Can I build an ADU on a property with a septic system in California?

Usually yes, but your county environmental health department, or the local agency that runs the onsite wastewater program, has to approve the wastewater plan before your building permit can move forward. That agency decides whether your existing septic system can carry the ADU or whether you need an upgraded or new system, based on soil testing, bedroom count, and available space.

Who reviews it

Septic systems are formally called onsite wastewater treatment systems (OWTS). The State Water Resources Control Board sets a statewide OWTS Policy, and local agencies carry it out through their own approved programs, often called a Local Agency Management Program (LAMP). Your county's program sets the sizing, setback, and soil rules your project is measured against.

What the process usually looks like

1. Find your existing system's records. Many counties keep as-built drawings. San Diego County, for example, points owners to its online document library.
2. Hire an OWTS professional to evaluate the tank, the leach field, and the site.
3. Test the soil. Percolation tests and borings show how fast effluent soaks in and whether there is enough soil above groundwater or rock. State ADU law lets a local agency require a percolation test completed within the last five years, or within the last ten years if it has been recertified (Gov. Code section 66323(f)).
4. Get the wastewater approval first. San Diego County advises getting its OWTS layout approval before you apply for a building permit, and its planning department rejects building site plans that don't show well and septic locations.
5. Protect the reserve area. Counties commonly require space set aside for a future replacement field. San Diego County and Los Angeles County both call for a 100 percent reserve or future expansion area.

If public sewer is nearby

The plumbing code requires buildings with plumbing to connect to a public or private sewer, and a private system is used where no public sewer is available in an adjoining street or right-of-way (CPC 713.1 and 713.2). Under Section 713.4, a public sewer may be treated as not available when it is more than 200 feet from the proposed building, and local agencies may set their own rules. If a sewer main runs in your street, ask whether you must connect before you design around septic.

If you do connect, state law limits connection fees and capacity charges. An ADU created within existing space generally isn't treated as a new residential use for those charges, and any charge for other ADUs must be proportionate to the unit's burden, based on square footage or drainage fixture units (Gov. Code section 66311.5). See ADU impact fees for more.

What it means for your plumbing plan

Your plans should show the septic tank, leach field, reserve area, and any wells on the site plan, the building sewer route from the ADU to the tank, and fixture and bedroom counts that match what environmental health approved. Mismatches between the plumbing sheets and the approved septic layout are an easy way to stall plan check. Read what utilities an ADU needs and the California ADU permit guide for the bigger picture.

Sources: waterboards.ca.gov, sandiegocounty.gov, admin.publichealth.lacounty.gov, california.public.law, gocodebook.com.