Can my ADU share plumbing with the main house?

Yes — in most cases an ADU can share both water supply and sewer with the main house, and for a single ADU this is the normal, code-compliant, and most economical approach. The unit still needs its own complete, independent fixtures; "sharing" refers to the service connections, not the bathroom.

What sharing actually means

• Shared water supply: the ADU taps off the existing potable service rather than getting a new meter — provided the service has capacity for the combined demand.
• Shared sewer lateral: the ADU's drain ties into the home's existing building sewer through a wye or tee, as long as gravity slope (1/4 inch per foot) and lateral capacity allow.
• Shared gas, where used: the ADU can branch off the existing gas system if the meter and piping can carry the added BTU load.

What you cannot share is the dwelling itself: to be a legal ADU the unit must have its own permanent provisions for cooking, bathing, and sanitation. A JADU is the one exception that may share a bathroom with the primary home.

When sharing works well — and when it doesn't

Before you commit to sharing

1. Camera the sewer lateral ($150–$350). A deteriorated line should be replaced before you add ADU load — far cheaper while trenches are open.
2. Check fixture-unit and service capacity for both water and sewer with your districts.
3. Confirm separate-service rules — some sewer/water districts require an independent lateral or meter for a new dwelling unit.
4. Verify gravity slope to the tie-in; a downhill or below-grade unit may need a sewage ejector.

Shared-versus-separate rules, capacity studies, and connection fees vary by city, water district, and sewer authority — confirm in writing with each during early planning.

Our plumbing plans document the connection method, slope, and tie-in detail your plan checker needs, whether you share or run separate service. For a detached unit, pair them with the Full MEP package, or read the ADU plumbing guide for more.