What is an ADU?

History, Laws & Updates

Early Concepts (Pre-1980s)

Before ADUs were officially recognized in state law, secondary housing units called “granny flats” and converted garages emerged organically in response to multigenerational living needs. These were often built informally without permits, especially in urban areas where extended family living arrangements were common. However, the lack of regulation meant they had no legal protection or uniform standards. It was only years after that the first state ADU law was enacted.

Initial Legal Recognition (1982)

Era of Local Control with Minimal State Interference

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This era marks California's first attempt at formally addressing the housing shortage through accessory dwelling units. However, it did so while preserving significant local control, which limited the effectiveness of the new law.

To address the housing shortage, California introduced SB 1534, its first ADU law. However, the law heavily deferred to local governments, giving them three options:

• Local ADU ordinances: Cities could adopt local ordinances allowing ADUs and retain control over development standards like height, setbacks, and floor area.

• State minimum standards: Cities that didn’t adopt an ordinance defaulted to state ADU standards allowing only attached ADUs with a maximum 10% floor area increase.

• ADU bans: Cities could ban ADUs entirely, but only with findings showing adverse impacts on public health, safety, or welfare.

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Raising Minimum Standards (1986–1994)

Gradual State Encouragement of ADU Development

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During this period, the state incrementally raised the baseline ADU standards to enable slightly larger and more flexible housing solutions. 

• AB 4343 (1986): Increased state minimum standards to allow detached ADUs up to 640 sq ft and allowed a 15% increase in floor area for attached ADUs.

• AB 3529 (1990): Further expanded state minimums to permit detached ADUs up to 1,200 sq ft or a 30% increase for attached ADUs.

• AB 3198 (1994): Limited parking requirements to one space per ADU or per bedroom unless additional need was justified, and required cities to allow efficiency units as ADUs.

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Ministerial Approval & Permit Access (2002)

First Real Attempt to Streamline Permitting

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Although state reforms had been ongoing, it wasn’t until AB 1866 that a real procedural breakthrough happened. This law aimed to prevent cities from stalling ADU approvals through discretionary review.

• AB 1866 (2002): Required that all ADU permits be approved ministerially (without hearings or discretionary review).

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Major Expansion Era (2016–2019)

State Asserts Control to Remove Local Barriers

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California entered a new phase of aggressive reform to break down local regulatory barriers. For the first time, the state imposed clear limits on how much control local governments could exert over ADUs.

2016:

• SB 1069, AB 2299, AB 2406:

  • Created “Exemption ADUs” for internal conversions not subject to local zoning rules.

  • Required local governments to approve compliant ADUs within 120 days.

  • Eliminated covered parking replacement and parking near transit.

  • Mandated utility connection fees be proportional to actual impact.

  • Side and rear setbacks were limited to 5 feet to simplify placement on existing lots.

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2017:

• SB 229, AB 494: Allowed ADUs to be included in plans for new single-family homes instead of only after homes were built.

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2019:

• SB 13, AB 670, AB 881:

  • Prohibited cities from imposing minimum lot sizes for ADUs.

  • Reduced setbacks to 4 feet for side and rear yards.

  • Eliminated fire sprinkler requirements unless required for the main dwelling.

  • Waived owner-occupancy requirement through 2025.

  • Reduced review time to 60 days.

  • Created minimum size protections where cities could not apply standards that prevent 800 sq ft ADUs.

  • Required cities to allow at least one ADU and one JADU per lot.

  • Capped impact fees for ADUs under 750 sq ft.

  • Prevented HOA and common interest developments from unreasonably restricting ADUs.

  • Required HCD oversight of local ordinances and expanded ADU types for multifamily lots.

  • Set maximum ADU size thresholds where local ordinances must permit at least 850 sq ft for ADUs with one or fewer bedrooms, and 1,000 sq ft for two-bedroom units.

  • Expanded the Exemption ADU program to include:

    • One detached ADU per single-family property up to 800 sq ft, 16 ft tall, with 4-ft side and rear setbacks.

    • Conversion of nonresidential space in multifamily buildings into multiple ADUs, adding up to 25% more housing units.

    • Two new detached ADUs per multifamily property under 800 sq ft, 16 ft tall, and meeting 4-ft setback requirements.

    • Permission to pair a new detached ADU with a JADU on single-family lots, allowing up to three units on what was previously a single-unit parcel.

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Triplex Zoning & Homeownership Tools (2021)

Reframing Single-Family Zoning & Expanding Rights

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This era reimagined single-family zoning altogether. With the passage of the HOME Act and similar reforms, homeowners gained new rights to build more than one housing unit on lots once reserved for a single home.

• AB 68 (2019): Legalized Junior ADUs (JADUs) up to 500 sq ft and introduced strong statewide mandates requiring local agencies to approve one ADU and one JADU per lot in single-family zones. It streamlined the permitting process and preempted local zoning standards that limited ADU development. This bill significantly contributed to California's move toward de facto triplex zoning by allowing up to three units—primary residence, ADU, and JADU—on a single-family lot.

• SB 9 (2021): California HOME Act allowed qualifying homeowners to split their lots and build either two 800-square-foot homes or one duplex. In total, up to four units may be developed on what was previously a single-family parcel. The law also prohibits local governments from blocking these developments based on subjective design standards or additional parking requirements.

• AB 1584 (2021): Voided certain deed restrictions and covenants that prohibited ADUs.

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Streamlining & Clarification (2022)

Closing Loopholes and Codifying ADU Rights

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Policymakers addressed growing frustration with inconsistent interpretations of ADU law. These updates further narrowed the ability of cities to delay or deny projects, ensuring that homeowners could build by right.

• SB 897 & AB 2221 (Effective 2023):

  • Enforced a 60-day approval/denial deadline.

  • Required cities to use only objective and measurable development standards.

  • Allowed ADUs in detached garages and permitted demolition of garages with the ADU application.

  • Established a base height limit of 16 ft for detached ADUs, with allowances up to 18 ft near transit or on multifamily lots, and up to 25 ft for attached ADUs.

  • Barred fire sprinkler mandates in the main house due to ADU construction.

  • Protected unpermitted ADUs built before 2018 unless they posed a health/safety risk.

  • Allowed enclosed uses like garages to count toward JADU eligibility.

  • Barred permit denials based on unrelated zoning/code violations.

  • Mandated that JADUs without bathrooms must have a separate entrance from the main home, along with interior access to the main living area.

  • Required permitting agencies that deny an ADU or JADU application to issue a full written response within the 60-day timeframe, identifying all deficiencies and describing how the applicant can correct them to secure approval.

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New Ownership Options & Enforcement (2023)

ADUs Become Separate Legal Properties

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This period marked a major step toward expanding ADU viability as a form of homeownership. Meanwhile, state agencies took stronger roles in enforcing ADU laws locally.

• AB 1033: Allowed separate sale of ADUs as condominiums (if allowed locally).

• AB 976: Permanently prohibited owner-occupancy requirements for ADUs.

• AB 1332: Required each local agency, by January 1, 2025, to develop a program for the preapproval of accessory dwelling unit plans

• AB 434: Empowered HCD to enforce non-compliant jurisdictions.

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Supercharging ADU Growth (2024)

The Most Ambitious Pro-ADU Reforms Yet

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The 2024 reforms represent California’s boldest commitment yet to maximizing housing density and accelerating ADU adoption statewide. With incentives like tax exemptions and high-density ADU allowances, the state removed many final barriers.

• SB 1211: Allowed up to 8 detached ADUs on multifamily lots (not to exceed the number of existing units).

• SB 1164: Provided 15-year property tax exemption for new ADUs.

• AB 3057: Extended CEQA exemption to Junior ADU ordinances.

• SB 477: Renumbered ADU/JADU laws into Government Code §§ 66310–66342.

• AB 2533:

  • Extended protections for unpermitted ADUs built before Jan 1, 2020.

  • Required cities to create a checklist of substandard housing conditions.

  • Allowed homeowners to request third-party inspections before applying.

The Future of ADUs in California

California’s journey toward universal ADU access has been decades in the making, but in many ways, it's just beginning. What started as a cautious attempt to increase backyard housing options has evolved into one of the boldest statewide housing initiatives in the country. These reforms haven’t just changed zoning codes, they’ve sort of redefined what it means to live, invest, and age in place in the Golden State of California.

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Today, ADUs are no longer a fringe concept. They’re a foundational pillar of California’s response to the housing crisis—offering families multigenerational living options, landlords new sources of passive income, and communities a gentler path to increasing density.

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Looking ahead, we can expect more innovation at both the state and local levels:

• The 2025 rollout of preapproved ADU plans may dramatically reduce the cost and friction of design and permitting.

• Tax incentives, such as those introduced under SB 1164, may expand further to boost affordability.

• We may also see greater coordination with climate policy, as ADUs can play a critical role in smart growth, infill housing, and reducing suburban sprawl.

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If the last decade is any indicator, one thing is clear: ADUs are no longer optional, they’re essential. Stay informed with the latest news and updates about ADUs.