How close can an ADU be to a septic tank or leach field in California?

There's no single statewide number. Your county's onsite wastewater rules set the setbacks, and a common pattern is about 5 feet from a building to a septic tank and 8 feet to a leach field. Setbacks to wells, streams, and property lines are larger, and you also have to keep the septic reserve (replacement) area clear of the ADU.

One county's numbers as an example

El Dorado County publishes these minimum setbacks:

• Septic tank: 5 feet from any building, 5 feet from property lines, 5 feet from a swimming pool, 50 feet from streams, springs, lakes, or reservoirs, and 100 feet from a domestic well or spring.
• Leach lines: 8 feet from buildings, 8 feet from property lines, 10 feet from a swimming pool, 50 feet from a seasonal stream or swale, 100 feet from a flowing stream, and 100 feet from a well.
• Slopes: leach lines need extra distance from cut or fill slopes; El Dorado uses four times the height of the cut or fill.
• Surfaces: leach lines can't go under asphalt, concrete, or areas used by vehicles, or in fill material.

Other counties use different tables, so treat these as an example, not your answer.

The setback people forget: the reserve area

Most counties require land set aside for a future replacement leach field. San Diego County requires a 100 percent reserve area to be delineated, and Los Angeles County requires a 100 percent future expansion area to be identified and tested. Los Angeles County also requires testing when an ADU is proposed within a previously approved future expansion area or its setbacks. In practice, the ADU footprint, its driveway, and new patios have to stay out of both the active field and the reserve area.

Other things that eat into the space

• The building sewer from the ADU to the tank, which needs a sloped route and cleanouts.
• Water service lines and any well, which have their own separation rules.
• Fire access, grading, and stormwater features that environmental health and the fire authority also review.

How to find your real numbers

1. Pull your county's OWTS or LAMP setback table from the environmental health website.
2. Get the as-built drawing of your existing system, or have an OWTS professional locate the tank and lines.
3. Plot the tank, field, reserve area, wells, and property lines on a scaled site plan before you place the ADU.

What it means for your plumbing plan

Your site plan and plumbing sheets need to dimension the ADU's distance from the tank, leach field, reserve area, and any well, and show the building sewer route between them. San Diego County's planning department rejects building permit site plans that lack well and septic locations, so this is not optional detail. See California ADU requirements, our plumbing plans, and the ADU plumbing guide.

Sources: eldoradocounty.ca.gov, sandiegocounty.gov, admin.publichealth.lacounty.gov.