California ADU Laws: What You Can Build (and the Tax Impact)

California state law (Government Code §65852.2, plus the junior-ADU statute §65852.22) requires every city and county to allow accessory dwelling units on residential lots and to approve a complying ADU ministerially — meaning by-right, without a public hearing or discretionary review, generally within 60 days. The state has steadily overridden local restrictions, so a proposal that meets the state standards must be approved even where a city once banned or limited ADUs.

Under state law a detached ADU of up to 800 sq ft (16 ft tall, with 4-ft side/rear setbacks) must be allowed on essentially any residential lot. Larger ADUs are generally allowed up to about 1,200 sq ft detached, or up to 50% of the primary home's floor area for an attached ADU, subject to your local ordinance. A junior ADU (JADU) is capped at 500 sq ft and must be created within the existing walls of the main home.

Yes, but only partly. Building an ADU is 'new construction,' so the county assessor adds the ADU's market value to your assessment as a new base-year value — but the rest of your property keeps its existing Proposition 13 value. This 'blended' assessment means your tax goes up by roughly the ADU's value times your area's rate, not a reassessment of the whole property. Converting existing space to a JADU adds less.

Often not. State law bars cities from requiring replacement or new parking for an ADU in many situations — including when the ADU is within a half-mile walking distance of public transit, in a historic district, part of the existing primary residence or an accessory structure, or where on-street permit parking is required. Otherwise up to one space per ADU (or per bedroom) may be required.

If your property is on a septic system rather than a sewer, an ADU can require a percolation test and possibly a larger or upgraded system, since it adds bedrooms and wastewater load — a real cost to budget on rural parcels. Local health departments handle this. On sewer, you connect the ADU to the existing service (a connection fee may apply).

No. Cities and counties must permit ADUs and can only adopt reasonable objective standards within the limits state law sets. If a local ordinance conflicts with state ADU law, the state law controls, and the state Department of Housing and Community Development (HCD) reviews local ordinances for compliance.