California ADU Guide: what homeowners need to know

What is an ADU?

An ADU, or accessory dwelling unit, is a smaller secondary home on the same lot as a primary residence. In California, ADUs are an important housing option for family use, rental use, and multigenerational living.

The California Energy Commission describes an ADU as an attached or detached residential dwelling unit with complete independent living facilities on a lot with a proposed or existing primary residence.

Why homeowners build ADUs

Most homeowners look into ADUs for one or more of these reasons:

• Housing for parents, adult children, or extended family
• Potential rental income
• Flexible backyard living space
• Better use of an existing garage or conversion opportunity
• Long-term property value and housing flexibility

California's ADU policies are part of a broader housing effort to support smaller and more flexible housing options statewide.

Common ADU types

The most common ADU paths are:

California ADU law basics

California ADU law is statewide. That matters because the basic legal framework does not depend entirely on one city's preference or internal practice. Cities still administer applications and review projects, but their local ordinances must fit within state law.

For homeowners, the main takeaway is simple: there is a statewide ADU baseline, but local review still matters.

Where local rules still matter

Even with statewide ADU law, your city or county can still affect the project in important ways. Local agencies usually control:

• Application checklists
• Submittal formatting
• Department workflow
• Local objective standards where allowed
• Inspection and permit process
• City-specific web guidance and handouts

That is why California ADU Co. always separates statewide law from local implementation. Local practice can change and needs to be checked carefully.

Where SB 9 fits

SB 9 is often mentioned alongside ADUs, but it is not the same thing. SB 9 deals with certain housing developments of up to two primary units in a single-family zone, certain urban lot splits, or both, when statutory conditions are met.

SB 9 requires ministerial approval of certain qualifying projects, but that does not mean every lot qualifies or that SB 9 replaces the standard ADU path.

For many homeowners, SB 9 is relevant only if they are thinking beyond a standard ADU and evaluating lot split or multi-primary-unit options.

Where Title 24 fits

For most homeowners, “Title 24” usually means the California Energy Code side of the project. ADUs are subject to the Energy Code, and the permit application date determines which code cycle applies.

The 2025 Building Energy Efficiency Standards apply to permit applications submitted on or after January 1, 2026. The 2025 code expands the use of heat pumps in newly constructed residential buildings, encourages electric-readiness, and strengthens ventilation standards.

That means homeowners should not treat energy compliance as a late detail. It should be part of early project planning.

What usually shapes cost and timeline

The biggest cost and timeline factors usually include:

• Detached new build vs conversion
• Site access and layout
• Utility connection work
• Grading or structural issues
• City process and review time
• Title 24 and other compliance needs
• How complete and coordinated the design set is

Costs vary too much by city, site, and design to present one statewide number responsibly.

What to do first

A strong first-step checklist for most homeowners looks like this:

1. 1Identify the city or county with jurisdiction
2. 2Learn the statewide ADU basics
3. 3Review your local planning page
4. 4Define your goal for the ADU
5. 5Compare the likely ADU type
6. 6Ask which code cycle applies to your permit timing
7. 7Build the right team for the project stage

If the project may involve SB 9, treat that as a separate review track rather than assuming it works the same way as a standard ADU.

Recommended next steps

From here, most readers should go to one of these pages next: