How are water and sewer capacity charges calculated for an ADU?

It depends on the type of ADU. For an ADU or JADU converted from existing space, utilities generally cannot require a new connection or charge a related connection fee or capacity charge. For other ADUs, a charge is allowed, but it must be proportionate to the ADU's burden on the system, based on its square footage or its drainage fixture unit (DFU) count, and cannot exceed the reasonable cost of providing service.

The rules in Government Code section 66311.5

• Not a new residential use. A local agency, special district or water corporation cannot treat an ADU or JADU as a new residential use when calculating connection fees or capacity charges for utilities, including water and sewer, unless the unit was built with a new single-family home.
• Conversions within existing space. For units created under section 66323(a)(1), such as an ADU or JADU inside the existing home or an ADU in a converted garage or other accessory structure, the utility cannot require a new or separate connection directly to the unit or impose a related connection fee or capacity charge, unless the unit is built with a new single-family home or is later conveyed separately as a condominium.
• Other ADUs. For new detached or attached ADUs, the utility may require a new or separate connection. Any connection fee or capacity charge must be proportionate to the burden of the ADU on the water or sewer system, based on either its square feet or the number of its drainage fixture unit values as defined in the Uniform Plumbing Code, and cannot exceed the reasonable cost of providing the service.

Connection fees and capacity charges are not impact fees, so the impact-fee exemption for ADUs of 750 square feet or less does not cover them. They follow these separate rules.

What a DFU-based charge means

The plumbing code assigns each fixture a drainage fixture unit value that reflects the load it puts on the drainage system. HCD describes the charge as proportionate to the ADU's burden based on its square footage or DFU count compared with the primary dwelling. Ask your district which method it uses and for its written charge schedule before your plumbing design is final.

Electric and gas are handled differently

HCD notes it has no authority over electric and gas service. If you believe an investor-owned utility is not following connection or fee rules, you can file a complaint with the California Public Utilities Commission; for a publicly owned utility, use that utility's own complaint process.

What this means for your MEP plans and permit set

Your plumbing plans are the record of the ADU's fixtures, so an accurate fixture schedule and DFU count supports a correct charge and avoids disputes. The plans also show whether the ADU ties into the existing building sewer and water service or gets its own connection, which affects whether a charge applies at all. See fixture unit counts, impact fees and waivers, and separate water meters. Charges and methods vary by district, so confirm them in writing with your water and sewer providers.

Sources: leginfo.legislature.ca.gov, leginfo.legislature.ca.gov, hcd.ca.gov.