Can a garage conversion use existing plumbing?

Usually there's nothing to reuse — most California garages have no plumbing at all. Converting one into an ADU means designing and installing a complete new water-supply and drain-waste-vent (DWV) system from scratch, almost always by cutting into the concrete slab.

Why the slab is the central challenge

Garages sit on a concrete slab-on-grade with no below-slab drainage. Adding a kitchen sink, a full bathroom (toilet, sink, tub/shower), and a water heater requires cutting and trenching the slab to install drain lines, P-traps, and vent stub-ups below grade, then patching the concrete back. Typical slab cutting, pipe, and patching runs roughly $2,000-$5,000. A post-tension slab adds an important wrinkle: cutting one can sever the tensioning cables, so it requires structural assessment before any work begins, which adds cost and time.

Three site conditions that drive the plumbing design

• Elevation relative to the sewer lateral — if the garage floor is at or above the existing sewer lateral, drain lines must run deeper than floor level to keep the required slope, or a sewage ejector pump must be added.
• Distance to the lateral — drains must hold a minimum 1/4-inch-per-foot slope all the way to the connection; a garage at the back of a deep lot can require significant trenching.
• Water-supply run length — long runs from the house to a back-of-lot garage may need upsized supply lines to maintain pressure.

What the plumbing plans must show

To pass plan check and inspection, the plumbing set documents fixture locations and a fixture schedule, water-supply routing and sizing, the full DWV system with vents and cleanouts, the sewer tie-in, the water heater (type, efficiency, T&P valve, seismic strapping), backflow prevention, and CALGreen low-flow fixture compliance. Missing vent design is one of the most common plumbing corrections, so it must be engineered in from the start.

A pre-start sewer camera inspection of the existing lateral is strongly recommended — discovering a failed lateral after the slab is cut is an expensive surprise.

The water heater and fixture decisions

A garage conversion also introduces a water heater where none existed. Heat pump water heaters score best under the 2025 energy code but want roughly 700 SF of surrounding air volume; compact conversions often turn to tankless units instead, and for very small ADUs the code allows an electric tankless alternative under the prescriptive path when space rules out a heat pump unit. Whichever you choose, the unit's location affects both plumbing routing and the electrical load, so it's decided alongside the rest of the MEP design rather than after. All new fixtures must also meet CALGreen low-flow limits — 1.28 gpf toilets, 1.8 GPM kitchen faucets and showerheads — which the fixture schedule documents for the reviewer.

Budgeting realistically

Because the plumbing is built from scratch, it's frequently the most unpredictable line item on a conversion. Beyond the slab cut, watch for sewer-lateral condition, possible sewer-capacity fees charged by some districts, water-service sizing, and post-tension slab complications. None of these are reflected in the plan price itself — they're construction and utility costs — but a well-designed plan set anticipates them so they don't appear mid-build.

Requirements vary by jurisdiction and water provider — confirm sewer connection rules, capacity fees, and backflow requirements with your local building department and water district. Our plumbing plans handle the slab-cut DWV layout and tie-in details reviewers expect; start your order to get them prepared alongside your mechanical and electrical sheets.