Can the water and sewer lines to a detached ADU share one trench?

Often yes. The 2025 California Plumbing Code restricts a shared trench only when the sewer or drain pipe is clay or another material not approved for use inside a building (Sections 609.2 and 720.1). In that case the water pipe must sit at least 12 inches above the top of the sewer, on a solid shelf cut into one side of the trench, at least 12 inches away horizontally.

Reading the rule

• It turns on pipe material. The restriction applies to building sewers or drains made of clay or materials not approved for use within a building. "Within a building" means inside the fixed limits of the foundation (720.1).
• Most new ADU sewers are plastic. ABS and PVC drainage pipe are approved for use inside buildings, so a new ABS or PVC building sewer generally doesn't trigger the shelf requirement.
• Old pipe changes the answer. If your water line runs alongside or crosses an old clay lateral, both conditions apply, and wherever the water pipe crosses over that sewer it must be at least 12 inches above it.

Local standards may be stricter

Your water purveyor and sewer agency may have their own separation standards, especially near the meter and the public sewer, and some cities amend these sections. Confirm before you dig.

Adding gas or electrical to the trench

• Gas: underground gas piping needs enough clearance from other underground structures to avoid contact and allow maintenance, and at least 12 inches of cover (CPC 1210.1). Plastic gas pipe also needs a tracer wire.
• Electrical: conduit and cable burial depths come from the California Electrical Code, not the plumbing code, so coordinate depths with your electrical plan.

Don't stack pipes directly on top of each other. Even where code allows a shared trench, give each line its own bedding and room for future repair.

Practical trench tips

• The sewer needs continuous slope; the water line doesn't. Over a long run they drift apart in depth, which makes a stepped trench or separate shelf easy to build.
• Plan the sewer first, because its slope sets the depth at the tie-in.
• Keep the trench open until the inspector has seen the pipe, the separations, and any test.

What it means for your plumbing plan

Your plans should show the route of each underground line and a trench section with pipe materials, depths, separations, and the sewer slope. A clear trench detail answers the inspector's questions before they're asked. See how ADU sewer connection works, gas line requirements, and our plumbing plans.

Sources: gocodebook.com, gocodebook.com, gocodebook.com, gocodebook.com.