California State ADU law (Gov. Code 66310 to 66342): what it allows on a lot
State ADU law (Gov. Code 66310 to 66342) is California law, and the official source is at hcd.ca.gov.
On a lot with a house the city or county must approve, with no hearing and within 60 days, one accessory dwelling of at least 800 square feet with 4 foot side and rear setbacks and a height of 16 feet or more (18 near major transit, 20 next to a two story multifamily building), plus one junior unit inside the house. No owner occupancy rule may be set. Size limits where there is a local ordinance: at least 850 square feet, 1,000 with two or more bedrooms; 1,200 for a detached unit without one.
Whether it applies to one lot turns on where the lot is and what it is zoned; the city, county or state office named in the law decides. Mapped areas are approximate. This is a plain reading of public law with its source, not legal advice.
Free, no account. ParcelSheet shows the lot's mapped size, parcel lines and flood zone, and this law on the lot's California sheet.
Other California laws on a lot: SB 9 lot split and two homes.