Does a detached ADU need its own water meter?

Usually no — most California ADUs, including detached units, share the existing water meter and service with the main house, and state ADU law specifically discourages mandatory separate-meter requirements. That said, whether a separate meter is required, allowed, or simply worth installing depends entirely on your local water district, and detached units are the type most likely to trigger a district requirement, so confirm before you design.

When a shared meter is fine

For the typical single ADU, the existing meter and service line can carry the added demand because CALGreen low-flow fixtures keep the new load modest. A complete ADU bathroom group plus a kitchen adds a defined fixture-unit count, and an existing 3/4-inch or 1-inch service serving a single-family home often has spare capacity. Sharing is simpler, cheaper, and the most common outcome.

When a separate meter may be required or wise

• District policy. Some water districts require — or offer — a dedicated meter or sub-meter for a new dwelling unit, particularly for detached ADUs.
• Capacity limits. If the existing service is undersized for the combined demand, you may need a larger service or a second tap, which can trigger a meter.
• Separate billing. If you plan to rent the ADU and want the tenant billed directly for water, a separate meter or a private sub-meter makes that clean.
• Connection/capacity fees. A new meter usually carries a district connection fee that can range from modest to several thousand dollars plus the installation.

How to decide — a quick checklist

1. Call your water district and ask, in writing, whether a separate meter is required, optional, or prohibited for your detached ADU.
2. Confirm the existing service size and whether it can carry the added fixture-unit demand.
3. Get the connection/capacity fee schedule so you can compare shared vs. separate on real numbers.
4. Decide whether direct tenant billing matters to you — if so, weigh a separate meter or private sub-meter.

Meter requirements and fees vary widely by jurisdiction and water district — and ADU law limits when separate connections can be mandated. Confirm with your local building department and water utility before finalizing design.

Our plumbing plans document the water service point of connection and supply sizing either way, so your set reflects the path your district approves. See the ADU plumbing guide for the bigger picture on shared versus separate utilities.