California ADU rules: a starting point for your project

Which official reference is used here?

The official HCD ADU resource links to its March 2026 handbook. The sources on this page were checked on October 25, 2026. A later law or agency instruction may change what applies; the review date is not a promise that this page updates automatically. California HCD: ADU resources and updates

Before relying on a number or exception, identify your proposal: existing or proposed primary home, single-family or multifamily property, new construction or conversion, and ADU or JADU. A headline rule may address only one of those situations.

Open the official HCD handbook (PDF)

Selected statewide points

Read these with the cited handbook sections
Topic	Selected point	Handbook section
Review	Covered ADUs use ministerial review against objective standards, not discretionary hearings.	Permit applications, pp. 34–36
Completeness	HCD describes a written completeness determination within 15 business days of receipt.	Permit applications, pp. 34–35
JADU occupancy	For JADUs, HCD distinguishes shared from separate sanitation when explaining owner-occupancy requirements.	Owner-occupancy, pp. 32–33
Occupancy approval	HCD states that a Certificate of Occupancy is needed before residential occupancy.	Permit applications, p. 35

These are selected summaries from HCD’s March 2026 handbook. Ask the reviewing agency or a qualified adviser about exceptions and the provisions applicable to your application.

Why this page does not give one size for every lot

A question such as “How large can my ADU be?” needs more context than a statewide maximum copied into a calculator. Ask your design team which approval pathway it is using and what the relevant dimensions mean for the proposed layout.

Have the answer tied to a drawing or concept: where the building sits, what existing space will remain, and which dimensions are still provisional. If you are comparing two design options, ask whether the same pathway and assumptions apply to both.

What kind of unit is being proposed?
Which current rule and approval pathway is the team relying on?
What property facts have been confirmed?
What still needs an agency answer before the design or price is reliable?

The point of this conversation is to make the basis visible. It should not require the homeowner to independently design a code-compliant building.

Do not read a review deadline as a move-in date

A completeness determination, a decision on an application and the full journey to occupancy are different events. Ask which event a timeline refers to and which documents or decisions must come first.

Keep your own schedule in ordinary terms: preparing the design, submitting the application, responding to review, obtaining the needed permissions, building, connecting services and finishing approvals. Ask the project team to identify where the largest uncertainties remain.

If a deadline seems to have passed, keep the submission receipt, written status and agency correspondence. Seek a project-specific explanation rather than assuming that silence gives permission to begin construction. Read the permit stages and records.

Separate permissions from the cost of the work

Ask the team to identify agency fees and utility charges separately from the physical construction. If someone says a fee is waived or reduced, ask which fee, under which rule, for which proposal. A saving on one charge does not tell you whether a connection or upgrade has been priced.

The site quote should show what happens if a provider requires work that has not yet been confirmed. For meters, gas choices and sprinkler questions, use the ADU utilities guide. It keeps those assumptions connected to scope and price.

What still needs a local answer?

Use the official jurisdiction directory to find the agency covering the property. Confirm the current application instructions, any property-specific review and the intended-use requirements for the finished ADU.

Read location caveats before generalizing a local bulletin. San Diego’s Bulletin 400 flags that some provisions have a different effective status in the Coastal Overlay Zone. A rule copied without that qualification could mislead a homeowner. City of San Diego: Information Bulletin 400, January 2026

If two sources appear to disagree

Save both sources and record the exact issue. Ask the project professional or reviewing agency to identify the applicable provision and explain the difference. The source may concern a different unit type, property type, date or approval pathway.

For unresolved state-law questions, the HCD ADU resource offers technical-assistance information. California HCD: ADU resources and updates Do not treat a general article, including this page, as a binding interpretation for your property. Legal disputes or time-sensitive application issues need qualified advice.

Questions about this topic

Are the rules identical in every California City?

Do not assume identical local procedures or site-specific requirements. Establish the state pathway and then read the current instructions for the jurisdiction responsible for the property.

Does this page confirm that I can build an ADU?

No. It explains how to start reading the rules. Your property, proposed design and applicable review still need to be assessed.

Does the review date mean every recent law is covered?

No. It records when the cited sources were checked. This is a selected summary, not an exhaustive legislative update or a guarantee about later effective dates.

Sources and review notes

Official references checked on October 25, 2026. Inline links identify the source for specific statements.

Prepared for PrepareToBuild.info as an editorial draft, assisted by AI. Not yet reviewed by a licensed construction or legal professional. Practical examples are illustrative, not actual quotes, legal terms or completed projects. Confirm requirements with the responsible agency and get qualified advice about your property, contract or finances.