An Accessory Dwelling Unit (ADU) is a second, independent home on a single-family lot — detached, attached, or a garage conversion. A Junior ADU (JADU) is a smaller unit (up to 500 sq ft) carved out of the existing house with its own entrance. California has spent years stripping away local barriers, so for many buyers an ADU is the difference between an affordable purchase and an unaffordable one.
The economics are real but not automatic. A detached ADU in Southern California commonly runs $150,000 to $300,000+ to build, and rental income depends on local rents, financing, and how long permitting takes. The state’s ministerial-approval timeline (60 days for a complete application) helps, but utility connection fees, a separate meter, fire-sprinkler triggers on the main house, and septic capacity in rural areas can quietly reshape the budget.
County matters. Setbacks, maximum size, owner-occupancy rules for JADUs, and impact fees vary by jurisdiction, and unincorporated county land follows different standards than an incorporated city. A lot in the wildland-urban interface may also face Chapter 7A fire-hardening requirements on the new structure.
Before you bank on it: confirm the lot’s zoning and overlay, verify utility and septic capacity, get a real build quote — not a per-square-foot guess — and treat projected rent conservatively. An ADU adds value and flexibility; it is not free money.