An accessory dwelling unit is the most flexible thing you can add to a property you already own: a rental, a place for parents or adult kids, a home office that's actually separate from the house. In Riverside and San Bernardino counties, where lots tend to be bigger than in Los Angeles, a detached unit is usually on the table.
There are three main paths. A detached ADU is a stand-alone building, typically the most expensive but also the most valuable. A garage conversion reuses an existing structure and foundation, which saves money if the slab and framing are sound. A junior ADU (JADU) carves up to 500 square feet out of the existing house and shares a wall.
California law is on your side. Cities must approve or deny a complete ADU application within 60 days, can't require more than four-foot side and rear setbacks for a detached unit, and can't demand replacement parking when you convert a garage. Local rules still apply on top of that, which is why a clean, complete plan set matters more than anything else in the timeline.
Where the budget goes: foundation and framing are the biggest structural line items, followed by utility connections (sewer, water, and electrical service can vary enormously by lot), then rough-ins and finish. The best way to control cost is to decide finishes early and keep changes off the job site once framing starts.
Our advice: start with a site walk. Setbacks, lot coverage, utility locations, and slope decide what fits before any drawing gets made. That walk is free, and it's the difference between an estimate you can plan around and a guess.