Will adding an ADU raise my property taxes?

Adding an ADU typically raises your assessed value, since a permitted structure that increases a property's fair market value gets reflected at the next reassessment. The size of that change depends on the local assessor's valuation of the added square footage and any improvements, not a figure a general answer can quote. Property tax is billed on 40 percent of fair market value, so the increase in your bill follows the increase in assessed value. Ask your county tax assessor's office how they will treat the specific addition before you build.