What is stormwater management for Accessory Dwelling Units (ADUs) in California?

Importance of Stormwater Management in ADUs

Environmental Impact

Unmanaged runoff from ADUs can overwhelm stormwater facilities and pollute nearby waterways, carrying oil, chemicals, and debris into rivers and beaches. Nationally, an estimated 10 trillion gallons of untreated stormwater runoff flow into U.S. waterways every year (Surfrider Foundation). To reduce this impact, homeowners should consider stormwater treatment devices such as rain gardens, permeable paving, or rainwater harvesting systems that help manage discharge onsite.

“Ocean Friendly Gardens is Surfrider’s sustainable landscaping and education program that provides beautiful, inexpensive and natural solutions to reduce polluted runoff and support resilient coasts.” — Surfrider Foundation

Regulatory Compliance

Under California ADU regulations, cities require homeowners to manage runoff from secondary dwellings according to project size. For example, any ADU project creating 10,000 sq. ft. or more of impervious surfaces is classified as a Priority Development Project under San Diego’s DS-560 Storm Water Checklist. Even smaller ADUs tied to a single-family dwelling still require the DS-560 checklist, which “must be completed and submitted with the permit application” (City of San Diego). Additional local rules apply: Encinitas requires a recorded covenant for onsite basins, while Signal Hill mandates a Low Impact Development (LID) stormwater management plan for projects adding ≥500 sq. ft. of new surface.

Property Protection

Beyond municipal code, effective stormwater management protects a homeowner’s investment. Poor drainage can damage foundations, erode soil, and cause costly disputes with neighbors if runoff flows off-site. Professional estimates show that stormwater management costs between $2,000 and $30,000 for residential properties, depending on site work, soil, and design complexity (Soils Inc.). By budgeting early for drainage as part of ADU construction costs, homeowners avoid compliance issues and ensure their stormwater management devices work effectively over the long term.

Unmanaged runoff pollutes waterways—use rain gardens, permeable paving, or basins to protect both your property and the local environment.

Key Components of Stormwater Management

Drainage Systems

Effective drainage systems are the backbone of ADU stormwater management and are often reviewed as part of a city’s stormwater management manual during the building permit process. Common solutions include gutters and downspouts, site grading, bio-retention basins, and permeable pavers. Bio-retention areas typically range from 150 to 300 sq. ft. for private residences, though exact sizing depends on roof area, soil type, and rainfall volumes (Wikipedia – Green Infrastructure).

Gutters should generally be 4–6 inches wide to handle residential roof runoff, ensuring water is safely directed into drains, infiltration areas, or an approved stormwater treatment facility (Designing Idea). Proper grading is equally important: water should flow away from the ADU foundation and into landscaped or engineered basins, with pre-treatment features (like filter strips or gravel inlets) to keep systems functioning effectively as part of a long-term Stormwater Maintenance Plan.

Infiltration vs. Runoff

California’s CalGreen standards and the San Diego Regional Water Quality Control Board’s Order R9-2013-0001 require projects to prioritize infiltration over direct runoff, so rainwater soaks into the ground instead of overwhelming the city’s MS4 permit system. This is especially important because, as the City of San Diego notes:

“Urban runoff discharged from municipal stormwater conveyance systems has been identified by local, regional and national research programs as one of the principal causes of water quality problems in most urban areas.” (City of San Diego)

Designing ADU sites for infiltration—through rain gardens, swales, or permeable paving—is both a regulatory requirement and an environmental necessity, reducing the need for costly stormwater discharge permits later.

Water Retention & Detention

Stormwater retention strategies help slow, store, and reuse rainwater onsite rather than letting it rush into the street or overwhelm city infrastructure. Homeowners can use:

• Infiltration basins – shallow, landscaped areas that absorb runoff.
• Rain gardens – decorative planted depressions that filter and infiltrate water.
• Permeable paving – driveways and walkways that allow water to seep through.
• Rain barrels or cisterns – storage containers that capture roof runoff for later use.

These stormwater management devices not only reduce flood risk but also align with the Stormwater Quality Program and Stormwater Quality Design Manual requirements many local governments enforce during ADU permitting.

Drainage design must consider roof size, lot slope, and soil conditions—choose the right BMPs to meet code and avoid flooding risks.

Understanding Regulations and Requirements

Municipal Regulations

Stormwater management requirements for ADUs vary across San Diego County municipalities, and homeowners need to be aware of local thresholds:

• Encinitas – ADUs larger than 500 sq. ft. of new impervious surface require stormwater mitigation, typically through a bio-retention or separated stormwater basin. Homeowners must also sign a recorded covenant, sometimes filed as a Notice to Title, to maintain the drainage feature as part of their property obligations.
• Santee – The City of Santee mandates that “every project over 500 sq. ft. is required to fully complete and implement a Storm Water Quality Management Plan (SWQMP)” (City of Santee – ADU Permit Guidance PDF). This means most detached ADUs trigger the requirement, and the SWQMP must be included with the building permit or alteration permit application.
• City of San Diego – All ADU projects must include the DS-560 Storm Water Requirements Applicability Checklist, which “must be completed and submitted with the permit application” (City of San Diego). Larger projects that qualify as Priority Development Projects (creating ≥10,000 sq. ft. of impervious surfaces or exceeding lot coverage and Floor Area Ratio thresholds) require an additional DS-564 Storm Water Quality Management Plan (SWQMP) (City of San Diego DS-560 Form PDF).

Building Code Requirements

At the state level, CalGreen requires all new residential projects, including ADUs, to incorporate Best Management Practices (BMPs) that promote infiltration of stormwater on-site. Local governments enforce these rules through forms and project classifications:

• DS-560 – required for all ADUs in San Diego to evaluate compliance with the Stormwater Quality Program.
• DS-564 (SWQMP) – required only if an ADU project is classified as a Priority Development Project (≥10,000 sq. ft. impervious area).

These forms ensure ADUs meet both Clean Water Act requirements and local water quality standards through the Stormwater Quality Maintenance Program and Stormwater Quality Design Manual.

Zoning Laws

In addition to stormwater thresholds, zoning overlays can impose stricter drainage requirements:

• Coastal Zone Overlays often require extra protections to safeguard beaches and sensitive marine environments.
• Hillside or Steep Slope Overlays may mandate engineered grading, slope stabilization, or enhanced drainage plans due to erosion risks.

Homeowners should review their property’s zoning designations early in the ADU planning process. Factors such as gross floor area, on-site parking, and permanent location within these overlays can increase the complexity of the utility application and permitting process, making early site planning critical.

Stormwater rules vary by city—always confirm thresholds like San Diego’s DS-560 or Santee’s 500 sq. ft. SWQMP requirement before building.

Planning for Stormwater Management in ADU Projects

Site Assessment

Before designing a stormwater system, homeowners should evaluate the site to understand how water will flow once the ADU is built. A thorough site assessment includes:

• Checking slope and grading to ensure water flows away from structures and doesn’t impact a combined sewer basin.
• Conducting a soil percolation test to determine how well the ground absorbs water.
• Measuring the impervious area added (roof, driveway, patio) since this dictates runoff volume and may be reviewed during a Minor Project Application or plumbing permit process.

Designing for Stormwater Control

A civil engineer plays a crucial role in ADU drainage design. They can:

• Develop a grading plan that prevents water from pooling near the ADU or neighboring lots.
• Size bio-retention basins or infiltration areas based on roof size, soil type, and rainfall intensity.
• Ensure compliance with city-specific forms such as DS-560 or DS-564 in San Diego, which are often required alongside a Single-Family Dwelling Application or ADU Worksheet.

Planning stormwater measures early in the design process helps homeowners avoid costly permit delays or redesigns later. Incorporating features like gutters, swales, or permeable paving from the start ensures smoother approvals and reduces unexpected costs, such as impact fees or an ADU System Development Charge.

Utility Connections and Challenges

In addition to managing runoff, ADU projects often need to address utility applications and connections that intersect with drainage planning:

• Sewer Tie-Ins – Many ADUs can connect to the property’s existing side sewer line. Cities like San Diego typically allow this if the line has capacity and is up to code. A civil engineer or plumber must confirm that the added flow won’t overwhelm existing infrastructure or require alley improvements to connect.
• Septic System Considerations – For properties on septic, adding an ADU may require decommissioning or upgrading the system, especially if the ADU is within 10 feet of the existing septic tank or leach field. In such cases, homeowners often need to switch to a municipal sewer connection and coordinate with the city for water line and water meter upgrades.

Homeowners should review both stormwater and sewer infrastructure together to avoid conflicts—such as placing a retention basin in an area slated for a sewer lateral or shut-off valve installation.

Homeowner Stormwater Planning Checklist for ADUs

Step 1: Assess Your Site

• Measure slope and drainage paths using a site plan or GIS map.
• Test soil percolation to understand infiltration capacity on California residential parcels.
• Calculate new impervious area (roof + driveway) since this affects runoff and future development rights.

Step 2: Confirm Regulatory Requirements

• If ≥500 sq. ft. added → mitigation likely required (e.g., Santee SWQMP).
• In San Diego City → submit DS-560 checklist with permit.
• If ≥10,000 sq. ft. impervious → project is Priority (DS-564 SWQMP required). Homeowners should also check height restrictions, lot coverage, and scenic standards if applicable.

Step 3: Hire a Professional

• Engage a civil engineer for grading plans, basin sizing, and reviewing connections like backwater valves or shut-off valves.
• Consult with an ADU specialist familiar with electrical permits, stormwater rules, and ADU Worksheet requirements.

Step 4: Design for Stormwater Control

• Plan gutters/downspouts sized at 4–6″.
• Choose solutions like rain gardens, permeable paving, or infiltration basins, considering nearby street trees and landscaping limits.
• Ensure basins are properly graded and include pre-treatment in line with the Stormwater Quality Maintenance Program.

Step 5: Coordinate Utilities and Drainage

• Check if ADU can tie into an existing side sewer or electrical service panel without overloading.
• If on septic → confirm decommissioning or upgrade requirements.
• Avoid conflicts between sewer laterals, water line, electrical panels, and stormwater basins.

Step 6: Submit and Maintain

• File required forms (DS-560/DS-564, covenants if required by city).
• Budget $5K–$10K for onsite drainage as part of ADU costs, along with other expenses such as ADU System Development Charge or impact fees.
• Maintain gutters, rain gardens, or basins to ensure long-term performance, which can protect rental income streams from ADUs or short-term rentals.

Always plan grading and stormwater systems early—proper slope and drainage design prevent costly permit delays and foundation issues.

Techniques and Strategies

Integrating Civil Engineering Solutions

Civil engineers play a central role in ensuring ADU stormwater systems meet Best Management Practices (BMPs) and comply with city code. They prepare grading plans, calculate runoff volumes, and size infiltration basins appropriately for each property. In Encinitas, for example, ADUs larger than 500 sq. ft.—whether a detached unit, Bonus Unit, or attached by a common wall or covered breezeway—must include a mitigation basin, and homeowners must sign a recorded covenant committing to maintain it long-term (City of Encinitas). Partnering with an engineer early in the ADU design process ensures both compliance and efficient site performance.

Green Infrastructure Options

Homeowners can integrate green infrastructure into their ADU projects to manage runoff while improving property aesthetics. Common strategies include:

• Rain gardens that capture and filter runoff with native plants.
• Permeable paving for driveways or walkways that allow water to soak through.
• Rainwater harvesting with barrels or cisterns for irrigation reuse.

The Surfrider Foundation’s Ocean Friendly Gardens program highlights how these solutions can reduce polluted runoff while creating attractive landscapes:

“Ocean Friendly Gardens is Surfrider’s sustainable landscaping and education program that provides beautiful, inexpensive and natural solutions to reduce polluted runoff and support resilient coasts.” — Surfrider Foundation

These approaches not only help meet municipal stormwater requirements but also contribute to long-term sustainability and can reduce reliance on costly stormwater treatment facilities.

Stormwater Best Management Practices (BMPs)

Every ADU project in California is expected to include BMPs to prevent flooding, erosion, and pollution. These fall into two categories:

Permanent BMPs:

• Infiltration basins or rain gardens.
• Permeable surfaces for driveways/walkways.
• Maintenance covenants (e.g., Encinitas requires property owners to maintain stormwater features in perpetuity, even if they later convert the ADU to a short-term rental).

Temporary BMPs (during construction):

• Erosion control blankets.
• Silt fences or fiber rolls to trap sediment.
• Gravel entrances to minimize dirt tracking, protecting utility lines like electrical service panels or overcurrent devices from sediment impacts.

Together, these practices ensure ADU projects comply with CalGreen mandates and city-level stormwater standards while protecting neighboring properties and local waterways. They also support utility coordination, since features like Shared meter or Separate meter connections may run alongside stormwater infrastructure and require careful planning.

Use the right tools and techniques—integrating permeable paving, basins, and rain gardens ensures long-term compliance and sustainability.

The Role of Professionals

1. Partnering with ADU Experts

For many homeowners, the first and most important step is working with an experienced ADU design-build firm. At Better Place Design & Build, our team specializes in end-to-end ADU projects, guiding homeowners through stormwater planning, permitting, and construction. We integrate stormwater compliance into the broader ADU design process—helping clients understand city requirements, coordinate drainage solutions, and ensure each project meets both regulatory and budget needs.

By managing everything under one roof, ADU specialists save homeowners from delays and costly redesigns while ensuring drainage features like basins, rain gardens, or permeable paving are seamlessly included in site plans. This full-service approach is especially valuable for senior citizens or homeowners seeking a property tax exemption when building an ADU for multigenerational housing.

2. Working with Civil Engineers

Civil engineers provide the technical expertise required to size stormwater features correctly and prepare compliance documents. They:

Size infiltration basins based on impervious area, soil infiltration rates, and rainfall patterns (often ranging 30–75 sq. ft. for typical ADUs, depending on the roof and driveway area).
Draft Storm Water Quality Management Plans (SWQMPs) when required, ensuring compliance with municipal thresholds such as San Diego’s DS-560 and DS-564 forms.
Coordinate directly with city reviewers, streamlining the permitting process and reducing back-and-forth revisions.

Their role is especially important in areas with stricter thresholds like Encinitas, where homeowners must sign covenants to maintain stormwater basins, or Santee, where any project over 500 sq. ft. requires a full SWQMP (City of Santee). Civil engineers may also provide site evaluations related to tree removal, water line coordination with the Water Bureau, or managing constraints near a hazardous waste facility.

3. Collaborating with Environmental Consultants

Environmental consultants focus on sustainable design and compliance with Best Management Practices (BMPs). They help homeowners and builders by:

• Designing rain gardens and native landscaping that filter and absorb runoff.
• Certifying that BMPs meet California’s CalGreen and Regional Water Quality Control Board requirements.
• Advising on sustainable strategies like rainwater harvesting, permeable surfaces, and integration with programs such as Surfrider’s Ocean Friendly Gardens.

Consultants also ensure projects remain compliant through construction, often reviewing temporary BMPs such as silt fences, fiber rolls, and erosion blankets to prevent sediment pollution during grading and building. They may also work with cities or counties (e.g., Placer County, El Dorado County, or the City of South Lake Tahoe under TRPA Code) where environmental regulations tied to the Growth Management Act or RCW 36.70A.680 require stricter oversight of development projects.

Rely on licensed ADU experts and civil engineers to size basins, draft SWQMPs, and meet strict local stormwater codes.

Common Challenges and Solutions

Decommissioning Outdated Systems

One frequent challenge during ADU development is dealing with outdated or incompatible septic systems. In California, an ADU cannot be placed within 10 feet of an existing septic tank or leach field, which often forces homeowners to either tie into the municipal sewer system or decommission the septic altogether. Septic decommissioning involves pumping out the tank, crushing or filling it, and capping it per city health codes. If sewer tie-in is available, engineers or ADU specialists coordinate connections to the property’s existing side sewer, ensuring the added flow does not exceed line capacity. Addressing septic issues early prevents costly mid-project redesigns and ensures compliance with local health and building codes.

Handling Complex Site Conditions

ADU sites in San Diego often face unique challenges such as hillside lots, clay soils, and coastal overlays. Each of these conditions complicates stormwater management:

• Hillside lots – Require engineered grading and slope stabilization to prevent erosion or slope failure. Retention basins may need to be stepped into the hillside.
• Clay soils – Have low infiltration capacity, limiting the effectiveness of traditional infiltration basins. Engineers often recommend larger bio-retention areas, amended soils, or underdrain systems to handle runoff.
• Coastal zones – Properties near beaches or lagoons may fall under stricter Coastal Commission oversight, requiring advanced drainage strategies and environmental review.

In all cases, the civil engineer’s role is critical. They adjust design specifications—such as sizing infiltration basins beyond the typical 30–75 sq. ft. range—and ensure that stormwater features meet both city requirements and site-specific constraints. Environmental consultants may also be brought in to address sensitive habitats or coastal compliance issues.

Address challenges early—grading, septic tie-ins, or poor soil infiltration can be solved faster with civil engineer input at the planning stage.

Conclusion

Building an ADU in San Diego or anywhere in California requires more than just good design—it also requires a plan for stormwater management. By managing runoff responsibly, homeowners not only stay compliant with local and state regulations, but also protect their property from flooding and foundation damage while contributing to healthier waterways and sustainable communities.

Key Homeowner Checklist:

1. Know your thresholds: ≥500 sq. ft. typically requires mitigation; ≥10,000 sq. ft. = Priority Development Project (City of San Diego DS-560).
2. Complete required city forms: DS-560 (all projects), DS-564 (Priority projects).
3. Hire a civil engineer for grading plans and basin sizing.
4. Budget $5K–$10K for onsite drainage solutions.
5. Maintain systems—gutters, basins, rain gardens—to keep them effective long-term.

At the end of the day, stormwater planning is about compliance, protection, and sustainability. The good news is you don’t have to navigate this alone.

👉 Contact local ADU specialists like Better Place Design & Build or a licensed civil engineer to design and implement a compliant stormwater plan for your ADU project. Taking this step early ensures smoother permitting, long-term property protection, and peace of mind knowing your project is built right from the ground up.

Successful ADU projects balance compliance, property protection, and sustainability—budget early for drainage to avoid costly redesigns.