California law and local agency review both shape what you can build in your backyard. State standards set the baseline for size, setbacks and approval timing, while local agencies check for code compliance – but local zoning rules cannot be more restrictive than what state law allows. Understanding these requirements before you start can help you avoid costly redesigns and permitting delays.
How state law and local agencies shape ADU approval
The first step is understanding which rules govern your Accessory Dwelling Unit (ADU) approval process – state law, local zoning or both. State law sets minimum standards while local agencies review plans for compliance with local rules. The California Department of Housing and Community Development (HCD) publishes state guidance to help identify common compliance questions.
A qualifying ADU application generally moves through a ministerial approval process where the local agency reviews for code compliance. State law requires the agency to act on complete applications within 60 days of the submission date. State law also preempts local ordinances that conflict with statewide ADU standards.
What California reviewers check before approving an ADU
Early review matters when your design approaches the legal limits for unit size or placement. California law sets a maximum of 1,200 square feet for detached ADUs in most areas. State standards also require 4-foot side and rear setbacks for most detached units.
- Lot and zoning limits affecting unit placement
- Building code compliance for fire, egress and safety
- Utility connections for water, power and sewer
- Access needs such as paths, stairs and address assignment
Utility access and zoning compliance often affect total costs more than the primary floor plan. You can avoid expensive revisions by keeping the architect, contractor and permit drawings aligned.
Tenant considerations when building on an occupied property
Building an ADU where tenants already live raises questions about construction access and your habitability obligations under California law. California law generally requires you to give tenants at least 24-hour written notice before entering. A written agreement about noise and work hours can help reduce potential complaints.
Before you start: Key legal details to confirm
Confirm the permit record and certificate of occupancy terms before you market the unit to tenants. California law treats a junior ADU (JADU) differently by limiting the size to 500 square feet. A real estate attorney can help you clarify legal rights before work begins.