Anyone who gives you a clean price-per-square-foot rule for ADUs is overselling their data, in the Bay Area or anywhere else. The honest reason: there aren't many comparable sales with ADUs to study yet, and ADUs vary enormously. One is a full second dwelling with a kitchen, bath, and living space; another is little more than a room with a sink and a toilet. Those aren't worth the same, so you can't take the main home's price per square foot and multiply it across the added footage. What actually drives an ADU's value: - The local market. - What the unit can legally be used for. A permitted, rentable unit that produces income is worth far more than a bonus room. - What a genuinely comparable property with a similar addition recently sold for. Construction cost doesn't convert to value one for one either. Spending, say, $100,000 to build an ADU doesn't automatically add $100,000 at appraisal; appraisers work these case by case from what the market and comparable sales support. If rental income is part of your plan, confirm the local rules on renting the unit before you build. On a purchase, the appraisal is where the real number gets settled.