ADU plumbing is one of the line items that surprises homeowners the most. It’s not a simple extension of your existing system — an ADU requires a fully independent plumbing system, and California has strict requirements for what that means.
Here’s what it actually costs, based on projects completed in Thousand Oaks, Calabasas, Newbury Park, and the surrounding Conejo Valley.
ADU Plumbing Cost Ranges in the Conejo Valley
| ADU Type | Plumbing Cost (Rough Estimate) |
|---|---|
| Studio or 1-bed (400–600 sq ft) | $12,000 – $22,000 |
| 1–2 bed with full kitchen + bath (600–900 sq ft) | $18,000 – $32,000 |
| 2-bed with 2 baths (900–1,200 sq ft) | $25,000 – $45,000 |
| Garage conversion (1 bath, kitchenette) | $10,000 – $18,000 |
These are total installed costs — rough-in, finish plumbing, water heater, and coordination with inspections. They don’t include civil work (connecting to the street), which is handled by others.
What’s Included in ADU Plumbing
Rough-In
The rough-in covers everything inside the walls and under the slab before drywall goes up:
- Supply lines (hot and cold) from the point of connection to every fixture
- DWV system — drain, waste, and vent lines for every fixture
- Stub-outs at each fixture location
This is where most of the labor and material cost lives.
Finish Plumbing
After drywall, cabinets, and tile are complete, finish plumbing includes:
- Setting toilets, sinks, tubs, and showers
- Installing faucets and valves
- Connecting the water heater
- Final trim on all fixtures
Water Heater
Most ADUs in the Conejo Valley go tankless for space reasons. A tankless unit sized for a small ADU adds $2,000–$3,500 to the budget installed.
Connection to Existing Main Line
The ADU needs to tie into your existing water supply and sewer lateral. Depending on the distance and condition of existing lines, this adds $1,500–$6,000.
What California Code Requires for ADU Plumbing
Separate Water Meter (Sometimes)
Some jurisdictions in Ventura and Los Angeles County require a separate water meter for ADUs. Check with your local building department early — this affects your permit and your utility connection cost.
Backflow Prevention
California code requires backflow prevention on the ADU connection. This is standard and included in most plumbing bids.
Water Heater Seismic Strapping
All water heaters in California must be seismically strapped per state code. This is non-negotiable and should be included in any legitimate bid.
Expansion Tank
If your system is a closed system (common in Conejo Valley homes on city water with a pressure regulator), an expansion tank is required. This is a code item, not an upsell.
What Affects Your ADU Plumbing Cost
Distance from existing systems. An ADU attached to your home costs less to connect than a detached unit at the back of a large lot.
Slope and drainage. In some Thousand Oaks and Calabasas hillside properties, getting gravity drainage to work requires creative routing or a sewage ejector pump.
Fixture count. Every fixture — toilet, sink, shower, tub, dishwasher connection — adds to the DWV complexity and rough-in time.
Soil conditions. Rocky soil in parts of the Conejo Valley can make trenching significantly more expensive than in softer areas.
The Right Time to Involve a Plumber
The best time to bring in a plumber on an ADU project is before permits are submitted. Early plumber involvement means:
- The plumbing layout is coordinated with architectural plans from the start
- No costly plan revisions after permit approval
- Correct placement of cleanouts, vents, and connections
Calling a plumber after framing is already up creates unnecessary constraints and usually costs more.
Berkeley Plumbing — ADU Plumbing in the Conejo Valley
Berkeley Plumbing has completed ADU plumbing projects throughout Thousand Oaks, Newbury Park, Calabasas, Agoura Hills, Westlake Village, Camarillo, and Malibu. We work with homeowners, architects, and general contractors at the design stage to ensure plumbing is planned correctly before walls go up.
Upfront pricing before any work begins. Call or text (310) 733-7808.
Get an Estimate
Upfront pricing before any work begins. No surprises.
Request an Estimate Call (310) 733-7808✓ C-36 Licensed · CSLB #1115151
✓ Serving Conejo Valley & Malibu
✓ Upfront pricing, always