Unpermitted ADU Rules in California

Across California, thousands of homeowners have built accessory dwelling units (ADUs) without permits. Some were motivated by housing needs, others by rental income, and many by frustration with complicated permitting systems. Until recently, these units existed in a legal gray area, exposing owners to fines and enforcement. In January 2025, Assembly Bill 2533 (AB 2533) went into effect, promising a clear statewide process to bring unpermitted ADUs “out of the shadows.” But while the law creates a path to legalization, its implementation looks very different from city to city.

AB 2533: What the Law Says

AB 2533 was designed to simplify the process of legalizing unpermitted ADUs and apartment conversions. The bill directs jurisdictions to adopt inspection checklists based on Section 17920.3 of the California Health and Safety Code, which defines substandard housing conditions. The intent is straightforward: cities should inspect unpermitted ADUs only for health and safety issues, not for zoning or design details.

On paper, the promise is simplicity and consistency. In reality, homeowners will face very different processes depending on their city.

The Role of Health and Safety Checklists

Every city adopting AB 2533 has created its own version of a health and safety inspection checklist. These checklists are the documents inspectors use to determine whether an unpermitted ADU has conditions that make it unsafe.

The length and style of these checklists vary widely. Anaheim uses a one-page form that appears simple but is highly prescriptive. Other jurisdictions publish ten-plus page checklists that are more detailed but, paradoxically, sometimes less strict. Understanding whether your city has a “friendly” or “unfriendly” checklist is the first step in preparing for legalization.

Examples of City Differences

Natural Light Requirements

• El Dorado County takes a straightforward approach by omitting natural light minimums altogether, recognizing that the code does not require them for existing ADUs.
• Vacaville includes the language “minimum amounts of natural light required by this code” even though the code provides no specific requirement.
• Milpitas follows Section 17920.3 closely, which lists “lack of minimum amounts of natural light and ventilation required by this code” as a condition that can render a building substandard—but it does not add extra quantitative requirements such as percentages or thresholds, staying true to the general code language.
• Concord likewise includes the phrase “lack of minimum amounts of natural light and ventilation required by this code” in its checklist—mirroring the code without inventing stricter requirements(https://www.cityofconcord.org/DocumentCenter/View/12739/Unpermitted-ADU-Substandard-Conditions-Checklist-PDF).

Mold and Moisture Rules

• Vacaville simplifies the requirement to “visible mold growth,” a clear and enforceable standard.
• Anaheim deviates completely, requiring inspectors to verify a vapor barrier under the slab—something impossible to confirm without destructive testing.
• Milpitas relies on the explicit phrasing from Health & Safety Code Section 17920.3: “visible mold growth… excluding the presence of mold that is minor and found on surfaces that can accumulate moisture as part of their properly functioning and intended use” (https://www.milpitas.gov/DocumentCenter/View/7100/Health-and-Safety-Code-Section-179203-substandard-building-2). This means Milpitas keeps the technical nuance—distinguishing between serious mold and expected moisture accumulation—without introducing unworkable new testing requirements.
• Concord also reflects this nuanced standard: “visible mold growth, as determined by a health officer or a code enforcement officer… excluding the presence of mold that is minor and found on surfaces that can accumulate moisture as part of their properly functioning and intended use”(https://www.cityofconcord.org/DocumentCenter/View/12739/Unpermitted-ADU-Substandard-Conditions-Checklist-PDF).

Final Clauses and Exceptions

• Vacaville includes protective language reminding inspectors not to require changes that would make a structure less safe, such as removing load-bearing walls.
• Anaheim, by contrast, warns that inspectors may cite additional issues not listed in the checklist, leaving homeowners with uncertainty even after passing an inspection.
• Milpitas offers a balanced, pragmatic approach through its Safe ADU Legalization Program: after an initial courtesy inspection, homeowners may apply for a five-year delay of enforcement on non–health-and-safety code violations (e.g. design or zoning issues) even if those issues are identified—allowing time to address them while avoiding immediate punitive enforcement (https://www.milpitas.gov/277/Safe-ADU-Legalization-Program). This provides homeowners with flexibility and protects against hasty punitive actions that go beyond critical safety concerns.
• Concord echoes the Health & Safety Code’s exception clause verbatim in its checklist: "[A] condition that would require displacement of sound walls or ceilings to meet height, length, or width requirements ... shall not by itself be considered sufficient existence of dangerous conditions making a building substandard..." (https://www.cityofconcord.org/DocumentCenter/View/12739/Unpermitted-ADU-Substandard-Conditions-Checklist-PDF) This insertion ensures inspectors and homeowners alike understand that structural adjustments, when not dangerous, do not automatically signal non-compliance—aligning closely with the state code’s intent.

Case Study: Milpitas (Santa Clara County)

Milpitas provides an excellent example of how AB 2533 is being implemented in practice. The city has published a “Safe ADU” AB 2533 Fact Sheet (https://www.milpitas.gov/1329/Safe-ADU-AB2533-Fact-Sheet) outlining the rules for property owners seeking to legalize unpermitted ADUs or JADUs.

Key highlights from the Milpitas approach include:

• No denial of permits based on past violations. If your unpermitted ADU or JADU was built before January 1, 2020, the city cannot deny a legalization permit solely because the unit does not comply with current building standards or local ordinances.
• No impact or connection fees. Property owners are not required to pay impact fees or utility connection charges unless utility infrastructure upgrades are necessary to correct health and safety issues under Section 17920.3.
• No penalties for past unpermitted work. Applicants will not be fined simply for having the unpermitted unit. Instead, the city is required to approve permits that bring the unit up to health and safety standards.
• Inspection and recommendations. When an application is filed, a city inspector may visit the property to assess compliance with health and safety standards. Instead of issuing automatic violations, the inspector provides recommendations on how to correct any issues.
• Milpitas’ fact sheet reflects the intent of AB 2533 by focusing on health and safety while reducing financial and legal barriers. For homeowners in Santa Clara County, this represents a relatively supportive pathway to bring unpermitted ADUs into compliance.

Beyond the Checklist: Documentation Requirements

The inspection checklist is only one part of the legalization process. Cities may also require supporting documentation such as:

• Full construction drawings and floor plans.
• Plot plans showing the ADU’s placement on the lot.
• Proof that the ADU was built before January 2020, to qualify under AB 2533.

These additional requirements can make the process more burdensome, especially for homeowners who built their ADUs years ago and lack records.

Best Practices for Homeowners

• Study your city’s checklist and compare it to the state Health and Safety Code. This helps you understand whether local rules align with the law.
• Prepare documents early. Even if not listed on the checklist, many cities will request plans, permits, or proof of construction date.
• Look out for overreach. If a city adds unrelated requirements (like mobile home rules), ask for clarification.
• Seek professional guidance. Local inspectors, housing consultants, or ADU specialists can help navigate the process efficiently.

Key Takeaways

AB 2533 establishes a long-awaited path for legalizing unpermitted ADUs in California. But implementation varies widely: some cities create streamlined, reasonable checklists while others add confusing or overly strict requirements. Homeowners who understand both the state code and their local checklist will be better equipped to protect their rights and bring their ADUs into compliance.

Conclusion

Legalizing an unpermitted ADU may not be as simple as AB 2533 initially promised, but it is more achievable than ever before. By learning your city’s approach, preparing the right documentation, and knowing how to interpret inspection checklists, you can navigate the process successfully. While the rules differ across California, the opportunity to secure long-term, legal use of your ADU is now firmly within reach.