A look into Sonoma County ADU septic requirements

Oct 16, 2024

Building an Accessory Dwelling Unit (ADU) in Sonoma County can be a great way to add living space, generate rental income, or accommodate family members. However, one crucial aspect to consider before proceeding with the construction is the existing septic system. Since many properties in Sonoma County rely on septic systems rather than municipal sewer services, it's essential to ensure that your current system can support the additional wastewater load from the new ADU. Below are the primary considerations regarding septic systems and the limitations you may encounter when building an ADU.

1. Understanding Your Existing Septic System

a. Types of Septic Systems

First, familiarize yourself with the type of septic system you have. Sonoma County primarily uses two types of onsite wastewater treatment systems (OWTS):

1. Standard OWTS: This includes a septic tank for primary sewage treatment and a drainfield for effluent dispersal into the soil. The tank typically operates on gravity flow or a pump system to move effluent from the tank to the drainfield.

2. Non-Standard OWTS: These systems utilize alternative methods for wastewater treatment and dispersal. Examples include mound systems, shallow trench systems, and subsurface drip systems. Non-standard systems can offer better performance in challenging soil conditions but may require more maintenance​.

In order to add an ADU to an existing septic system, the system must be labeled as Code Compliant on a findings report or Class I if a findings report was done prior to August 31, 2018. This was when the PRMD adopted the new OWTS Manual replacing Class I, II and III classifications with either the classification of Code Compliant or a Legal Non-Conforming systems.

b. Capacity Assessment

The first thing to check is the capacity of your existing septic tank. The capacity is often determined by the number of bedrooms in your main dwelling, which directly correlates to wastewater generation. Generally, the septic system must have enough capacity to accommodate both the primary residence and the proposed ADU. Septic capacity per bedroom amount can be seen in the 2022 California Plumbing per Table H 201.1(1) and in Onsite Wastewater Treatment System (OWTS) Manual version 8.0 Table 8.2.

To determine if your septic system can handle the additional load from an ADU, you may need to consult the OWTS Manual, which outlines the regulations governing septic systems in Sonoma County. You can obtain this manual from Permit Sonoma to review the requirements specific to your situation​

OWTS Manual Version 8.0

2. Inspecting Your Existing Septic System

a. Physical Inspection

Conduct a thorough inspection of your existing septic tank and components. Look for signs of wear, cracks, or leaks that could compromise the integrity of the system. Common indicators of a failing septic system include:

• Sewage Backup: If you experience frequent backups in your home, it may signal that your septic system is overloaded or failing.

• Pooling Water: Any standing water near the drainfield or tank can indicate a problem with the system.

• Unpleasant Odors: Foul smells around the septic area can be a sign of a leak or malfunction.​

*Note: This is no replacement for the septic findings report that is required for an ADU addition. These reports are to be done by a Licensed Professional Engineer or Registered Health Specialist.

List of qualified septic consultant in Sonoma County

b. Pumping Records and Maintenance History

Review any past pumping records and maintenance history. Regular pumping is crucial for preventing solids from accumulating in the tank and clogging the drainfield. Generally, septic tanks should be pumped every three to five years, but the frequency may vary based on usage, system design, and the number of occupants​.

If your system has not been maintained adequately, it may require immediate servicing before you can apply for an ADU permit. In some cases, the county may mandate that your septic system be pumped or repaired before issuing construction approvals​

3. Local Regulations and Permitting

a. Permitting Requirements for ADUs

Before you can start construction on your ADU, you must obtain the necessary permits from Sonoma County. The permitting process can vary based on your property’s specific characteristics and existing infrastructure. The county has established guidelines that dictate the size and scope of ADUs, including zoning regulations and setback requirements​

In addition to general building permits, your septic system will likely require a separate evaluation and approval. This process typically involves submitting plans for the ADU and proving that your septic system can accommodate the additional waste generated by the new unit​

b. Environmental Health Considerations

Sonoma County prioritizes public health and environmental protection. Therefore, you may be required to conduct a comprehensive site evaluation to assess soil conditions, groundwater levels, and proximity to water sources. The OWTS Manual outlines specific criteria that your septic system must meet to ensure compliance with local health and safety standards​

This evaluation may include:

• Soil Percolation Tests: These tests assess the soil's ability to absorb effluent. Poor soil conditions can limit the effectiveness of your septic system, which may necessitate a non-standard OWTS.

• Setback Requirements: Ensure your septic system meets minimum distance requirements from property lines, wells, and other structures. In Sonoma County, specific setbacks are outlined in the OWTS Manual​

4. Limitations and Challenges

a. Inadequate Capacity

One of the most significant limitations when adding an ADU is the possibility that your existing septic system lacks the capacity to handle additional wastewater. If this is the case, you may have a few options:

• Upgrade the Septic System: This option may involve significant costs and can be a lengthy process. Upgrading may include replacing the septic tank with a larger one or expanding the drainfield.

• Bedroom Swap: If upgrading is not feasible, inquire about a "bedroom swap" with your local planning department. This option allows you to convert an existing bedroom in your main residence to non-bedroom use, thus freeing up capacity for the ADU​

b. Soil Conditions

The soil conditions on your property can greatly affect your septic system's performance and the feasibility of adding an ADU. Poor soil permeability can lead to inadequate wastewater treatment, increasing the risk of system failure and public health issues. If the soil is not suitable for traditional septic systems, you may need to explore alternative systems, which can be more complex and costly to install​

c. Existing system is not usable

In cases where the primary septic system can not handle another room, a new septic tank and leach field can be designed for the ADU, provided that the existing leach field and reserve area are documented and there is adequate space for the new system, along with its own reserve area, without violating setback or property boundary requirements. Proper planning ensures that the systems remain independent and functional without compromising environmental health or local regulations.

Alternatively, in situations where space is limited or soil conditions are poor, an Aerobic Treatment Unit (ATU) can be integrated into the existing system. An ATU is more compact, produces cleaner effluent, and offers better performance in challenging soil compared to installing an additional septic tank and leach field for the ADU. However, ATUs tend to be more expensive than standard systems.

5. Steps for Moving Forward

a. Consult Professionals

Given the complexities involved, it is advisable to consult with professionals experienced in septic systems and ADU construction. This may include licensed septic contractors, civil engineers, and architects who can help you navigate the requirements and ensure compliance with local regulations​

• Septic System Evaluator: A professional can conduct a thorough assessment of your existing system, helping you understand its capacity and condition.

  List of qualified septic consultant in Sonoma County

• Architect or Designer: They can assist you in designing the ADU to fit your property while complying with local zoning regulations.

  BP Design Associates possesses extensive expertise in ADU regulations, particularly those involving well and septic systems, ensuring your project complies with all local requirements and avoids potential setbacks.

  Contact us

b. Engage with Permit Sonoma

Engaging early with Permit Sonoma is crucial. They can provide information about local regulations, the permitting process, and any potential challenges you may face. Early communication helps streamline the process and avoids delays when submitting your application​

Conclusion

Building an ADU in Sonoma County can significantly enhance your property's value and utility, but it requires careful planning, particularly regarding your existing septic system. Thoroughly inspecting the system, understanding its capacity, and navigating local regulations are critical steps in the process. By addressing these factors upfront, you can avoid costly setbacks and ensure a smoother path to realizing your ADU project.

For more detailed guidance and the most current information regarding septic systems and ADU regulations in Sonoma County, consult the Sonoma County Permit Sonoma website or contact the Well and Septic Division directly.