.
Does a deck count?
It depends. Some ordinances distinguish between decks that allow rainfall to pass through and solid impervious surfaces. Others treat them differently.
Does gravel count?
It can. Gravel should not automatically be assumed to be pervious for stormwater purposes.
Does my existing driveway count?
Possibly. Existing impervious area may be relevant to the calculations, and previous additions to the driveway may matter under cumulative provisions.
I received an exemption for an earlier project. Why does it matter now?
Because some municipal ordinances establish cumulative thresholds. A later project can cause the property to cross a threshold that changes the applicable requirements and, depending on the ordinance, can require previously exempt improvements to be included in the stormwater management calculation.
Do I need stormwater management if my property has never flooded?
Possibly. Stormwater requirements are generally triggered by the proposed development and the applicable ordinance, not simply by whether the property currently experiences a drainage problem.
Can the BMP go underground?
Often, yes. Underground infiltration beds and trenches are common residential solutions where site conditions and municipal requirements permit them.
Can I put the BMP anywhere in my yard?
Usually not. Setbacks, grading, utilities, buildings, property lines, septic systems, wells, soils, and other constraints can limit where it can be located.
Do I need an infiltration test?
Sometimes. It depends on the municipality, the BMP, and the site.
Will I have to maintain it forever?
Potentially, yes. Many stormwater facilities are permanent improvements with continuing O&M obligations that remain with the property.
Will the Township inspect it later?
It may. Inspection rights and procedures vary by municipality and are often addressed in the ordinance and O&M agreement.
Does every residential project require an engineer?
No. Some projects are exempt or can use simplified procedures. Others require engineered plans, calculations, reports, certifications, or other professional services.
Start With the Requirements, Not the Stormwater Facility
For a homeowner, stormwater management can seem backwards. You may already know exactly where you want the garage, patio, pool, or addition, only to discover that the stormwater requirements affect the size, location, cost, or approval process for the project.
Elsa Associates approaches residential stormwater projects in the opposite order.
We first determine what the municipality considers impervious area, whether previous improvements matter, which threshold the project falls under, what exemptions or simplified procedures are available, and what supporting requirements apply. We then evaluate the site and develop an appropriate stormwater solution when one is required.
The goal is not to design more stormwater infrastructure than necessary. It is to understand the requirements, find a practical way to satisfy them, and help move the project through the approval process.
Planning a Home Improvement?
If you are planning a garage, addition, patio, pool, driveway, new home, or other improvement in Central Pennsylvania, Elsa Associates can review your property and the applicable municipal requirements to determine what level of stormwater management, if any, applies.