California law requires cities to allow an ADU of at least 850 square feet for a studio or one bedroom, and 1,000 square feet for two or more bedrooms, under Government Code Section 66321. A detached ADU of up to 800 square feet is allowed by right. Interior livable space is measured under SB 543, effective January 1, 2026.
Last verified: October 18, 2026 against the California Legislature bill text (leginfo.legislature.ca.gov) and the recodified Government Code Section 66310 and following, including Sections 66321, 66323 and 66313 as amended by SB 543 (2025), cross-checked against the HCD ADU Handbook (March 2026) and HCD ordinance review letters issued in 2026.
An accessory dwelling unit, or ADU, is a second, smaller home on a lot that already has a house. A junior ADU, or JADU, is a unit of up to 500 square feet created inside the walls of an existing house. This page is the definitive answer to how big an ADU can be in California, size class by size class, with the Government Code section for each number. For the broader statewide law overview covering permits, setbacks, parking and fees, see the California ADU law and permits pillar. For the bill-by-bill 2026 changes, see the California ADU law tracker.
How big can an ADU be in California?
California sets ADU size two ways: a floor that every city must allow, and a size that is allowed by right on any qualifying lot. By right means the use is allowed without a special permit or discretionary review.
First, a city cannot cap ADU size below 850 square feet for a studio or one bedroom unit, or below 1,000 square feet for a unit with two or more bedrooms, under Government Code Section 66321(b)(2). The statute uses the words “eight hundred fifty square feet” and “one thousand square feet.” A city can allow larger, but it cannot force you smaller than these floors.
Second, a detached, new construction ADU of up to 800 square feet of livable space is allowed by right under Government Code Section 66323(a)(2)(A). The statute uses the words “not more than 800 square feet of livable space.” At or below 800 square feet, a city cannot deny the ADU on lot coverage, floor area ratio, or open space grounds.
A city also cannot set a minimum size that prohibits an efficiency unit, under Government Code Section 66321(b)(1). An efficiency unit is a small dwelling with limited kitchen and room requirements, and California protects it from minimum size rules.
California ADU size limits table
| Unit type | Minimum a city must allow | Always allowed by right | Statute | Source |
|---|---|---|---|---|
| Studio or one-bedroom ADU | 850 square feet | Detached ADU up to 800 square feet of livable space | Gov. Code 66321(b)(2); 66323(a)(2)(A) | California Government Code (leginfo) |
| ADU with two or more bedrooms | 1,000 square feet | Detached ADU up to 800 square feet of livable space | Gov. Code 66321(b)(2); 66323(a)(2)(A) | California Government Code (leginfo) |
| Efficiency unit | Cannot be prohibited by a minimum size rule | Same as above | Gov. Code 66321(b)(1) | California Government Code (leginfo) |
| Junior ADU (JADU) | Up to 500 square feet within existing walls | Not applicable | Gov. Code 66313(d) | California Government Code (leginfo) |
How is ADU interior livable space measured?
Interior livable space is defined as space in a dwelling intended for human habitation, under Government Code Section 66313(e). SB 543 (2025) added this definition, effective January 1, 2026. Because the measurement is of interior livable space, it does not count the thickness of the exterior walls. That definition sets the basis for the 500 square foot junior ADU cap and for the 750 square foot interior livable space threshold that waives ADU impact fees. When you calculate whether your ADU meets a size cap, you measure the interior habitable area, not the outer footprint.
How much space is a junior ADU?
A junior ADU, or JADU, is no more than 500 square feet of interior livable space, created within the walls of an existing or proposed single-family residence, under Government Code Section 66313(d). The statute uses the words “no more than 500 square feet of interior livable space.” A JADU is carved out of the existing house rather than built as a separate structure, which is why its cap is measured within existing walls. A single-family lot may have one ADU plus one JADU, under Government Code Section 66323(a)(1) together with Section 66323(a)(2). From January 1, 2027, AB 956 allows two detached new-construction ADUs instead, but a city then does not have to approve a JADU on that lot.
How do height and stories relate to ADU size?
Height limits shape how many stories an ADU can have and therefore how much floor area fits within its footprint. California sets three height tiers.
- A detached ADU on a lot with a single-family dwelling may reach 16 feet, under Government Code Section 66321(b)(4)(A).
- A detached ADU within one-half mile walking distance of a major transit stop or high-quality transit corridor may reach 18 feet, plus up to two additional feet to match the primary dwelling roof pitch, under Government Code Section 66321(b)(4)(B).
- An ADU attached to the primary dwelling may reach 25 feet or the local zoning height limit, whichever is lower, and a multistory ADU is limited to two stories, under Government Code Section 66321(b)(4)(D).
The 800 square foot by right size is measured as livable space, so a two story ADU within the applicable height tier can reach 800 square feet across two floors while keeping a smaller footprint.
Can setbacks or lot coverage shrink an 800 square foot ADU?
No. For a detached ADU of up to 800 square feet built under the always-allowed rule, a city cannot use lot coverage, floor area ratio, or open space limits to force it below 800 square feet, under Government Code Section 66323(a)(2)(A). The ADU must still meet four-foot side and rear yard setbacks, under Government Code Section 66323(a)(2). For new ADU construction generally, a city may require no more than a four-foot side and rear setback, under Government Code Section 66314(d)(7), and the statute uses the words “a setback of no more than four feet from the side and rear lot lines shall be required.” When you convert an existing structure into an ADU, no setback may be required at all, under Government Code Section 66314(d)(7).
How many and how big on a multifamily lot?
Multifamily size and count rules differ from single-family lots. On a lot with an existing multifamily building, a city must allow at least one ADU converted from within the existing building, up to 25 percent of the existing units, under Government Code Section 66323(a)(3)(B). A lot with an existing multifamily dwelling may also add up to 8 detached ADUs, not exceeding the number of existing units on the lot, under Government Code Section 66323(a)(4). That detached multifamily allowance was enacted by SB 1211 (2024), which covers the multifamily counts in detail.
How California ADU size limits connect to other rules
California ADU size limits sit inside a larger 2026 framework. The California ADU law tracker lists the four bills that took effect for 2026, including SB 543, the bill that defined how interior livable space is measured. AB 2533 addresses unpermitted ADUs. SB 1211 sets the detached ADU counts on multifamily lots. For the full statewide rules on setbacks, parking, fees and approval timelines, see the California ADU law and permits pillar. You can also browse the ADU laws library for other states.
What changed in 2026?
The main size-related change for 2026 is how interior livable space is measured. SB 543 (2025) added the definition of interior livable space at Government Code Section 66313(e), effective January 1, 2026, as space in a dwelling intended for human habitation. That definition anchors the 500 square foot JADU cap and the 750 square foot impact fee threshold. The 850 and 1,000 square foot minimums a city must allow, and the 800 square foot by right detached ADU, remain the current standards.
What changes in 2027?
No change to the size limits above is chaptered as of October 18, 2026. The 2027 change we have confirmed is about count, not size: AB 956 (Stats. 2026, ch. 791), effective January 1, 2027, lets a single-family lot have two detached new-construction ADUs instead of one. A second 2027 change touches the 750 square foot fee line: SB 1117 (Stats. 2026, ch. 809) means impact fees on an ADU over 750 square feet start at the space above 750, for up to two ADUs on a lot. The California ADU law tracker has the details.
Frequently Asked Questions
How big can an ADU be in California?
A California city cannot cap an ADU below 850 square feet for a studio or one bedroom, or below 1,000 square feet for two or more bedrooms, under Government Code Section 66321(b)(2). Separately, a detached ADU of up to 800 square feet of livable space is allowed by right under Government Code Section 66323(a)(2)(A).
What is the minimum ADU size a California city must allow?
A city must allow at least 850 square feet for a studio or one bedroom ADU and at least 1,000 square feet for an ADU with two or more bedrooms, under Government Code Section 66321(b)(2). A city also cannot set a minimum size that prohibits an efficiency unit, under Government Code Section 66321(b)(1).
How big is a junior ADU in California?
A junior ADU is no more than 500 square feet of interior livable space, created within the walls of an existing or proposed single-family residence, under Government Code Section 66313(d).
How is ADU interior livable space measured in California?
Interior livable space is space in a dwelling intended for human habitation, under Government Code Section 66313(e), a definition added by SB 543 and effective January 1, 2026. Because it measures interior habitable space, it does not count the thickness of the exterior walls.
Can a city use setbacks or lot coverage to shrink my ADU?
No. For a detached ADU of up to 800 square feet built under the always-allowed rule, a city cannot use lot coverage, floor area ratio, or open space limits to force it smaller, under Government Code Section 66323(a)(2)(A). The ADU must still meet four-foot side and rear yard setbacks.
How tall can an ADU be in California?
A detached ADU on a single-family lot may reach 16 feet, or 18 feet near qualifying transit, and an attached ADU may reach 25 feet or the local zoning height, whichever is lower, under Government Code Section 66321(b)(4). A multistory ADU is limited to two stories.
Sources
- California Government Code, Section 66310 and following (ADU law), including Sections 66321, 66323 and 66313: https://leginfo.legislature.ca.gov/faces/codes_displayText.xhtml?division=1.&chapter=13.&part=&lawCode=GOV&title=7.&article=2.
- California Legislature, SB 543 (2025), text and history: https://leginfo.legislature.ca.gov/faces/billTextClient.xhtml?bill_id=202520260SB543
- California Department of Housing and Community Development, Accessory Dwelling Unit Handbook (March 2026): https://www.hcd.ca.gov/sites/default/files/docs/policy-and-research/adu-handbook-update.pdf
- California Department of Housing and Community Development, Review of Gardena’s ADU Ordinance (May 2026), applying Gov. Code 66323(a)(2)(A): https://www.hcd.ca.gov/sites/default/files/docs/policy-and-research/ordinance-review-letters/gardena-adu-findings-05082026.pdf
- California Legislature, SB 1211 (2024), text and history: https://leginfo.legislature.ca.gov/faces/billTextClient.xhtml?bill_id=202320240SB1211
- California Legislature, AB 956 (2026), Stats. 2026, ch. 791, bill text and history: https://leginfo.legislature.ca.gov/faces/billNavClient.xhtml?bill_id=202520260AB956
Disclaimer
This page is general information, not legal advice. ADU law changes often and cities apply it differently. Confirm the current size rules with your local planning department before you build.
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