In brief

  • State law sets the floor. Cities can be more generous than state law, but not stricter on the points it covers.
  • The law was renumbered. ADU rules now live in Government Code §§ 66310–66342. Anything still citing § 65852.2 is out of date.
  • 2026 brought real changes. A cleanup bill (SB 543) changed how size is measured, tightened permit deadlines and clarified unit combinations. AB 1154 narrowed owner-occupancy rules for junior ADUs.
  • A single-family lot can often hold more than one unit. HCD reads the law to require at least a converted ADU, a junior ADU and a detached ADU together, plus a locally regulated unit.
  • The rules are the easy part. Easements, utilities, lot shape and hazard zones decide most real-world outcomes, and public parcel data often can't answer those questions on its own.

What an ADU is

An accessory dwelling unit is a complete second home on the same lot as a primary residence. It has its own permanent space for living, sleeping, cooking and a bathroom. Granny flats, in-law units and backyard cottages are all ADUs under California law (HCD Handbook, Glossary).

The law recognizes four basic forms:

Type What it is State size rule (interior livable space) Worth knowing
Detached A new, freestanding structure on the lot Cities must allow at least 850 sq ft (1,000 sq ft with 2+ bedrooms); 1,200 sq ft default where a city has no compliant ordinance Can be two stories if the height allowance and building code permit it
Attached An addition that connects to the main house, including above a garage or in a new basement Same floor as detached; cities may cap at 50% of the main house, but never below the floor Height up to 25 ft or the zone's limit, whichever is lower
Converted Space carved from the existing house or an existing garage, shed or other accessory structure No unit size limit applies No setbacks required; an accessory structure may grow 150 sq ft for entry and exit
Junior ADU (JADU) A unit of up to 500 sq ft inside the walls of a single-family home, including an attached garage 500 sq ft maximum Needs only an efficiency kitchen; may share the main bathroom

Since January 1, 2026, square footage means interior livable space: bedrooms, bathrooms, kitchens, laundry rooms, closets and interior stairs. Because the cap is measured inside the unit, the building's exterior footprint can be somewhat larger than the number on the permit.

What California guarantees

State law sets minimums that every city and county must honor, including charter cities. If a city's ordinance conflicts with state law, or the city fails to submit it to the state on time, the ordinance can be void and state standards apply instead (HCD Handbook, pp. 29–30).

800 sq ft detached ADU 4' 4' PLAN · REAR YARD 16'min. ELEVATION
The state floor. No local lot coverage, floor-area or front setback rule may block an 800 sq ft ADU with 4 ft side and rear setbacks (Gov. Code § 66321(b)(3)). Detached height is at least 16 ft (§ 66321(b)(4)).
Topic What the state guarantees Code section
Approval Ministerial: no hearing, no discretionary review, objective standards only § 66317
Completeness check City must say within 15 business days whether the application is complete, and list what's missing § 66317(a)(2)
Decision Approve or deny within 60 days of a complete application; a denial must list every deficiency § 66317
Minimum buildable unit No lot coverage, FAR, open space or front setback rule may block an 800 sq ft ADU with 4 ft side and rear setbacks § 66321(b)(3)
Side and rear setbacks No more than 4 ft; none for conversions or rebuilds in the same footprint § 66314(d)(7)
Detached height At least 16 ft; 18 ft within a half-mile of major transit or on a multistory multifamily lot, plus 2 ft to match the main roof pitch near transit § 66321(b)(4)
Parking At most one space per unit or bedroom; none near transit, for conversions, in historic districts and other cases; no replacement when a garage is converted §§ 66314(d)(10)–(11), 66322
Owner occupancy Cannot be required for an ADU § 66315
Impact fees None for ADUs up to 750 sq ft or JADUs up to 500 sq ft; proportional above that § 66311.5
School fees None for units under 500 sq ft of interior livable space § 66311.5(c)(3)
HOAs CC&Rs cannot prohibit or unreasonably restrict an ADU or JADU, and cannot charge fees for one Civ. Code §§ 714.3, 4751

How many units? HCD says single-family lots must be allowed a converted ADU, a JADU and a detached ADU of up to 800 sq ft together, plus at least one locally regulated ADU under § 66314 (HCD Handbook, pp. 17–18). Multifamily lots get converted units in non-livable space (up to 25% of existing units) plus up to eight detached ADUs on existing multifamily lots, capped at the number of existing units.

Junior ADUs changed in 2026. Owner occupancy is now required only when the JADU shares a bathroom with the main house. JADUs can no longer be rented for under 30 days (AB 1154).

SB 9 (2021) is a separate law that allows lot splits and up to two primary homes per lot. It can combine with ADU law, but a city need not approve a lot split if the lot would end up with more than four units total.

What cities still control: the San José example

Cities may add objective standards to locally regulated ADUs: front setbacks, design and materials, lot coverage, maximum size above the state floor. They may not apply those standards to the state-mandated § 66323 units (HCD Handbook, p. 19). Cities can also be more generous than the state.

San José lets owners sell an ADU separately. State law (AB 1033) lets cities opt in to selling ADUs as condominiums. San José adopted its ordinance in 2024, the first city in California to do so (City of San José). The state's first ADU condo sale closed there in June 2026: a 749 sq ft home on Josefina Street (City of San José news release). The path runs through a city checklist and a Public Works parcel map, and the ADU needs a certificate of occupancy before mapping (KQED).

The city also staffs ADU Ally positions to help owners through permitting and condo conversion.

Renting and selling

Renting is the default use. Cities may require rentals of 30 days or longer, and state law now requires that minimum for JADUs. No city may require the owner to live on site for an ADU. For a JADU, the owner must live on site only if it shares a bathroom with the main house.

Selling separately is the exception. An ADU can be sold apart from the main house only in two cases. One is a city that has adopted a condominium ordinance under § 66342, as San José has. The other is a narrow path for qualified nonprofit builders selling to low- and moderate-income buyers under § 66341. Everywhere else, the ADU transfers with the property.

Financing has caught up. FHA, Fannie Mae and Freddie Mac programs can count ADU rental income toward a borrower's qualifying income, under program-specific rules (HCD Handbook, pp. 23–24).

State law guarantees the right to build; it does not guarantee that a particular backyard can hold the building. These are the conditions that most often change the answer on a real parcel:

  • Easements. A recorded utility or access easement can run through the only buildable area. Most easements appear in title documents, not on public parcel maps.
  • Lot shape and access. Narrow, irregular or flag lots can technically meet setbacks yet leave no workable footprint or construction access.
  • Utilities. New detached ADUs may need a separate connection. Electrical panel capacity and sewer lateral condition often drive cost more than zoning does.
  • Hazard and environmental overlays. Fire hazard severity zones, flood zones, slopes, protected trees and the Coastal Zone can add requirements or reviews.
  • Existing unpermitted work. Cities generally cannot deny an ADU over unrelated nonconforming conditions that don't threaten health and safety. Unpermitted ADUs built before 2020 can usually be legalized, with health-and-safety exceptions (HCD Handbook, pp. 32, 41).
  • New construction needs solar. Newly built detached ADUs generally fall under the state Energy Code solar requirement; conversions and additions do not.

For a step-by-step way to check these conditions on any California lot, see how to check ADU feasibility in California. For a San José property, our page on ADU feasibility in San José walks through the City's own checklist questions, from fire access to flood zones.

What this article does not establish

  • Whether an ADU fits on any specific property. That depends on survey, title, utilities and site conditions this guide does not examine.
  • San José's current local dimensional standards. We have not verified them against the municipal code for this article, and we found conflicting third-party summaries.
  • How a city will interpret an ambiguous provision. HCD's handbook is the state's reading of the law, not a court ruling. Cities sometimes disagree, especially on unit combinations.
  • Rules after the review date. California amends ADU law almost every year, usually effective January 1. This article reflects law in effect as of January 1, 2026.

Sources

This article is general information, not legal, architectural or financial advice. Confirm requirements for a specific property with the local planning department and a qualified professional before relying on them.