Short answer: in California, an accessory dwelling unit (ADU) that meets your city's objective standards must be approved ministerially — no public hearing — and the city has to approve or deny a complete application within 60 days when there is already a house on the lot. State law also limits how small a city can make its size, height, setback and parking rules. What it does not do is remove the need for a building permit: every ADU still goes through plan check, inspections and a final sign-off, and your city's own ADU ordinance fills in the details.

This guide summarizes the statewide rules that apply in both Greater Los Angeles and the Bay Area, explains where local rules take over, and lists the questions to settle before you pay for drawings. It is a plain-English summary, not legal advice — the Government Code sections and the state's ADU Handbook linked at the end are the authority, and your city's planning counter has the final word on your lot.

The Four Kinds of ADU

  • Detached ADU: a new, separate building — the classic backyard cottage.
  • Attached ADU: an addition built onto the existing house with its own entrance, kitchen and bathroom.
  • Conversion ADU: an ADU created inside existing space — most often a garage, but also a basement, an accessory structure or part of the house.
  • Junior ADU (JADU): a unit of no more than 500 square feet of interior livable space created within a single-family home. It needs its own entrance and an efficiency kitchen, but may share a bathroom with the main house.

Which type fits is usually decided by the lot, not by preference: a garage that is already in the right place is often the most direct route, while a detached unit depends on yard depth, utility routes and access for construction. Our ADU and garage conversion service page explains how we assess this on a site visit.

How the Approval Process Works

State law requires ADU applications that meet objective standards to be reviewed ministerially, without discretionary review or a hearing. Under Government Code §66317, the permitting agency must approve or deny a complete application within 60 days if there is an existing single-family or multifamily dwelling on the lot; if it does neither, the application is deemed approved. If the application is denied, the agency must return written comments listing what is deficient and how it can be fixed. Two details matter in practice:

  • The clock starts with a complete application. Missing drawings, reports or fees mean the 60 days have not started. A careful first submittal is the single biggest schedule lever a homeowner controls.
  • Corrections are normal. Plan check comments and resubmittals are part of the process, and the applicant can ask for a delay, which pauses the clock.

Every city must also offer a program for pre-approved detached ADU plans (AB 1332, 2023). Choosing a pre-approved plan can shorten design and review, but the site work — utilities, grading, foundation, and fire-department requirements — is still specific to your property.

A typical sequence looks like this:

  1. Feasibility: confirm zoning, lot lines, easements, utility capacity and any hillside, fire-zone or coastal-zone overlays.
  2. Design and engineering: architectural drawings, structural calculations, energy (Title 24) compliance, and any soils or survey work the site needs.
  3. Plan check: submit, respond to corrections, pay fees, and receive the building permit.
  4. Construction and inspections: foundation, framing, rough mechanical/electrical/plumbing, insulation, drywall and final inspections.
  5. Final sign-off: the certificate of occupancy or final inspection approval, after which the unit can be lived in.

How Big Can an ADU Be?

Cities may set minimum and maximum sizes, but under Government Code §66321 a city cannot set a maximum below 850 square feet of interior livable space, or 1,000 square feet for an ADU with more than one bedroom. A city also cannot use lot coverage, floor-area ratio, open-space or percentage-of-house rules to prevent an ADU of at least 800 square feet with four-foot side and rear setbacks.

The figures you often see quoted — 1,200 square feet for a detached ADU and 50% of the main house for an attached one — are the state's fallback limits in §66314 for agencies without a compliant ordinance. Many cities allow larger units. Since January 1, 2026, size limits are measured as interior livable space (SB 543).

Height and Setbacks

A city may limit height, but not below these minimums in §66321(b)(4):

  • 16 feet for a detached ADU on a lot with a single-family or multifamily home;
  • 18 feet for a detached ADU within one-half mile walking distance of a major transit stop or high-quality transit corridor, plus up to two more feet so the roof pitch can match the main house;
  • 18 feet for a detached ADU on a lot with a multistory multifamily building;
  • 25 feet or the height limit for the main house, whichever is lower, for an attached ADU — and a city is not required to allow more than two stories.

For new construction, a city may not require more than four feet from the side and rear lot lines. No setback can be required for a conversion of existing living area or an existing accessory structure, or for a structure rebuilt in the same location and to the same dimensions — which is one reason garage conversions are popular on tight lots. Front setbacks, building separation, and fire-code clearances can still shape where a new unit goes.

Parking, Fees, Sprinklers and Occupancy

  • Parking: a city may require at most one space per ADU or per bedroom, whichever is less, and cannot require any where the ADU is within one-half mile walking distance of public transit, among other exemptions (§66322). When a garage or carport is converted into an ADU, the lost spaces do not have to be replaced (§66314).
  • Impact fees: an ADU with 750 square feet or less of interior livable space is exempt from impact fees charged by the city, special districts and water corporations; above that, fees are charged in proportion to the size of the main house. Plan check, permit and utility connection charges are separate and still apply.
  • Fire sprinklers: not required in an ADU if they are not required in the main house, and adding an ADU cannot trigger sprinklers in the existing home. Local fire departments still review access and water supply, especially in high fire hazard areas.
  • Owner occupancy: cities may not require the owner to live on the property for an ADU. For a JADU that shares a bathroom with the main house, owner occupancy can be required.
  • Selling separately: an ADU generally cannot be sold apart from the main house. A narrow condominium option exists only where a city has adopted it, so do not plan on it without confirming.

Local Rules Still Matter

State law sets floors and ceilings; your city's ordinance sets the rest. In practice, many of the differences between cities across Los Angeles and the Bay Area come from overlays rather than from the ADU ordinance itself: hillside grading rules, very high fire hazard severity zones, coastal-zone permits, historic districts, sewer and storm-water requirements, and the utility's process for a new or upgraded electrical service. Check these early:

  • your city's current ADU ordinance and its submittal checklist;
  • whether the property is in a hillside, fire, coastal or historic overlay;
  • whether the existing electrical panel and sewer lateral can serve a second unit;
  • whether your homeowners' association has design guidelines (it cannot prohibit an ADU outright, but it can apply reasonable rules).

Common Mistakes

  • Designing before checking the site. A floor plan that ignores the sewer route, the electrical service or a utility easement often has to be redrawn.
  • Converting an unpermitted structure without a plan for it. Existing work may need to be opened up, upgraded or documented.
  • Budgeting only for the building. Utility connections, a panel upgrade, site work and permit fees are real costs — see our ADU cost guide for how they add up.
  • Assuming the 60-day clock started. It starts when the application is complete, not when it is first submitted.

Questions to Ask an ADU Contractor

  • Who prepares the drawings and engineering, and who responds to plan check corrections?
  • What site conditions — utilities, access, slope, soil — could change the price, and how are they priced in the contract?
  • Is the contractor's CSLB license active, and which classification does it carry? (Our contractor licensing guide shows how to check.)
  • What does the payment schedule look like, and is each payment tied to completed work?
  • Who schedules inspections and the utility work, and who coordinates with the city until final sign-off?

Next Step

If you are weighing a garage conversion against a new detached unit, the fastest way to an answer is a site visit: we look at the lot, the existing structures, the utilities and the local overlays, and tell you which options are realistic before you spend money on drawings. Request a free estimate or call (310) 310-4108 in Los Angeles or (415) 316-7307 in the Bay Area.

Sources

  • California Department of Housing and Community Development (HCD), Accessory Dwelling Unit Handbook (March 2026 update) — hcd.ca.gov/building-standards/adu
  • California Government Code §§66314 (standards and fallback limits), 66317 (60-day approval), 66321 (size, height and setback limits), 66322 (parking exemptions) and 66323 — leginfo.legislature.ca.gov
  • SB 543 (2025): interior-livable-space measurement and JADU size, effective January 1, 2026
  • AB 1332 (2023): pre-approved ADU plan programs