As of January 1, 2026, California ADU size limits are measured using interior livable space. SB 543 clarified the statewide measurement standard and also added a 15-business-day deadline for agencies to determine whether an ADU or JADU application is complete. For San Diego homeowners, the changes affect how ADU size, permitting, and several fee thresholds are calculated.
The 60-second version
SB 543 took effect January 1, 2026 and changed several parts of California ADU and JADU law. It clarifies that size limits use interior livable space, meaning space intended for human habitation. It also requires a permitting agency to determine whether an ADU or JADU application is complete within 15 business days.
If an application is incomplete, the agency must identify the missing items and explain how the applicant can make the application complete. If the agency does not make the required timely completeness determination, state law provides a process for the application to be deemed complete.
SB 543 also confirms impact-fee protections based on interior livable space: an ADU with 750 square feet or less is exempt from impact fees, and a JADU with 500 square feet or less is exempt from impact fees. State law also specifies that an ADU or JADU with less than 500 square feet of interior livable space does not increase assessable space for purposes relevant to school developer fees. The law also clarified JADU fire-sprinkler rules.
- Interior livable space standard
- 15-business-day completeness check
- 750 sq ft ADU impact-fee threshold
- 500 sq ft JADU threshold
- Effective Jan. 1, 2026
Why this matters for California ADUs
Before SB 543, important parts of ADU law were interpreted differently across jurisdictions. The 2026 changes put several of those rules directly into state law and give homeowners, designers, and permitting agencies a clearer statewide standard.
The biggest practical changes are the definition of interior livable space, the 15-business-day completeness check, updated impact-fee language, and procedural limits on how completeness comments are handled. These changes do not replace local zoning, building, coastal, fire, or site-specific requirements.
What “interior livable space” actually means
California Government Code defines interior livable space as space in a dwelling intended for human habitation, including living, sleeping, eating, cooking, or sanitation. California HCD's March 2026 ADU Handbook specifically states that interior livable space includes bedrooms, bathrooms, kitchens, laundry rooms, closets, and interior stairs.
That is different from the building-code concept of “habitable space.” For SB 543 size calculations, homeowners should use the state ADU-law definition of interior livable space.
| Examples that count as interior livable space | Examples that are not interior livable space |
|---|---|
| Living and dining areas | Exterior wall thickness |
| Bedrooms and bathrooms | A garage that is not part of the ADU's interior living area |
| Kitchen | Separate accessory structures outside the ADU |
| Interior hallways and circulation | Open exterior areas such as uncovered patios |
| Laundry rooms and closets | Exterior site area outside the dwelling |
| Interior stairs | Spaces that are outside the dwelling's interior livable area |
The size limits, restated with the current measurement standard
SB 543 did not create new statewide ADU size caps. It clarified that the applicable statutory size limits are measured using interior livable space. Local jurisdictions can allow more generous standards than the statewide protections.
| Unit type | Statewide protection | Measurement |
|---|---|---|
| Studio / 1-bedroom ADU | A local maximum cannot prevent an ADU of at least 850 sq ft. | Interior livable space |
| ADU with more than one bedroom | A local maximum cannot prevent an ADU of at least 1,000 sq ft. | Interior livable space |
| Detached ADU, statewide by-right baseline | State law protects the ability to construct at least an 800 sq ft ADU subject to applicable state standards, including state setback and height provisions. | Interior livable space |
| Attached ADU | A local percentage limit, such as 50% of the primary dwelling, cannot reduce the allowed ADU below the state-protected 850 / 1,000 sq ft minimums where applicable. | Interior livable space |
| JADU | No more than 500 sq ft and contained within a single-family residence. | Interior livable space |
Two more SB 543 changes worth knowing
A 15-business-day completeness check
A permitting agency must determine whether an ADU or JADU application is complete and provide written notice within 15 business days after receiving it. If the application is incomplete, the written notice must identify the incomplete items and explain how the applicant can make the application complete.
If the agency does not make the required timely completeness determination, state law provides a process for the application to be deemed complete. This is a completeness check, not a requirement to issue the full permit in 15 business days.
For a qualifying completed ADU application involving an existing single-family or multifamily dwelling, the normal state ministerial approval or denial timeline is generally 60 days. Separately, the City of San Diego states that qualifying applications using preapproved detached-ADU plans are subject to a 30-day review under its preapproved-plan program. That 30-day program is separate from SB 543.
SB 543 also limits completeness-review comments so an applicant is not repeatedly given entirely new completeness requirements that were not part of the original application checklist.
Fee protections, clarified
SB 543 ties the relevant size thresholds directly to interior livable space and keeps impact fees separate from other project costs.
| Unit size | What state law says |
|---|---|
| ADU, 750 sq ft or less | No impact fees based on the ADU's interior livable space threshold. |
| JADU, 500 sq ft or less | No impact fees based on the JADU's interior livable space threshold. |
| ADU above 750 sq ft | Impact fees may be charged proportionately in relation to the primary dwelling, subject to applicable law. |
| ADU or JADU under 500 sq ft | State law specifies that it does not increase assessable space for purposes relevant to school developer fees. |
Impact fees are only one category of project cost. Permit fees, plan-check fees, utility connection or capacity charges, school developer fees where applicable, and other project costs are separate. The useful change is that the state thresholds are now expressly tied to interior livable space.
What this means in San Diego specifically
City of San Diego
The City must apply the state interior-livable-space standard when reviewing ADU and JADU size limits and the related state fee thresholds. The City currently identifies SB 543 as effective January 1, 2026.
SB 543 does not change the number of units allowed under the City's ADU Home Density Bonus Program. Unit count, lot coverage, floor area ratio, height, setbacks, fire requirements, coastal rules, and other development standards remain separate issues. The new measurement standard may affect how an individual ADU's regulated size and applicable fee thresholds are calculated.
Unincorporated San Diego County
State SB 543 requirements also apply in unincorporated San Diego County. Property-specific zoning, development standards, utility conditions, fire requirements, and County permitting rules still need to be reviewed for the individual lot.
North County and coastal jurisdictions
SB 543 is state law, so its statewide ADU provisions apply in incorporated cities such as Encinitas, Carlsbad, Oceanside, and other San Diego-area jurisdictions. Local development standards and coastal permitting requirements can still apply. SB 543 does not replace California Coastal Act review or local coastal-program requirements.
How we use SB 543 on your project
- Feasibility and size limitsWe confirm the applicable state and local ADU standards for your property and jurisdiction.
- Design around interior livable spaceWe lay out the ADU using the current state measurement standard while also accounting for setbacks, lot coverage, structure size, utilities, access, and other site constraints.
- Prepare a complete permit packageWe prepare the required plans and documentation so the permitting agency can perform its completeness review.
- Review fee thresholdsWe check whether the proposed interior livable area falls within relevant impact-fee and school-fee thresholds.
- Design through constructionWe carry the approved design into construction under the same design-build process.
Want to know what SB 543 means for your lot?
We'll review your property's jurisdiction, applicable ADU size standards, design constraints, and fee thresholds during a free feasibility check.
Get a free feasibility check Call (619) 404-0125SB 543 FAQ
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Does SB 543 change the City of San Diego ADU Bonus Program?
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Keep exploring
Not legal advice. This page summarizes California SB 543 and related ADU law for general informational purposes. ADU requirements can vary by jurisdiction and property, and permitting rules change over time. IL Total Design & Build provides design, permitting coordination, and construction services; it is not a law firm or government permitting authority. Prepared by IL Total Design & Build, CSLB #1058676. Last updated October 2026.
