Question 1

What exactly is an ADU in California?

Accepted Answer

An accessory dwelling unit (ADU) is a self-contained home with its own kitchen, bathroom, and sleeping area on the same lot as a primary residence. In California, ADUs can be detached, attached, or created by converting existing space such as a garage. Junior ADUs (JADUs) are a related, smaller option created entirely within a single-family home and max out at 500 sq ft.

Question 2

How many ADUs can I build on a single-family lot in California?

Accepted Answer

State law requires cities to allow a combination that commonly yields up to three units on a single-family lot: one JADU (interior only), one detached ADU that fits the state 800-sq-ft standards, and one ADU created within existing space. Whether all three can be combined depends on meeting the statewide exemption criteria and local objective standards. Always confirm your local ordinance and lot conditions before planning the exact mix.

Question 3

How many ADUs are allowed on a multifamily property?

Accepted Answer

For a lot with existing multifamily buildings, California law allows interior conversion ADUs in non-livable areas (at least one, and up to 25% of the existing unit count). In addition, as of 2025, the state allows up to 8 detached ADUs on existing multifamily lots, provided you do not exceed the number of existing primary units. On lots with proposed (new) multifamily buildings, up to 2 detached ADUs are allowed, plus interior conversions when the building exists.

Question 4

What size ADU can I build in California?

Accepted Answer

Cities must allow at least 850 sq ft for a studio/1-bedroom ADU or 1,000 sq ft for ADUs with 2+ bedrooms. Separately, California's 'statewide-exempt' detached ADU must be approved at up to 800 sq ft with 4-ft side and rear setbacks, even if lot coverage, FAR, or front setbacks would otherwise block it. Many cities allow larger footprints, but these minimums apply statewide.

Question 5

What are the standard ADU height and setback rules?

Accepted Answer

Side and rear setbacks for ADUs can be no more than 4 feet. Minimum height allowances depend on context: at least 16 ft; at least 18 ft if the lot is within ½-mile of major transit or the ADU is above a garage, or for detached ADUs on lots with a multifamily, multistory building; and up to 25 ft (or the zone's primary height if lower) for ADUs attached to the primary home. Local codes can be more generous but not more restrictive than these floors.

Question 6

Do I need a permit, and how long will approval take?

Accepted Answer

Yes. ADUs are approved ministerially—no discretionary hearing—if you meet objective standards. Once your application is complete and there is an existing home on the lot, the city must approve or deny within 60 days (and a missed deadline can trigger deemed approval). If you use a locally pre-approved plan for a detached ADU, the turnaround is even faster: 30 days from a complete submittal.

Question 7

Do California ADUs require parking spaces?

Accepted Answer

Often no. Parking cannot be required if your ADU is within ½-mile walking distance of public transit, within a historic district, part of the primary home, or created by converting existing space (like a garage). If you convert a garage or carport, cities also cannot require replacement parking for spaces you remove to build the ADU.

Question 8

Will I need fire sprinklers or other major upgrades?

Accepted Answer

If the primary home does not require fire sprinklers, your ADU generally cannot be required to add them solely because you're building the ADU. Creating a conversion ADU also does not trigger sprinklers in an existing multifamily building. You'll still need to meet current building and safety codes for the new unit.

Question 9

What fees apply to ADUs in California (impact, school, and utility)?

Accepted Answer

Local 'impact fees' are prohibited for ADUs under 750 sq ft; above that size, fees must be proportionate to the primary home. School fees can be charged only if the ADU is larger than 500 sq ft. For conversion ADUs and JADUs, agencies generally cannot treat the unit as a new residential use for water/sewer capacity charges; any allowed connection fees must be proportionate to the ADU's burden.

Question 10

How much does it cost to build an ADU in California?

Accepted Answer

Costs vary widely by size, design, site work, and region. As a rough 2024–2025 snapshot, many detached ADUs land in the ~$150,000–$400,000+ range statewide, with common all-in costs of roughly $300–$600 per sq ft in high-cost markets. Conversions and preapproved/prefab designs can be less, while custom, larger, or hillside projects trend higher.

Question 11

Can I rent my ADU as a short-term rental (Airbnb)?

Accepted Answer

For ADUs approved under the statewide-exempt provisions, the minimum rental term is more than 30 days; they cannot be used as short-term rentals. Cities can separately regulate other ADUs, and many also prohibit ADU rentals under 30 days. Plan on leasing for 30+ days unless your local code clearly says otherwise.

Question 12

Do I have to live on the property (owner-occupancy)?

Accepted Answer

California removed owner-occupancy mandates for ADUs—neither the main home nor the ADU must be owner-occupied. JADUs are different: an owner-occupancy covenant is still required (with limited statutory exceptions). If you want flexibility, build an ADU rather than a JADU, or be prepared to meet the JADU owner-occupancy rule.

Question 13

Can I sell my ADU separately from the main house?

Accepted Answer

Possibly, but only if your city or county opts in. AB 1033 (effective 2024) lets local agencies adopt ordinances to condo-map and allow separate sale of ADUs; without that local ordinance, separate sale is not permitted (aside from limited nonprofit/TIC scenarios in state law). Check your local code to see if separate conveyance is enabled.

Question 14

Can my HOA block my ADU?

Accepted Answer

Homeowners associations cannot unreasonably restrict ADUs or JADUs. They can apply reasonable, objective rules (for example, design guidelines), but outright prohibitions or rules that effectively prevent a compliant ADU are not allowed under California Civil Code. Always review your CC&Rs, but state protections are strong.

Question 15

What happens if my city's ADU rules are stricter than state law?

Accepted Answer

State law controls. If a local ordinance conflicts with state ADU statutes, it's preempted, and HCD can require the city to fix it. Your completed ADU application must be approved or denied ministerially within the statutory timelines, and if a city misses the deadline, approval may be deemed granted under state law.

Question 16

How long does the feasibility/analysis phase usually take?

Accepted Answer

There's no state-mandated timeline for private feasibility studies; most homeowners complete zoning checks, utility scoping, and rough budgeting within a few days to a couple of weeks. The firm timeline starts once you submit a complete application: 60 days for ministerial approval, or 30 days if you use a locally pre-approved detached ADU plan. Your city may also publish pre-approved plans to speed design and review.

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