What water heater is best for an ADU?

For most new California ADUs the best choice is a heat-pump water heater, because it is highly efficient and aligns with the state's electrification direction. A tankless unit (gas or electric) is the strong runner-up where space is tight, and a standard tank heater still has its place. The "best" option ultimately depends on your fuel availability, space, hot-water demand, and Title 24 path.

The main options compared

How to choose

• Check your city's fuel rules. Many California jurisdictions have all-electric ordinances for new construction, which point you to a heat-pump or electric option and can eliminate gas-line work entirely.
• Match capacity to demand. A studio or 1-bath ADU has modest hot-water needs; a 2-bath unit with simultaneous use benefits from a larger HPWH or a properly sized tankless.
• Mind the electrical load. Heat-pump and especially electric tankless units affect your panel sizing and load calculation — coordinate with your electrical plans.
• Plan placement for CALGreen. Locate the heater centrally so hot-water runs stay short and meet CALGreen delivery limits; a far-corner heater can force an added recirculation pump.

What every ADU water heater needs

Regardless of type, California requires seismic strapping — typically two metal straps lag-bolted into framing — plus listed connectors, a drip pan with drainage where it sits over finished space, and code-compliant venting for combustion units. These details belong on the plumbing sheet from the start.

Fuel ordinances, rebates, and equipment requirements vary by jurisdiction and utility — confirm with your local building department before specifying a heater.

Our plumbing plans note the water heater type, location, connections, and strapping, and the efficiency choice is coordinated with your mechanical plans and Title 24 report (a $240 add-on). See the ADU plumbing guide for a deeper comparison.