Accessory Dwelling Unit Handbook: What HCD Publishes

What the Accessory Dwelling Unit Handbook actually is

The Accessory Dwelling Unit Handbook is published by the California Department of Housing and Community Development (HCD). The current version is dated March 2026 and runs about 50 pages. The handbook is the only state-level document that puts the whole ADU and JADU framework into plain language, with definitions, FAQ entries, and a Resources section that links to the underlying Government Code sections (Gov. Code, 66310 to 66342).

Homeowners do not have to read the handbook to build an ADU, but the document is the official reference every city planning desk and every HCD technical assistance response leans on. When a city says an ADU application is ministerial, when an HOA says a covenant cannot block the unit, or when a builder says an 800 sq ft backstop applies, the rule can usually be traced back to a section the handbook explains in writing.

The handbook is not a regulation. State ADU law lives in the Government Code, and the handbook cites those sections by number. The legal text controls; the handbook is the readable map. HCD updates the handbook when the law changes. The December 2025 Addendum is folded into the March 2026 PDF.

What changed in the March 2026 update

The March 2026 update adds a December 2025 Addendum that summarizes every bill that took effect on January 1, 2026. The four that matter most to a single-family homeowner are SB 543 (Chapter 520, Statutes of 2025), AB 1154 (Chapter 507, Statutes of 2025), SB 9 (Chapter 510, Statutes of 2025), and the new section numbers carried over from SB 477 (Chapter 7, Statutes of 2024).

SB 543 changed how impact fees apply. Under Gov. Code 66311.5, a permitting agency shall not impose an impact fee on an ADU that has 750 sq ft of interior livable space or less, and any impact fee on a larger ADU must be charged proportionately in relation to the primary dwelling. The same bill moved the 800 sq ft ministerial backstop into Gov. Code 66321(b)(3) and tied size limits to interior living space instead of gross floor area.

AB 1154 rewrote the JADU rules. A JADU with shared sanitation now requires owner occupancy; a JADU with its own bathroom does not (Gov. Code 66333(b)). The same bill adds a 31 night minimum on JADU rentals, so short-term rental platforms can no longer list a JADU (Gov. Code 66333(g)). SB 9 (Chapter 510, Statutes of 2025) lets HCD treat a local ordinance as null and void if the city fails to submit it within 60 days or fails to respond to findings within 30 days, after which the city must apply State ADU Law directly.

The update also tracks the new section numbering. SB 477 re-numbered the ADU chapters in March 2024, and the March 2026 handbook reads from Gov. Code 66310 through 66342. Many older city handouts still cite the pre-2024 numbering, and the handbook's Resources section includes a crosswalk table so a reader can map an older citation to a current one.

Who the handbook is written for

HCD writes the handbook for local governments, homeowners, attorneys, architects, and the public. The Glossary section defines the words the law uses (ADU, JADU, accessory structure, livable space, multifamily dwelling) and ties each definition back to a Government Code subdivision. A homeowner who reads the Glossary first can read the rest of the document without guessing what a term means.

For a single-family homeowner in Orange County or Los Angeles County, the most useful parts are the size and setback entries (size floors and the 800 sq ft backstop), the parking and impact fee entries (one space per ADU, 750 sq ft fee cliff), the owner occupancy entries for a JADU, and the rental term entry for short-term rentals. The Frequently Asked Questions section reads like a checklist, with one short answer per question and the underlying code section at the end.

The Resources section at the back of the handbook is a separate reference. It links to the HAU findings letters, the ADU Portal, the Senate and Assembly bill texts, and a Summary Table of Statutory Changes that lists every bill since 2016 with the affected Government Code section. The Summary Table is the right place to look when a city handout cites a section number that does not match the current law.

What the handbook does not decide

The handbook does not replace a city's zoning ordinance. Gov. Code 66325 lets State ADU Law preempt a local rule that conflicts with the handbook's underlying statutes, but only the specific conflicting section is preempted. A homeowner still has to apply at the city, pay the city's plan check fee, and meet any local standard that is not in conflict with state law.

The handbook does not list a price. ADU cost depends on the lot, the utility routes, the foundation, the design, the permit conditions, and the finish selections. L Square Design & Build's current configured prefab model prices run from $180,000 for a 600 sq ft Model 600 to $354,000 for a 1,200 sq ft Model 1200, and those are model prices, not a final project total. The handbook never quotes a number; it points to the rules a city must follow when reviewing a permit application.

The handbook is not a contractor license. Building an ADU in California still requires a licensed contractor under CSLB rules. The handbook does not endorse any contractor, and the homeowner is responsible for verifying the license, the bond, and the references before signing a contract.

How a homeowner should use the handbook on a real project

Start with the Glossary and the Frequently Asked Questions. Both are short and both are written for a non-lawyer. The Glossary explains what a JADU is, what 66323 Units are, and what livable space includes. The FAQ section answers the size, parking, fee, and rental questions in one paragraph each, with the exact Government Code subdivision at the end of the answer.

Open the Summary Table of Statutory Changes when a city handout cites a section number that does not match the handbook. The crosswalk table maps the new Government Code sections to the old ones, so a homeowner can read an old city reference and find the current rule. The Resources section also links to every Senate and Assembly bill that touched ADU law since 2016, with chapter and statute citations.

For a project on a real lot, the handbook is the second read, not the first. The first read is the property: lot size, slope, sewer route, panel capacity, existing structures, and the city file. The handbook explains what the city is allowed to ask for and what it is not. The lot explains what the project will actually cost. L Square reads both before producing a written proposal for an Orange County or Los Angeles County homeowner.