Can my city set stricter ADU rules than the state?

Only within the limits of state law. A city may adopt objective standards, but not below the state minimums, and it must send its ordinance to HCD within 60 days of adoption. An ordinance that is not submitted is void, and only state ADU law applies.

Does an ADU need its own Title 24 report?

It needs energy documentation in almost every case. It is not always a separate model: a garage conversion done with an addition to the house can be modelled together, with the house, addition and ADU as separate zones.

Does a garage conversion ADU need solar panels?

No. Converting unconditioned space is an addition under the Energy Code, and additions are not required to install PV.

Is HCD or the Energy Commission in charge of ADUs?

Both, for different things. HCD enforces state ADU law on size, setbacks, fees and approvals; the Energy Commission sets the Energy Code that decides how the unit performs. Your city or county applies both when it reviews the permit.

California ADU Rules and Title 24

California's ADU rules are state law, summarised in HCD's ADU Handbook. A city must allow side and rear setbacks of no more than four feet, cannot cap size below 850 square feet (1,000 with more than one bedroom), and must approve or deny a complete application within 60 days. The Energy Code is a separate layer that almost every ADU also has to meet.

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Two sets of rules, two authorities

Where an ADU can go and how big it can be is state ADU law, in Government Code sections 66310 to 66342. Those sections were renumbered in March 2024 by SB 477. The Department of Housing and Community Development (HCD) reviews local ADU ordinances and, since 1 January 2024, has had authority to enforce state ADU law. None of this is part of Title 24. Title 24 governs how the unit is built, including its energy performance.

State law sets the floor. A city or county may be less restrictive and may apply objective standards within the limits the law allows. This page summarises the rules people ask about most, from the HCD ADU Handbook updated March 2026. It is a summary, not legal advice. Your planning and building departments have the final say on your lot.

Size

A local maximum must allow at least 850 square feet, or 1,000 square feet for more than one bedroom.
Without a compliant local ordinance, a new detached ADU may be up to 1,200 square feet, and an attached ADU up to 50 per cent of the main house, while still allowing 800 square feet.
Lot coverage, floor area ratio, open space and front setbacks cannot rule out an 800 square foot ADU with four-foot side and rear setbacks.
Converting existing space is not subject to unit size limits. Sizes are measured as interior livable space.

Setbacks, height and number of units

Side and rear setbacks of no more than four feet, and none for an ADU inside an existing structure or rebuilt in the same place and dimensions.
A local height limit cannot be lower than 16 feet for a detached ADU, or 18 feet within a half mile of a major transit stop or high-quality transit corridor, plus 2 feet for a roof pitch matching the main house. For an attached ADU it is 25 feet or the main house's zoning limit, whichever is lower.
On a single-family lot, at least one converted ADU, one JADU and one new detached ADU of up to 800 square feet must be allowed together.

JADUs, timelines and fees

A JADU is up to 500 square feet, entirely within a single-family home; an attached garage counts. One per lot, and no parking can be required.
Applications are reviewed ministerially, without a hearing. Completeness is decided in writing within 15 business days, and a complete application is approved or denied within 60 days where there is an existing dwelling on the lot.
No impact fees on an ADU of 750 square feet or less, or a JADU of 500 or less. Larger units pay in proportion to the main house. School districts may charge on units over 500 square feet.
Parking is capped at one space per unit or bedroom, whichever is less, and none in several cases, including within half a mile walking distance of public transit.

The energy side: how Title 24 classifies an ADU

The Energy Code has no special ADU category. The CEC's compliance manual classifies an ADU by how it is built. A newly constructed detached ADU is a new single-family building and meets the new-construction requirements in full. An ADU made by converting unconditioned space, such as a garage, is an addition. One made inside space that was already conditioned is an alteration. Do ADUs need Title 24? walks through the three, and garage conversions and JADUs have pages of their own.

Two energy-side rules are worth knowing early. A system serving an ADU must have its own thermostat. And the California Mechanical Code does not allow return air from one dwelling unit to be discharged into another, so an ADU generally cannot share the main house's ducted system.

Solar and ECC verification

Solar PV applies to newly constructed detached ADUs, subject to the code's exceptions, and not to conversions or additions. The details are in does an ADU need solar. Where a design relies on measures that need field verification, such as refrigerant charge on a heat pump, which the 2025 prescriptive package requires in every climate zone, the CF1R is registered and an ECC-Rater tests the work on site.

Does ADU size change the rules?

For zoning, yes, as above. For energy, size works mostly through classification, plus a few thresholds. An ADU built as an addition follows the addition limits for over 700 square feet, 700 or less, and 400 or less. A new dwelling unit of 500 square feet or less may use an electric water heater with point-of-use distribution instead of a heat pump water heater. One with one bedroom or less may use a 120-volt heat pump water heater.

Energy calculations also measure differently. Zoning counts interior livable space; the Energy Code's conditioned floor area is measured at the exterior surfaces of the exterior walls. The same unit has two sizes, and each rule uses its own.