Only new construction that adds value gets added to your assessment: additions, ADUs, a new bathroom, a pool, or converting a garage to living space. Repairing or replacing what is already there, like a new roof, a new kitchen in the same footprint or new windows, generally does not trigger reassessment under Proposition 13.
How the assessor finds out
Through the building permit. Once the permit is finaled, the county assessor values the new construction and adds it to your base-year value. The rest of the property keeps its Proposition 13 protection, which is the part people worry about and should not.
Rough math
Los Angeles County's effective rate runs roughly 1.1 to 1.25 percent. An ADU assessed at $200,000 adds about $2,200 to $2,500 a year. Against what the same unit rents for, that is a rounding error, but it should be in your numbers.
Los Angeles County figures, reviewed 2026-10-29 by America General Contractors, California contractor licence CSLB 1083262. Planning ranges, not a quote, and not legal advice. We confirm the current rules for your parcel before anything gets drawn.
