California State Laws Governing Perc Testing & Septic System Design
When building homes or other structures where public sewer service is not available, California law requires that onsite wastewater treatment systems (OWTS) — commonly called septic systems — be properly sited, evaluated, designed, permitted, and constructed to protect public health and groundwater quality. That obligation exists at the state level first, and then local agencies (counties or cities) implement those requirements through permits and codes.
This page summarizes what the state requires of every county before a septic system can be permitted, including perc testing requirements, design obligations, and the legal frameworks that govern them.
1. State Law Authority: California Water Code Section 13291
At the foundation of California’s septic law is Water Code §13291, which directs the State to adopt regulations or standards for permitting and operation of onsite sewage treatment systems (OWTS). These standards apply to:
Any septic system constructed or replaced
Any system subject to a major repair
Systems that discharge or pool wastewater on the surface
Systems that could affect water quality or cause a public nuisance
The statute also requires that these statewide regulations include minimum criteria for:
Siting
Construction
Performance
Monitoring
Corrective actions
Exemptions for certain types of systems
Importantly, Water Code §13291 does not prevent local governments or regional boards from adopting more protective standards than the statewide minimums.
2. The OWTS Policy: Statewide Standards for Septic Systems
To comply with Water Code §13290 et seq., the California State Water Resources Control Board adopted the Water Quality Control Policy for Siting, Design, Operation and Maintenance of Onsite Wastewater Treatment Systems (OWTS Policy).
What the OWTS Policy Does — Statewide Requirements
The OWTS Policy establishes the minimum statewide performance and protection standards that apply even before county or city rules are applied. Key points required under this policy include:
A. Risk-Based, Tiered Regulation
The policy sets a tiered approach based on system size, potential environmental risk, and proximity to sensitive waterbodies.
B. Minimum Requirements for All Septic Systems
At the state level, all septic systems, whether conventional or advanced, must include:
Site evaluation — including soil characteristics and groundwater depth
Proper siting — setbacks from wells, watercourses, and property boundaries
Design standards — appropriate system sizing based on flow and soil conditions
Performance expectations — system must not threaten water quality or public health
Operation & monitoring guidance — long-term oversight if required
C. Permit and Reporting Authority
Local agencies that issue septic permits must incorporate these standards or standards that are more protective into local permitting programs once approved by the relevant Regional Water Quality Control Board.
In areas where a local agency does not have an approved local program (LAMP), the Regional Board issues permits directly based on the OWTS Policy.
3. Perc Testing: State Expectations and Local Implementation
Statewide Requirements for Soil and Perc Testing
While California does not prescribe a single statewide perc test method (e.g., a single procedural standard like a universal number of holes or specific technique), under the OWTS Policy every site being considered for a septic system must have site evaluation that accounts for soil absorption characteristics including:
Soil permeability and texture
Presence and depth of groundwater
Soil horizons that affect infiltration performance
This evaluation typically includes a percolation test or other soil analysis that determines the soil’s ability to absorb effluent, which in turn dictates:
System type (conventional vs advanced)
Disposal field size
Soil dispersal design
Groundwater separation requirements
State policy expects that perc or similar soil test data be collected by a qualified professional and documented as part of the permit application.
Qualification of Professionals (State Basis)
Under the OWTS Policy, site evaluations and designs must be conducted by qualified practitioners. Although each county ultimately determines who qualifies (e.g., Civil Engineer, Environmental Health Specialist, Soil Scientist, Geologist), the state policy supports local authority to define professional qualifications consistent with statewide health and water quality goals.
4. What Every County Must Do Under State Law
A. Issue Permits Consistent with State Policy
Every county must ensure that septic permits:
Incorporate the OWTS Policy’s minimum standards
Include site and soil evaluations (including perc or equivalent)
Protect groundwater and surface water
Require adequate separation from limiting features (rocks, clay layers, high water table)
Require system construction and design based on documented soil conditions
Where local agencies have an approved Local Agency Management Plan (LAMP), they implement state policy through their permitting programs. Counties without a LAMP rely more directly on Regional Board oversight.
B. Enforce Professional Involvement
At a minimum, all counties must ensure septic evaluations and designs submitted for permitting are prepared and stamped by a licensed or certified professional consistent with state and local requirements. No county program can waive professional competency simply because a homeowner wants to self-manage a permit.
C. Provide Public Health & Water Quality Protection
Counties are required to enforce state mandates that septic systems:
Do not discharge to the surface
Do not cause groundwater degradation
Do not create a public nuisance
Meet setback and soil and site criteria
Follow performance standards for installation and operation
5. Summary: What California Requires Statewide (No Matter the County)
California law sets a baseline of requirements for septic systems that every county must follow:
✔ Authority: Water Code §13291 empowers the State to set minimum standards.
✔ Statewide Policy: The OWTS Policy establishes minimum siting, design, operation, and monitoring standards.
✔ Perc/Soil Evaluation: Every site must be evaluated for soil absorption and groundwater conditions as part of the permit process.
✔ Qualified Professionals: State policy mandates qualified evaluation and design.
✔ Permit Compliance: New, replacement, or major repaired systems must meet state standards regardless of local variations.
6. Next Steps for Builders and Designers
👉 Before any perc test or septic plan submission:
Confirm whether the county has an approved LAMP or relies on the Regional Board for OWTS permitting.
Ensure perc tests and soil reports are prepared by a qualified professional.
Check that design complies with both state OWTS Policy standards and local county code.
Include all required site data, separation distances, soil logs, groundwater data, and anticipated effluent flow in your permit application.
