Can I build a two-story ADU in California, and how does it change the MEP design?

Often, yes. State ADU law sets no limit on the number of stories, and HCD's ADU Handbook says a city cannot deny a detached two-story ADU that fits within the state height allowances and meets the building code; the practical limit is the height your lot is allowed. A second floor changes the MEP design mainly by stacking plumbing, splitting heating and cooling between levels, and adding stair, alarm, and egress items.

How tall can it be?

• Detached ADU: a city may not set a limit lower than 16 feet; 18 feet within a half mile of a major transit stop or high-quality transit corridor, plus up to 2 feet to align the roof pitch with your house; and 18 feet on a lot with an existing or proposed multistory multifamily building.
• Attached ADU: 25 feet or the height limit that applies to your house under local zoning, whichever is lower.
• Local ordinances can allow more. State figures are minimums a city must allow. Check whether your city's ordinance permits more height, which is often what makes a detached two-story unit practical.

Plumbing on two levels

• Stack upstairs bathrooms over downstairs wet walls where you can, so drains and vents run straight down.
• Upstairs fixtures add drainage load and pipe length; the plumbing plan shows sizing and vent routing through both floors.
• Place the water heater with the farthest fixtures in mind, since long hot-water runs mean longer waits and affect the water-heating inputs in the Title 24 report.

Heating, cooling, and ventilation

• Upper floors usually run warmer in summer, so room-by-room load calculations matter more than in a single-story unit. A ductless head on each floor, or a zoned ducted system, keeps both levels comfortable.
• For an attached two-story ADU built as an addition, the 2025 Energy Code's load-calculation table for additions (Table 150.2-C) uses separate infiltration values for two-story buildings when sizing equipment.
• A new detached two-story ADU is new construction, so it needs whole-dwelling ventilation, ECC-rater verification of the required items, and solar PV unless an exception applies.

Electrical and life safety

• Smoke alarms are needed on each story and interconnected where more than one is required; CO alarms apply where there are fuel-burning appliances or an attached garage.
• Plan stair lighting with controls at both levels.
• Upstairs sleeping rooms still need emergency escape openings, so keep outdoor units, line sets, and meters from blocking the area below those windows.
• A two-story unit may justify a larger subpanel; the load calculation decides.

What it means for your MEP plans

Expect a floor-by-floor layout for each discipline: plumbing isometrics or riser details, equipment and indoor head locations on both levels, and panel and circuit layouts that include stair lighting and interconnected alarms. Confirm your height allowance before the MEP layout begins, because dropping to one story changes nearly every route. Start with the California ADU requirements guide, maximum ADU size, and the ADU mechanical guide.

Sources: hcd.ca.gov, energy.ca.gov, energy.ca.gov.