Civil Engineering & Site Design
Septic System Design
A septic layout sized to your soils and ready for county review, so the house, well, and leach field fit the lot.
What is a septic system design?
A septic system design is a site-specific plan for an onsite wastewater treatment system (OWTS) that sizes and locates the septic tank and the leach field or other dispersal system from soil tests, expected wastewater flow, and local setback rules.
Often shortened to septic design, it is what the local permitting agency, usually county environmental health, reviews before issuing a septic permit. Percolation (perc) tests show how fast the soil absorbs effluent, and bedrooms or building use set the design flow.
A conventional system pairs a septic tank with a gravity leach field; poorer soils may need an engineered alternative. The layout is planned with the house, well, and driveway in the site layout.
Deliverables
What's included in a septic system design
Scope is set in the proposal. A design package typically covers:
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Soil & site evaluation
Soil profiles logged for depth, texture, restrictive layers, and signs of seasonal groundwater.
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Perc test results
Perc tests run to the county's method and count, with stabilized rates tabulated.
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Sizing calculations
Design flow from bedrooms or use, tank capacity, and dispersal area or trench length.
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Scaled site plan
Tank, dispersal and replacement areas, wells, streams and other surface water, structures, and property lines, all dimensioned.
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Replacement area
A separate, equivalent reserve area kept free for a future dispersal field.
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Construction details
Tank, riser, distribution, and trench cross-sections with the elevations the installer builds from.
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Alternative system specs
Treatment unit, pump, controls, and monitoring details when a conventional system won't work.
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Permit package
Signed plans and report for environmental health, plus revisions answering review comments.
When do you need a septic system design?
Start before a house plan or land purchase is final.
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New home without public sewer
Soils, slope, and setbacks decide the system type and its footprint.
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Adding an ADU or bedrooms
More bedrooms or a second unit add flow the system must handle.
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Replacing a failing system
Surfacing effluent or backups call for a permitted repair to current local standards.
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Dividing land served by septic
Some counties require soil profiles and perc tests on each proposed lot.
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Buying rural land
Testing before closing shows whether a conventional system, an alternative, or neither fits. See feasibility studies.
Where public sewer is available, a local septic program (LAMP) cannot approve a new septic system. Sewer may count as unavailable when it, or a building or drainage facility connected to it, is over 200 feet from the proposed building on a lot that abuts and is served by that sewer (OWTS Policy §9.4.9). A sewer main extension goes on improvement plans.
Process
How a septic design gets approved
Six steps from parcel check to installation.
Confirm the local rules
We identify the permitting agency and its rules, check for nearby impaired waters, and check whether public sewer is available.
Map the site
Property lines, slopes, wells, drainage, and structures are mapped, often from a topographic survey and boundary survey, so setbacks can be dimensioned.
Test the soil
Soil profiles are dug and perc tests run where and how the county specifies; some sites need wet-season groundwater checks.
Size the system
Design flow sizes the tank and dispersal field; both stay clear of the cuts and fills shown on the grading plans.
Submit for approval
Signed plans and the report go to the local permitting agency, usually environmental health, and we respond to its review comments.
Support installation
We answer installer questions and provide any designer inspection or sign-off the county requires.
California rules
California requirements for septic systems
State policy sets the framework; local agencies, mostly counties, apply it. As of September 2026:
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Water Code §§13290–13291.7 · OWTS Policy §§2.6, 9
One statewide policy, permitted locally
Septic systems fall under the State Water Board's Onsite Wastewater Treatment Systems (OWTS) Policy, last amended April 18, 2023. Local agencies permit most systems, usually under a Local Agency Management Program (LAMP); where none is approved, statewide Tier 1 standards apply. Systems that meet neither, or exceed 10,000 gallons per day, go to the Regional Water Board.
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OWTS Policy §8.1.1 · Bus. & Prof. Code §6731
Who may design a septic system
Under Tier 1, a qualified professional must design new systems and any replacement or expansion of the treatment or dispersal system. Each local agency's LAMP may change who qualifies. Civil engineering expressly includes sewerage (§6731).
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OWTS Policy §§7.1–7.4
Soil and percolation testing
Design starts with soil profiles in the dispersal and replacement areas, an estimate of the highest seasonal groundwater, and perc tests. Under Tier 1, rates must fall between 1 and 120 minutes per inch after presoaking. LAMPs set their own methods and number of tests.
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OWTS Policy §§7.7, 8.1.8, 8.1.9
Slope, replacement area, and cover
Under Tier 1, dispersal areas need natural slopes of 25 percent or less and a separate, equivalent 100 percent replacement area. Neither may be covered by paving, slabs, plastic sheeting, or anything else that blocks oxygen from the soil.
Requirements vary by county and city. Systems near listed impaired waters (Tier 3, generally within 600 feet) may need supplemental treatment before dispersal.
Sources & references
- Onsite Wastewater Treatment Systems (OWTS) Policy, as amended April 18, 2023 (State Water Resources Control Board)
- Onsite Wastewater Treatment Systems program page (State Water Resources Control Board)
- Accessory Dwelling Unit Permitting Guidelines, Onsite Wastewater Treatment (State Water Resources Control Board)
- Government Code §66314, accessory dwelling unit ordinances (California Legislative Information)
- Business and Professions Code §6731, definition of civil engineering (California Legislative Information)
Last reviewed .
Septic system design FAQs
Does a septic system design include the perc test and installation?
A septic system design typically includes soil profiles, perc tests, sizing calculations and signed plans, but not installation. Under the statewide Tier 1 standards, a licensed Class A, Class B, C-42 (sanitation) or C-36 (plumbing) contractor installs the system, or the owner may if the agency inspects and approves it before it is covered. Local programs (LAMPs) may set other installer rules.
Who can design a septic system in California?
The professionals who may design a septic system in California depend on the county. The State Water Board's OWTS Policy says qualified professionals may include registered environmental health specialists and licensed professional engineers or geologists, as their licenses allow, plus certified soil scientists for site evaluations. Each local agency may change that list in its Local Agency Management Program (LAMP).
What happens if my land fails a perc test?
Land that fails a perc test for a conventional leach field may still qualify for another system, such as pressure distribution, a mound, drip dispersal, supplemental treatment, or a seepage pit, where the county allows it. Under Tier 1, rates outside 1 to 120 minutes per inch are allowed only under an approved local program (LAMP) or through the Regional Water Board. Some sites cannot support any permitted system.
Can I add an ADU or a bedroom to a house on septic?
Adding an accessory dwelling unit (ADU) or a bedroom to a house on septic depends on whether the tank and dispersal field can take the added flow. An ADU on septic needs the local health officer's approval where required (Gov. Code §66314(d)(9)), and that review applies the OWTS Policy. Because systems are often sized by bedrooms, an added bedroom can trigger a similar review.
How far must a septic system be from a well or property line?
Septic setbacks are set by your local agency. Under the statewide Tier 1 standards, tanks and dispersal fields must be at least 5 feet from property lines and structures; 100 feet from wells, springs and flowing streams; 200 feet from lakes, ponds and wetlands; and 150 feet from a public water well when the dispersal system is 10 feet deep or less.
What affects the cost and schedule of a septic system design?
The cost and schedule of a septic system design depend on local testing and review rules, soil and groundwater conditions, slope and usable lot area, design flow, and whether supplemental treatment or wet-season testing is needed. Share the parcel number, planned bedrooms or use, and any existing system records; the proposal then sets scope and timing.