These are illustrative planning ranges for Southern California — not a quote, appraisal, or guarantee. Real cost depends on your lot, site conditions, permits and impact fees, and the contractor you choose; see what drives ADU cost and get real numbers from a licensed contractor. The monthly figure is a simplified amortized estimate — model real loan options in the loan calculator.
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How this estimate works
We start from Southern California's typical hard-construction cost — roughly $285–$350 per square foot for a ground-up detached ADU — and adjust for your ADU type and finish level. Converting existing space (a garage or an interior room for a Junior ADU) reuses structure, so it usually lands below new detached construction. Premium fixtures, cabinetry, and materials push the per-square-foot number up. Multiply by your size and you get a planning range — the same way an experienced advisor sanity-checks a project before you ever see a bid.
What it can't see is your specific lot: foundation and grading needs, how far utilities have to run, permit and impact fees, and the finish choices you'll actually make. Those swing the real number, which is why this is a starting point, not a quote. For the full picture, see what actually drives ADU cost.
Cost estimator questions
It's a planning range, not a bid. It reflects typical Southern California per-square-foot costs by type and finish, but it can't account for your lot's site conditions, utility runs, permit and impact fees, or your exact material choices — all of which move the real number. Use it to budget and sanity-check, then get a written estimate from a licensed contractor.
Because you reuse existing structure — foundation, walls, and roof — instead of building from the ground up. A garage conversion and a Junior ADU (an interior conversion) are typically the lowest-cost paths, while a new detached unit sits at the top of the range.
The headline range is hard construction only. The total-planning line beneath it adds typical allowances for design, engineering, permits, fees and contingency (roughly 25–30%) — see the soft-costs guide and impact-fees guide for detail. Financing costs, furnishings, and unusual site or utility work aren't included, so pad your budget further if those apply.
Sometimes — the low end generally assumes a simpler build, standard finishes, and a cooperative site. Tight lots, difficult access, long utility runs, or premium finishes push you toward the high end or beyond. An honest advisor read on your specific property will tell you which end is realistic before you spend on design.