Adding an ADU on a Septic Lot in LA County

Adding an accessory dwelling unit (ADU) or junior ADU to a house on septic is possible, but the septic answer has to come first. In unincorporated LA County, the County Department of Public Health, Environmental Health, treats an ADU or JADU as additional bedrooms on the system. That changes what the existing system has to handle.

Step one: can the existing system carry it?

Under the County's updated ADU and JADU requirements (June 2025), an existing conventional system can be used without an upgrade if the existing tank and dispersal system are adequately sized for the added bedrooms. If the system has been in use for more than 15 years, the County requires a system evaluation of the current system.

The County also requires a 100% future expansion area for the dispersal system to be identified and tested, if it was not tested before or if the ADU falls within a previously approved expansion area or its setbacks.

If the existing system can't carry the ADU

• Near an impaired water body: the County requires a non-conventional onsite wastewater treatment system (an advanced treatment system), regardless of percolation results.
• Not near an impaired water body: the existing system may be upgraded to serve both the house and the ADU, and a complete feasibility study is required for the expansion of the dispersal system.
• If the existing system can't be upgraded: a new, separate system has to be installed.

In the last two cases, whether the system can be conventional or must be advanced depends on the site's percolation rate.

Why it pays to start with the septic

The septic evaluation can decide where the ADU can go, how big it can be, and whether the budget needs an advanced treatment system. Doing it before the ADU design is finished means the septic answer shapes the plans, not the other way around.

Where we fit

We locate and expose the existing tank and pits, inspect the system, run hydraulic tests on seepage pits, and do the field work the evaluation and feasibility study need. The system design itself comes from a qualified designer, and we build to the approved design. On many projects we also handle the site work for the ADU: excavation, foundations, utilities and drainage.

Inside a city

These are the County's requirements for unincorporated areas. Cities such as Malibu run their own septic programs, so the first step is confirming which agency reviews your address. We check that before we price anything.