ADU building code: what California requires in 2026

The ADU building code in 2026

California rewrites its building code every three years. The Building Standards Commission published the 2025 edition of Title 24 on July 1, 2025, with an effective date of January 1, 2026. Plans submitted now are checked against it. If you have older plans drawn to the 2022 code, expect your designer to update them.

The 2025 edition also renumbered many sections. Ceiling height, escape windows and sprinkler rules all moved, so a section number from an old checklist or blog post may point to the wrong rule.

California Residential Code ADU scope

Most ADUs fall under the California Residential Code (CRC), Part 2.5 of Title 24. Section R101.2 applies it to detached one- and two-family dwellings and townhouses no more than three stories above grade, and to their accessory structures. That covers a backyard cottage, a garage conversion and an ADU attached to a single-family house, which together make a two-family dwelling.

An ADU added to an apartment building is outside that scope, so it follows the California Building Code (Part 2) instead. If you're adding units to a multifamily lot, see our multifamily ADU guide.

Cities have also lost most of their room to add stricter local rules for homes. According to the State Fire Marshal's 2025 rulemaking, the budget trailer bill AB 130 put a moratorium on new state and local building standards affecting residential units from October 1, 2025, to June 1, 2031, with limited exceptions such as wildfire home hardening.

What state ADU law changes

State ADU law doesn't replace the building code. HCD's ADU handbook is plain about it: even the ADUs a city must approve under section 66323 “must comply with building code and health and safety requirements for dwellings.” What the law does is block a few code-based requirements that cities used to stack on ADUs. For the zoning side of the law, see California ADU laws.

• No automatic occupancy change. Government Code section 66314(d)(8) says building an ADU isn't a Group R occupancy change under the local building code unless the building official finds a specific, adverse impact on public health and safety. The building code rules for detached dwellings still apply, as appropriate.
• Sprinklers follow the house. The same section says an ADU doesn't need fire sprinklers if the main house doesn't, and building one can't trigger sprinklers in the house. More below.
• Old violations on the house. According to the handbook, a city can deny an ADU for building code violations on the main house only if they threaten public health and safety and are affected by the ADU.
• Room to get in and out. Section 66323 lets a conversion of an accessory structure grow by up to 150 square feet just to fit an entrance or exit, such as a stair to a second-floor unit.

If your ADU was built without permits, the path to legalizing it runs through the same code. See unpermitted ADUs in California.

ADU fire separation

Fire separation is the building code rule people most often confuse with zoning. A setback is how far zoning makes you stay from the lot line. Fire separation is how much fire protection a wall needs given how close it sits to a lot line or another home. You can meet one and still owe the other.

Attached ADUs and conversions inside the house

An ADU attached to a house makes a two-family dwelling. Under CRC Section R302.3, the two units must be separated by fire-resistance-rated walls, floors or both, whether or not a lot line runs between them. The rating is 1 hour, or one-half hour if the building is sprinklered throughout. The separation has to be continuous: a rated wall runs from the foundation to the underside of the roof sheathing, or to a rated floor. Our attached vs detached ADU comparison covers the cost side.

Detached ADUs

A detached ADU is treated as its own dwelling. CRC Section R302.1 sets exterior wall protection by fire separation distance, measured to the lot line. Between two homes on the same lot, the code draws an imaginary line between them and measures each to it. The line has to sit where the existing house still complies.

In practice, a non-sprinklered detached ADU 10 feet from the house can usually keep ordinary walls and windows on the facing sides, because each building is 5 feet from the imaginary line. Pull it closer and the facing walls need a rating and fewer openings. The same logic applies on a 4-foot side setback: that wall can have openings, but no more than a quarter of its area. See ADU setbacks for the zoning side.

Do ADUs need fire sprinklers?

New homes in California need sprinklers. CRC Section R309.2 requires them in one- and two-family dwellings, then exempts an ADU that meets four tests: it fits the Government Code's ADU definition, the main house has no sprinklers, a detached unit is no larger than 1,200 square feet, and it's on the same lot as the main house.

State ADU law goes further. Section 66314 and section 66323 both say an ADU can't be required to have sprinklers if the main house isn't required to have them, and building the ADU can't trigger sprinklers in the house. Section 66323 says the same for junior ADUs and for existing multifamily buildings.

The test is what the code requires for the house, not its age alone. HCD's handbook gives the example of a house built decades ago that wasn't required to have sprinklers. Its ADU doesn't need them either. If that same house was recently altered enough to require sprinklers, any ADU created afterward needs them too. Fire zones have their own wrinkles; see building an ADU in a fire hazard zone.

Rooms, ceilings and egress

The livability rules in CRC Chapter 3 drive the floor plan more than most owners expect.

Escape windows matter most in conversions. A garage or basement often has no bedroom window that meets the size and sill limits, and cutting one in can mean new headers and, near a lot line, the opening limits above. Our garage conversion ADU guide walks through the upgrades. Ceiling height matters in two-story and attic units; see the ADU height limit guide for how 7-foot ceilings fit under a 16-foot cap.

A very small ADU can still be legal. HCD's handbook defines an efficiency unit, one allowed type of ADU, as at least 190 square feet of living space with a closet, kitchen sink, cooking appliance, refrigerator and a separate bathroom.

Energy code and solar

The energy code is the last big piece, and it treats new and attached units differently. According to the California Energy Commission's ADU FAQ:

• New detached ADUs must have newly installed solar panels under the prescriptive path, unless an exception applies. The panels can be on the same lot, and the code doesn't say whether they serve the ADU's meter or the house's.
• Attached ADUs are treated as additions, so solar isn't required.
• Garage conversions turn unconditioned space into living space and are generally treated as additions. Converting space that's already heated, like a finished basement, is an alteration.
• Gas appliances in a new detached ADU trigger electric-ready requirements so they can be swapped for electric later.

Energy compliance is handled on forms your designer prepares. Solar adds real cost to a detached unit; see what an ADU costs in San Diego.

ADU building code in San Diego

The City of San Diego enforces the state code and adds a few ADU-specific rules.

• Everything in Title 24 applies. IB-400 says ADUs must comply with all requirements of the California Building Standards Code, and that those rules may increase setbacks.
• Sprinklers. SDMC § 141.0302 repeats the state rule and adds the reverse: if the main house is protected by sprinklers, the ADU must be too. Bonus and affordable ADUs under the city's ADU Home Density Bonus always need sprinklers.
• Attached units and conversions. Technical Bulletin RESD 3-4 says an attached ADU always needs sprinklers when the house has them. With an unsprinklered house, one attached ADU on a single-family home is exempt, as are certain conversions of existing permitted space.
• Fire zones. In high and very high fire hazard severity zones, the city requires a 4-foot interior side and rear setback for new ADU structures, plus wildfire construction rules. See ADU in a fire hazard zone.

Plan check is where these rules are enforced, so your designer and builder carry most of the load. Our San Diego ADU permit guide covers the review steps, and pre-approved ADU plans can cut the code work if one fits your lot.

Before you submit plans

• Confirm the code edition. Plans should be drawn to the 2025 codes, not 2022.
• Find out if the house has, or must have, sprinklers. That one fact sets the ADU's sprinkler requirement and its fire separation distances.
• Measure to the lot line and the house. Walls, windows and eaves within 5 feet of either will need extra protection.
• Check every bedroom for an escape opening and every living space for 7-foot ceilings.
• Check your fire zone and Coastal Zone status early. See ADU coastal zone rules.
• Hire a licensed builder. Read how to hire an ADU contractor, check a contractor's license, or compare verified ADU builders in San Diego.

Common questions