Are utilities required for ADUs?

Yes. Because an ADU is a complete independent dwelling, it must have its own functioning electrical, water, and sewer (or approved septic) service — and gas only if the design uses gas appliances and the city allows them. Whether those utilities get dedicated meters or are shared with the main house depends on your jurisdiction and utility provider, so confirm the details with your local building department and utility early.

The three core utilities every ADU needs

• Electrical — a dedicated panel or properly sized subpanel, determined by a formal NEC Article 220 load calculation rather than a rule of thumb. All-electric ADUs with heat-pump HVAC, a heat-pump water heater, induction cooking, and EV charging often need 125–200 amps, which can require a service upgrade on homes with an existing 100-amp service.
• Water — connection to a potable water supply with correctly sized supply piping. Some water districts require a separate meter for a detached ADU; others allow a shared connection.
• Sewer/waste — connection to the municipal sewer (typically a tie-in to the existing lateral) or an approved septic system, with a drain, waste, and vent system that maintains proper slope and venting.

Separate vs. shared service

State law generally bars cities from requiring a brand-new, separate utility connection for an ADU created by converting existing space within the primary dwelling. For new detached ADUs, however, separate metering is frequently required or recommended, especially for rentals where independent tenant billing matters. The decision affects your plans either way — the MEP set must show the metering and connection configuration the reviewer expects.

Watch the hidden utility costs and timelines

Utility coordination is separate from your building permit and is often the longest item on the project schedule:

• Panel upgrades require the utility to de-energize the meter and can add weeks or months.
• Water and sewer connection fees are not covered by ADU impact-fee waivers and vary widely by district — some agencies have charged five figures for a new connection.
• A sewer-lateral camera inspection before design can reveal whether the existing lateral has the capacity and condition to take on the ADU's added fixture-unit load.

Start utility applications when you start the permit process — not when construction is nearly done. A surprise service upgrade discovered late can stall an otherwise finished project.

Your electrical plans document the service and load calculation, and your plumbing plans document the water and sewer connections — both included in the Full MEP Package. Because utility policies and fees change between code cycles and providers, verify current requirements with your specific utility before finalizing your budget.