ADU Sewer Connection in Los Angeles: Tying a New Unit Into an Old Sewer Lateral

November 9, 2026 • BBC Rooter & Plumbing

Every ADU set of plans we see has the same blank spot. The site plan shows the new unit at the back of the lot, a short dashed line leaving it, and a note that reads to existing sewer. Four words. No pipe material, no diameter, no depth, no grade, and no mention of the fact that the existing sewer under that driveway was laid when Eisenhower was president.

The water side of an ADU gets planned carefully, because a meter upgrade is a visible line item. The sewer side gets assumed. In Van Nuys, Reseda, Canoga Park and North Hills, where most of the garage conversions in the Valley are happening, that assumption is the single most common reason an ADU budget moves after construction starts.

What an ADU Actually Asks of the Sewer Line

A second unit adds a second kitchen, a second bathroom and in most cases a second laundry. In drainage terms that is a modest load. A 4-inch building sewer running at a quarter inch of fall per foot carries far more than one small dwelling adds, and plumbers who tell you the pipe is too small to take an ADU are almost always wrong about diameter.

Diameter is not what fails. Condition is. The pipe does not care whether the water came from one house or two, but a clay lateral with root hairs at every joint and a two-inch belly under the front lawn is running at a fraction of its rated capacity already. The house has been living with that line because one family's peak flow is a shower and a load of laundry. Add a second kitchen and a second washer on a different schedule and the line gets hit twice as often, at hours nobody was using it before. Lines that backed up once every two years start backing up every two months.

The materials you are likely to find

Age of the tract tells you most of it. Houses in the Valley built before the mid-1950s usually ran vitrified clay in short sections with mortared joints, which is the material that gives roots a way in. From the mid-1950s through about 1970 the common choice was cast iron inside and clay or cast iron out to the street. Orangeburg, the tar-impregnated fiber pipe that deforms into an oval, shows up in work from roughly 1945 to 1972 and is the one material that is never worth saving. ABS and PVC appear after that and are generally fine.

The 1971 Sylmar and 1994 Northridge earthquakes left a separate signature: joints that pulled apart at the connections while the barrel itself stayed sound. A line can pass a flow test and still have a half-inch offset that a camera finds in four minutes.

Two Legal Ways to Drain an ADU, and How to Choose

The City of Los Angeles allows either approach. The choice is an engineering one.

Option one: tie into the existing building sewer

A wye is cut into the existing lateral downstream of the house, and the ADU line joins it there. The trench is shorter and the permit is smaller, so the number comes in lower. It is the right answer when the camera shows a 4-inch line in sound condition at consistent grade with no standing water. Shortening the new run also means less pipe to maintain later.

The catch is that both dwellings now depend on one line. When that line fails, two households lose their drains on the same afternoon. The arithmetic of who pays for the repair afterward also gets complicated if the property is ever split or if the ADU is sold separately, which is the same problem we wrote about in shared sewer laterals between two properties.

Option two: run a second lateral to the city main

The ADU gets its own pipe, its own tap at the main and its own cleanout. This costs more up front because it means a longer trench and usually a street cut, and a street cut in the City of Los Angeles carries its own permit and its own restoration requirement. What you get is independence. A failure on one line leaves the other working.

A second lateral stops being optional when the existing line is 3-inch, when it is Orangeburg, when the camera shows a belly or offset joints that will need digging anyway, or when the ADU sits where the fall to the existing line does not work. In those cases the money spent tying into the old pipe is money spent twice.

The Question That Should Be Answered First

Before the designer picks a route, somebody has to go look. A camera inspection and a sonde locate of the existing lateral gives the design team five numbers it otherwise guesses at: material, diameter, depth at the proposed tie-in, grade along the run, and the location of the existing two-way cleanout. It also tells you whether the pipe crosses under the new footprint, which matters, because a sewer line running beneath a planned slab has to be rerouted before the concrete goes in.

That inspection costs a small fraction of a mid-construction change order. We run it as a recorded camera inspection so the designer and the plan checker both see the actual condition of the pipe for themselves.

Slope, Depth and the Long Flat Lot

A 4-inch building sewer is normally laid at a quarter inch of fall per foot. For a detached unit 80 feet behind the house, that is about 20 inches of extra depth at the ADU end. The deep flat lots in Reseda, Winnetka and North Hills usually have room for it. A garage conversion on a short lot where the garage already sits close to the house is easier still.

Hillside lots are where this stops being routine. On the slopes above Ventura Boulevard in Encino and Sherman Oaks, and on the north-facing streets in Porter Ranch, the house can sit above the street while the usable back corner of the lot sits below it. Gravity to the main may simply not exist from that corner. The answer is a sewage ejector pump in a sealed basin, pumping up to the gravity line, and it has to be in the design before the slab is poured. Discovering it afterward means breaking concrete.

Cleanouts, Permits and the Trench You Already Paid to Dig

Give the ADU its own cleanout. A long run of pipe at the back of a lot with no access of its own is a line that has to be worked from the street through a junction every time something goes wrong. The cost while the trench is open is small.

Sewer work on an ADU is permitted work, and a connection to the city main in Los Angeles also involves a sewer capacity charge assessed by LA Sanitation. The rates and the forms change, so confirm the current figure with the agency. A contractor's old invoice is not a current rate. We cover the general permitting picture in sewer line permits in Los Angeles.

One more piece of practical advice. If the camera shows the existing lateral is near the end of its life and the trench for the ADU line is going to run beside it anyway, replace it in the same dig. On sewer work the excavation costs several times what the pipe costs. Opening the same ground twice in three years is the most avoidable expense in this whole category of job.

BBC Rooter & Plumbing handles the sewer side of ADU projects in Van Nuys, Reseda, Canoga Park, Northridge, Winnetka, North Hills, Granada Hills, Chatsworth, Woodland Hills, Tarzana and across the San Fernando Valley. Camera inspection, locating, new laterals, cleanouts and main connections. Licensed contractor CSLB #720343. Call 818-280-9135, or read our frequently asked questions.