HCD ADU handbook: what California's 2026 guide says

What the HCD ADU handbook is

The Accessory Dwelling Unit Handbook is a 55-page PDF that HCD writes and updates as the Legislature changes ADU law. HCD says it wrote the handbook to help local governments, homeowners, attorneys, architects and the public understand and apply state law. The March 2026 edition references the law as amended by bills that took effect on January 1, 2026.

ADU law has changed almost every year since 2016. HCD says its goal is consistency and clarity in how those laws are applied, and the handbook is its main tool: one document with the current rules and HCD's answers to common questions. It also makes the case for ADUs with numbers: the handbook reports that ADUs permitted each year in California grew from 1,336 in 2016 to 30,354 in 2024, and that ADUs were more than 26.6% of all homes permitted statewide in 2024.

If you only want the rules themselves, our California ADU laws guide covers them in plain English. The handbook is the source to bring when a city reads a rule differently than you do.

What's in the handbook

The accessory dwelling unit handbook has five parts. The FAQ, which runs from page 16 to page 44 and is organized by topic, is the part most homeowners will use.

A few terms come up throughout. HCD uses “66323 Units” for the four kinds of ADUs and JADUs that Government Code section 66323 says a city must approve ministerially, such as one converted ADU plus one detached ADU of up to 800 square feet with 4-foot side and rear setbacks on a single-family lot. Those units are subject only to the standards in that section, so things like lot coverage, front setbacks and design standards don't apply to them. They still have to meet the building code. “Interior livable space” is the measurement state law now uses for ADU and JADU size.

Topics homeowners use most

• Fees. When impact fees can and can't be charged, and how utility connections work. See what an ADU costs in San Diego for the local numbers.
• Height, setbacks and parking. The minimums every city must allow. Our guides to the ADU height limit in California, ADU setbacks and ADU parking requirements go deeper on each.
• Permit timelines. The 15-business-day completeness check, the 60-day decision clock and what a denial must include.
• HOAs and multifamily lots. What an association can and can't restrict, and how many ADUs fit on an apartment property. See ADU HOA rules and multifamily ADUs.
• Unpermitted units. When a city can and can't refuse to legalize an older ADU. See our guide to an unpermitted ADU in California.

What's new in the California ADU handbook 2026

HCD's handbook page says it added an addendum in December 2025 summarizing the changes that took effect January 1, 2026, and the current edition is dated March 2026. The 2026 handbook covers five 2025 bills.

The 2026 edition also lists three renumbered sections, which matters if you're reading older city documents or articles.

Is the handbook the law?

No. The handbook explains and interprets statutes; the statutes are the law. The handbook itself sends readers to the Legislature's website for the complete text, and suggests a local permitting agency or a private attorney for questions it doesn't answer. It also says HCD may adopt guidelines that supplement or clarify state ADU law.

What gives the handbook weight is that the agency that wrote it can enforce the law it describes. According to the handbook:

• Ordinance review. Since 2016, HCD has reviewed local ADU ordinances. A city must send a new ADU or JADU ordinance to HCD within 60 days of adopting it. If HCD finds a problem, the city has 30 days to amend the ordinance or adopt findings explaining why it complies.
• Null and void ordinances. If a city misses either deadline, its ordinance is null and void and only state ADU law applies. While an ordinance is out of compliance, the city must apply state standards.
• Full enforcement since 2024. As of January 1, 2024, AB 434 added state ADU law to the list of housing laws HCD enforces. HCD can notify a city, and the Attorney General, when the city doesn't comply.
• Housing element pressure. Violations of state housing laws, including ADU law, can lead to revocation of a city's housing element certification and referral to the Attorney General. A city out of compliance with ADU law can't qualify for the state's Prohousing Designation.

So when you hand a planner a page of the handbook, you're not citing law. You're showing how the state agency that can escalate the issue reads the law.

Using the handbook with your city

The handbook works best as a reference you cite by page and section. If a plan checker asks for something you think state law forbids, find the FAQ that covers it, note the Government Code section it cites and ask the city which part of its code allows the requirement.

If that fails, HCD takes questions through its online ADU Portal, linked from HCD's ADU page under “Contact the ADU Team.” The handbook sets limits on that help:

• Work through the city's own process first, including its appeal.
• HCD may give technical assistance after an application has been submitted, but may not follow up with a city when no application is pending.
• Give as much detail as you can. HCD may ask for the denial and your correspondence with the city.

Before you write in, search HCD's technical assistance and enforcement letters dashboard by city. If HCD has already told your city its ordinance breaks state law on the point you're raising, that letter is stronger than a general FAQ answer.

What the handbook doesn't cover

The handbook explains the statewide floor. Your project is decided by local rules layered on top, and by facts the handbook doesn't try to address.

Local rules can be more generous than the state floor. In the City of San Diego, for example, the city's ADU bulletin allows attached and detached ADUs up to 1,200 square feet, sets a 31-day minimum lease for ADUs and lets owners sell an ADU separately as a condominium under conditions. The same bulletin says ADUs there are subject to the city's Development Impact Fee requirements. None of that is in the handbook. For the city's process, see our San Diego ADU permit guide.

The handbook also predates the 2026 legislative session. The Governor's office announced on September 29, 2026 that AB 956 and SB 1117, two ADU bills, were signed. The March 2026 edition doesn't reflect them. Our guide to California ADU laws for 2027 covers what they change.

Where to get it

Download the handbook free from HCD's ADU Handbook page. The page also offers the 2026 handbook in Spanish, Vietnamese and Traditional Chinese, a Simplified Chinese version, and recordings of HCD's 2025 and 2026 handbook webinars. The direct link to the English PDF is adu-handbook-update.pdf. Check the date on the cover so you know which edition you have.

Once you know the rules, the next decision is who builds it. Start with how to hire an ADU contractor, check a contractor's license, or compare verified ADU builders in San Diego.

Common questions