How to Coordinate Drainage and Grading for Your ADU Project

Why Drainage and Grading Matter for Your ADU

Drainage and grading are among the least glamorous but most critical aspects of ADU construction. Poor drainage can lead to foundation damage, standing water, soil erosion, mold growth, and even structural failure over time. In California, where seasonal rainstorms can dump significant rainfall in short periods, proper drainage design is not optional; it is essential.

Many homeowners are surprised to learn that drainage and grading requirements can significantly impact the design, cost, and timeline of their ADU project. A site that appears flat and well-drained to the naked eye may have subtle grade issues that require engineering solutions costing $5,000 to $20,000 or more. Conversely, properties with obvious drainage challenges can often be effectively managed with proper planning.

This guide covers everything California homeowners need to know about coordinating drainage and grading for an ADU project, from initial site assessment through permit approval and final construction.

Grading Basics: What Every ADU Homeowner Needs to Know

Grading refers to the shaping and leveling of the ground surface to direct water flow away from structures and toward designated drainage points. Proper grading is the first line of defense against water intrusion and foundation damage.

Minimum Grading Requirements

The California Building Code (CBC), based on the International Building Code, establishes minimum grading requirements for all residential construction:

These are minimum standards. In many situations, especially on sloped lots or properties with existing drainage issues, exceeding these minimums is advisable.

Site Assessment: What to Evaluate Before Design

Before designing your ADU's drainage system, a thorough site assessment should evaluate:

• Existing grade and slope: Where does water currently flow on your property? Are there low spots where water collects?
• Soil type: Clay soils (common in many parts of Los Angeles) drain poorly and can create perched water tables. Sandy soils drain quickly but may have erosion issues. Your geotechnical report will identify your soil type.
• Existing drainage infrastructure: Are there existing drain lines, catch basins, or French drains on the property? Where does the property drain to (street, alley, storm drain)?
• Neighboring properties: Water from neighboring properties may flow onto your lot. You cannot block this water without redirecting it to an approved drainage point.
• Tree roots and vegetation: Large trees can affect grading and drainage patterns. Root systems can also damage drain lines if not properly accounted for.

Creating a Drainage Plan for Your ADU

Most California jurisdictions require a drainage plan as part of the ADU permit application. The complexity of the drainage plan depends on the size of your project and local requirements.

What a Drainage Plan Typically Includes

• Existing conditions: A topographic survey showing existing grades, contours, structures, and drainage features.
• Proposed grades: The finished grade elevations around the ADU, showing how the ground will be shaped to direct water flow.
• Drainage flow arrows: Arrows indicating the direction water will flow across the site after construction.
• Drainage collection points: Locations of area drains, catch basins, trench drains, or other collection points.
• Drain line routing: The path of underground drain pipes from collection points to the discharge location (usually the street gutter or a storm drain connection).
• Detention or retention features: If required, details of any stormwater detention or retention systems, such as dry wells, rain gardens, or underground storage.

A civil engineer or landscape architect typically prepares the drainage plan. Costs range from $1,500 to $5,000 depending on the complexity of the site and local requirements.

Common Drainage Solutions for ADU Projects

1. Surface Grading

The simplest and most cost-effective drainage solution is proper surface grading. By shaping the ground to slope away from the ADU foundation at a minimum of 2% (1/4 inch per foot), surface water is naturally directed away from the building. This works well on lots with adequate overall slope and permeable soil.

Cost: $1,000 to $5,000 for grading work, often included in the general excavation and site preparation.

2. French Drains

A French drain is a trench filled with gravel and a perforated pipe that collects and redirects subsurface water. French drains are the most common drainage solution for ADU projects where surface grading alone is insufficient.

French drains are typically installed around the perimeter of the ADU foundation and along areas where water tends to collect. The perforated pipe at the bottom of the trench collects water that seeps into the gravel and directs it to a discharge point.

Cost: $25 to $50 per linear foot installed. A typical ADU perimeter drain of 80 to 120 linear feet costs $2,000 to $6,000.

3. Area Drains and Catch Basins

Area drains are surface-level drain grates installed in low spots or at the base of slopes to collect surface water. The water is directed through underground pipes to a discharge point. Catch basins are larger versions with a sump that traps sediment before water enters the drain line.

Cost: $200 to $500 per area drain installed, plus $10 to $20 per linear foot of connecting drain pipe.

4. Dry Wells (Infiltration Systems)

A dry well is an underground pit filled with gravel or a prefabricated chamber that collects stormwater and allows it to slowly infiltrate into the surrounding soil. Dry wells are particularly useful in areas where connecting to a storm drain is not feasible or where local ordinances require on-site stormwater management.

Cost: $1,000 to $3,000 per dry well, depending on size and soil conditions.

5. Rain Gardens and Bioswales

Rain gardens are shallow, planted depressions that collect and filter stormwater runoff. Bioswales are linear, vegetated channels that convey stormwater while allowing infiltration. Both are increasingly required or incentivized by California's stormwater regulations and can satisfy Low Impact Development (LID) requirements.

Cost: $1,500 to $5,000 for a typical residential rain garden or bioswale.

California Stormwater Regulations for ADU Projects

California has some of the most stringent stormwater regulations in the country. The specific requirements that apply to your ADU project depend on your jurisdiction and the size of the project.

Low Impact Development (LID) Requirements

Many California jurisdictions require new development projects, including ADUs, to incorporate Low Impact Development strategies. LID aims to manage stormwater at the source by mimicking natural hydrology. Common LID requirements include:

• On-site retention: Capturing and infiltrating a specified volume of stormwater on the property rather than discharging it to the street or storm drain.
• Permeable surfaces: Using permeable paving materials for driveways, walkways, and patios to allow water to infiltrate through the surface.
• Bioretention: Incorporating rain gardens or bioswales to filter and infiltrate stormwater runoff.
• Minimal impervious coverage: Limiting the total area of impervious surfaces (roofs, concrete, asphalt) on the property.

In the City of Los Angeles, projects that create or replace 500 square feet or more of impervious surface area are subject to LID requirements under the Bureau of Sanitation's Standard Urban Stormwater Mitigation Plan (SUSMP) and LID ordinance. Since most ADU projects add at least 500 square feet of impervious area (the building footprint plus any new paving), LID compliance is typically required.

Drainage and Grading in the Permit Process

Your ADU permit application will likely need to include the following drainage-related documents:

1. Grading plan: Showing existing and proposed grades, cut and fill quantities, and drainage patterns.
2. Drainage plan: Showing the location and size of all drainage features, pipe routing, and discharge points.
3. LID compliance documentation: If required, showing how the project meets local stormwater management requirements.
4. Soils and geotechnical report: Including soil permeability data that influences drainage design.
5. Erosion control plan: For sites with significant grading, showing how soil erosion will be prevented during and after construction.

Plan checkers pay close attention to drainage because water-related problems are among the most common and costly issues in residential construction. Expect questions and possibly correction requests related to your drainage plan, especially if your site has challenging conditions.

Common Drainage Challenges on ADU Sites

Flat Lots with No Natural Drainage

Properties with minimal slope present a drainage challenge because there is no natural direction for water to flow. Solutions include creating artificial grades (building up soil around the ADU to create slope), installing subsurface drain lines with mechanical slope, and using sump pumps in extreme cases.

Hillside Properties

Hillside properties have the opposite problem: too much water flowing too fast across the site. Retaining walls, terrace drains, and interceptor drains above the ADU may be needed to manage hillside runoff. Hillside drainage engineering is significantly more complex and expensive than flat-lot drainage.

Properties Between Neighboring Structures

On narrow lots where the ADU is close to both the main house and the property line, there may be limited space for surface drainage. In these cases, subsurface drainage systems (French drains, drain pipes) are essential, and the design must ensure that you are not directing water onto the neighbor's property.

Drainage and Grading Cost Summary

Plan Drainage Early, Save Money Later

Drainage and grading should be addressed in the earliest stages of your ADU design process, not as an afterthought during construction. Addressing drainage proactively during design costs a fraction of what it costs to fix drainage problems after the ADU is built.

Work with a civil engineer or experienced ADU contractor who understands local drainage requirements and can design a system that protects your investment for decades. The cost of proper drainage is modest compared to the cost of foundation repairs, water damage, or mold remediation caused by inadequate drainage.

Your ADU is a long-term investment in your property. Make sure it is built on a solid, dry foundation with a drainage system that will perform reliably for the life of the structure.

Common Drainage Problems and Solutions for ADU Projects

Problem: Standing Water Near the ADU Foundation

Standing water around the foundation is one of the most damaging drainage issues for any structure. Water that pools against the foundation can seep into the slab, cause soil expansion and contraction, and eventually lead to structural cracking. The solution is to establish positive drainage away from the foundation with a minimum slope of 2 percent (approximately 1/4 inch per foot) for the first 6 feet around the ADU. Install a French drain system along the foundation perimeter if natural grading alone cannot achieve adequate drainage.

Problem: ADU Construction Disrupts Existing Yard Drainage

Building an ADU changes the topography of your property, which can redirect water flow patterns that have existed for years. Water that previously drained to the street or a backyard swale may now pool against the main house, a fence, or a neighbor's property. Before construction begins, your civil engineer should prepare a grading plan that accounts for the new structure and maintains proper drainage for the entire lot. This is a common item flagged during plan check review.

Problem: Downspout Discharge Creates Erosion

Roof downspouts that discharge directly onto unpaved ground near the ADU can cause soil erosion, landscape damage, and localized flooding. Extend downspouts at least 4 feet from the foundation using splash blocks or underground drain pipes connected to a catch basin or the municipal storm drain system. For properties where the ADU shares a roof with the main house, coordinate the combined roof drainage to prevent overwhelming any single discharge point.

Problem: Clay Soil Causing Poor Percolation

Many California properties have clay-heavy soils that absorb water very slowly. This creates surface ponding even with proper grading. Solutions include installing area drains connected to a subsurface drainage system, using gravel trenches or dry wells to collect and slowly dissipate water, and incorporating permeable hardscape materials for walkways and patios around the ADU. A soils report completed early in the design phase will identify these conditions and inform the appropriate drainage strategy.

Seasonal Considerations for ADU Drainage in California

Wet Season Preparation (October through April)

California's rainy season can deliver significant precipitation over short periods, especially in Southern California. Before the wet season, inspect and clean all drainage infrastructure around your ADU, including gutters, downspout extensions, French drains, and catch basins. Verify that grading has not been disturbed by settling, landscaping, or foot traffic. Test your drainage system by running a hose at high volume to simulate heavy rainfall and observe where water accumulates.

Dry Season Considerations (May through September)

During California's long dry season, clay soils can shrink and crack, creating pathways for future water infiltration. Maintaining consistent moisture levels around the ADU foundation through light, regular irrigation prevents extreme soil movement. This is particularly important for slab-on-grade foundations common in California ADU construction.

Post-Construction Settlement Period

Newly graded soil around a completed ADU will settle over the first 6 to 12 months. This settlement can change drainage patterns and create low spots where water pools. Plan to re-evaluate and touch up grading after the first rainy season. Contractors who include a post-construction grading adjustment in their scope of work demonstrate an understanding of this normal settlement process. Discuss this during your contractor evaluation.

Frequently Asked Questions About ADU Drainage and Grading

Do I need a separate drainage plan for my ADU?

Most California jurisdictions require a grading and drainage plan as part of the ADU permit application, especially for detached units. This plan shows existing and proposed contours, drainage flow arrows, and the location of any drainage structures. For properties on slopes or in flood zones, additional engineering may be required. Your architect or civil engineer typically prepares this plan as part of the overall permit package.

How much does drainage work add to the ADU project cost?

Basic grading and drainage for a straightforward ADU project typically costs $2,000 to $5,000. More complex situations involving French drain systems, retaining walls, or connections to municipal storm drains can increase this to $8,000 to $15,000. Properties with significant slope or poor soil conditions may require even more extensive drainage infrastructure. Including drainage in your initial budget prevents unwelcome surprises during the contractor bidding process.

Can poor drainage void my ADU warranty?

Yes, many contractor warranties exclude damage caused by improper drainage or grading maintenance. If you modify the grading around your ADU after construction is complete, such as adding landscaping, raised beds, or hardscape, you may inadvertently void warranty coverage for foundation and moisture-related issues. Always consult your contractor before making changes that could affect drainage patterns.

What drainage requirements apply specifically to California ADUs?

California Building Code requires positive drainage away from all foundations and prohibits directing water onto neighboring properties. Many local jurisdictions have additional requirements for stormwater management, including Low Impact Development (LID) standards that mandate on-site water retention or infiltration. Green infrastructure elements like rain gardens, bioswales, and permeable pavement can satisfy these requirements while enhancing the appearance of your ADU landscape.

Should I install a sump pump for my ADU?

Sump pumps are generally not necessary for slab-on-grade ADU construction in California's climate. However, if your ADU includes a below-grade component such as a recessed entryway, below-grade utility room, or is built on a sloped lot where the downhill side has below-grade walls, a sump pump may be required by code. Your structural engineer and the plan check reviewer will identify whether a sump pump is needed based on your specific site conditions.