ADU plumbing in Los Angeles: sewer, water and gas for backyard units and garage conversions

Most backyard units drain by gravity into the existing house lateral, the pipe running from the house to the street sewer. The connection needs enough slope over the full run, so we check the depth of the lateral, the distance and the finished floor height of the unit before anything is designed around it.

We start with a camera inspection of the lateral. In houses built before the 1970s it is often clay or cast iron, and roots from street trees and backyard trees can crack it. If the line is damaged, adding a second unit's waste will turn a slow problem into backups for both homes. Repair or replacement then becomes part of the ADU budget.

When the yard falls away from the street, the unit may sit lower than the sewer. Then a sewage ejector pump and a pressure line become part of the design, and the pump needs power and a service route for the future.

• Check lateral material, depth and condition by camera
• Confirm slope from the ADU floor to the tie-in point
• Add a cleanout at the tie-in and near the unit
• Plan an ejector pump if gravity will not work

Most ADUs share the main house water meter, with a branch line run from the main to the unit and its own shutoff valve. The question is whether the existing meter and main line can supply both homes at the same time. Older homes often have a small main, and adding a second shower and washer can drop pressure when both are running.

We look at the meter size, the main line material and size, and the static pressure. If the main is old galvanized or undersized, replacing it during the ADU work is cheaper than doing it later. Our page on main water line replacement explains that job. Some owners choose a separate meter for billing, which involves the water utility and changes the cost and timeline.

If the ADU will have gas appliances, we total up the demand of every appliance in both homes and compare it to the meter and the pipe sizes. A tankless gas heater is a big load, and it often needs a larger line from the meter. That work ties into new gas line runs and a pressure test for the inspector.

Many owners now go all-electric in the unit to skip the gas work entirely. A heat pump water heater fits that plan well, as long as it has room to pull air and somewhere for condensate to drain. Our article on choosing a water heater type covers the tradeoffs, and planning the electric panel early avoids surprises.

Whichever heater you pick, a tank model has to be strapped in the upper and lower third, the same seismic rule that applies to the main house.

A garage conversion keeps the existing slab, so every drain has to be trenched into it. We place the bathroom and kitchen close together and near the side of the garage facing the house lateral, which cuts trench length. Garages also tend to sit lower than the house, which affects slope.

A new detached unit lets us lay drains before the slab is poured, which is cleaner and cheaper per fixture. The tradeoff is a longer trench across the yard for sewer, water and sometimes gas. Our article on planning backyard ADU plumbing goes deeper into layouts.