ADU Sewer Connections and Septic in Los Angeles.

Every ADU has a bathroom and a kitchen, so its wastewater has to go somewhere. Most LA homes are on the public sewer. Some, mostly in hillside and rural parts of the County, are on a septic system, which the regulators call an onsite wastewater treatment system (OWTS). This guide covers both.

LA ADU Guide is an information site. We are not a plumber, engineer or health department. Sewer and septic questions are lot-specific, so the answers come from the agencies below.

Which one do you have?

Check, before you design:

• Look at your utility bill for a sewer charge.
• Ask the sewer agency, or LA County Public Works, whether your street has a public main.
• Look for a record of a septic permit in your property file.

The answer decides which agencies you deal with.

If you are on the public sewer

What state law says about connections

Gov. Code section 66311.5 (formerly section 66324, renumbered by SB 543 effective January 1, 2026, according to a law firm summary) splits ADUs into two groups.

Under 66311.5(e), the fee is based on square footage or drainage fixture unit (DFU) values. Section 66311.5(b) says a utility may not treat an ADU as a new residential use for connection fees or capacity charges, unless it was built with a new single-family home.

In plain terms, a garage conversion should not trigger a new sewer connection fee. A new detached ADU may.

Does a new ADU need its own sewer lateral?

Not necessarily. The statute says a utility "may require" a separate connection for new units. Whether you need a new line from the ADU to the main, or can tie into your house's existing line, depends on the pipe size and slope, the distance, and what the plumbing code and the sewer agency allow. We could not find a single statewide rule, so your plumber or designer should check this against the plumbing code and the utility.

Costs here can be large because trenching, tie-ins and street work are involved. We do not have a sourced figure for Los Angeles, so we leave it out. Ask for the work to be a separate line in each bid.

Who to call

• City of Los Angeles: building permits come from LADBS, and sewer charges and connection permits from the city's sanitation bureau. Water is the Department of Water and Power.
• Unincorporated County: County departments and special districts.
• Other cities: each has its own sewer agency.

See impact fees and utility connections for the fee rules in full.

If you are on septic (OWTS)

In unincorporated LA County, LA County Public Health states that plan check for a septic system or other OWTS is "a requirement for obtaining a Building Permit." Its requirements document, last updated June 2025 per our search, includes an ADU and JADU section. Key points:

• Capacity. The existing system must handle both the house and the ADU. If it cannot, "either the existing OWTS must be upgraded or a new OWTS for the ADU/JADU must be installed."
• Bedroom equivalents. The County counts ADUs and guest houses as bedroom equivalents for sizing.
• Cesspools. The County says owners "may not increase potential wastewater flow to an existing cesspool," and that includes adding an ADU. Reconstruction needs prior Public Health approval.
• Sewer first. The County code says the director "shall not issue a permit for a new onsite wastewater treatment system" if a public sewer is available within 200 feet of the system, subject to parcel-size exceptions in that code.
• Impaired water bodies. Near an impaired water body, a non-conventional system (NOWTS) is required "regardless of the percolation result."
• Recent update. Public Health said it is updating ADU and extra-bedroom requirements after the Palisades and Eaton fires, with the Regional Water Quality Control Board. Check its page for the current version.

We could not open the County's separate ADU requirements PDF, so confirm these points with the Onsite Wastewater Treatment Program.

City of Los Angeles

LADBS has an OWTS bulletin (P/BC 2023-027). A search summary says the City allows ADUs to share the same OWTS as the main home. We could not open the bulletin, so read it directly and ask LADBS about your lot.

Sewer and septic planning checklist

• [ ] Public sewer or septic? Confirm with records and the agency.
• [ ] If sewer: ask whether the ADU needs a new lateral or can use the existing one.
• [ ] Ask the utility for a written fee estimate and the basis (square feet or DFU).
• [ ] If the ADU is a conversion, ask for confirmation that no new connection fee applies.
• [ ] If septic: ask Public Health about system capacity before design.
• [ ] If a sewer is within 200 feet, ask about the connection rule.
• [ ] Add trenching and tie-in work as its own budget line.
• [ ] Get all answers in writing.

Questions for your designer or plumber

1. Where does the existing house drain leave the building?
2. What are the pipe size and slope, and can they take more fixtures?
3. How far is the ADU from the main or the existing line?
4. Does anything block the route, such as a tree, easement or slab?

For overall cost drivers, see ADU cost in Los Angeles. Hillside lots may also face fire and access rules; see ADUs in fire hazard zones. More guides in the ADU costs hub. This page is general information, not legal advice.