Definition
An accessory dwelling unit (ADU) — also called a secondary unit, in-law unit, granny flat, or garage apartment — is a self-contained residential unit on the same parcel as an existing single-family or multifamily dwelling. ADUs have their own kitchen, bathroom, and sleeping area and are suitable for full-time habitation.
California law distinguishes between three types of ADUs:
- Attached ADU: Physically connected to the primary dwelling (e.g., a converted room addition).
- Detached ADU: A separate structure on the same lot (e.g., a new backyard cottage or converted detached garage).
- Junior ADU (JADU): A smaller unit (up to 500 sq ft) carved out of the interior of the existing single-family home, with its own entrance and efficiency kitchen. JADUs are subject to an owner-occupancy requirement and can share a bathroom with the primary unit.
State Law: Ministerial Approval Required
Under Government Code §§ 65852.2 and 65852.22, California requires all cities and counties to allow ADUs and JADUs by right on any lot that has an existing or proposed single-family or multifamily dwelling. "By right" means the agency must approve a qualifying ADU application ministerially — without a public hearing, without a conditional use permit, and without discretionary CEQA review.
Objective standards still apply — setbacks, height limits, lot coverage — but a city may not require owner-occupancy for ADUs (only JADUs), impose minimum lot size requirements, or deny an ADU based on neighborhood character concerns.
Key Requirements Cities Cannot Impose
- Maximum size below 850 sq ft (600 sq ft for studios/1-bedrooms) for detached ADUs
- Setbacks greater than 4 feet from side and rear lot lines for detached ADUs
- Replacement parking when a garage is converted to an ADU
- Owner-occupancy for standard ADUs (as opposed to JADUs)
- Design standards that are not objective or that prevent construction of an ADU of 800+ sq ft
Why ADU Compliance Is a Moving Target
The Legislature has amended ADU law almost every year since 2017 — adding new protections, removing local restrictions, and expanding ADU rights. Cities that haven't audited their ADU ordinances against current law are very likely applying standards that state law has since preempted. The consequences range from wrongful denials to deemed-approved claims to legal fees.
ADU ordinances are one of the highest-risk areas of California housing law compliance. Legislaide helps cities identify the specific provisions of their ADU ordinances that conflict with current state law — and tracks the annual legislative amendments that keep changing the rules.
Frequently Asked Questions
What is an ADU?
An accessory dwelling unit (ADU) is a self-contained secondary housing unit on the same parcel as an existing residential dwelling — with its own kitchen, bathroom, and sleeping area. California law requires cities to allow ADUs by right on qualifying residential parcels without discretionary review.
What types of ADUs does California law recognize?
California recognizes three types: attached ADUs (connected to the primary home), detached ADUs (a separate structure on the lot), and Junior ADUs (JADUs) — up to 500 sq ft carved from the interior of the existing primary home, which may share a bathroom and is subject to owner-occupancy requirements.
Can cities require a discretionary permit for an ADU?
No. California law (Government Code §§ 65852.2 and 65852.22) requires ministerial approval of qualifying ADU applications — without a public hearing, conditional use permit, or CEQA review. Cities may only apply objective standards and may not deny ADUs based on neighborhood character concerns.
What setback requirements apply to ADUs under California law?
For detached ADUs, cities may not impose setbacks greater than 4 feet from side and rear property lines. For ADUs created by converting an existing structure (like a garage), no new setback is required — the existing structure's setback governs. Cities imposing larger setbacks violate state law.
Do ADU approvals require replacement parking?
No. California law prohibits cities from requiring replacement parking when a garage is converted to an ADU. Cities also may not require new parking for ADUs located within one-half mile of public transit or in certain other circumstances. These parking prohibitions are mandatory — not optional.
Published June 1, 2026 by Legal Team at Legislaide.