Zillow pulls county record information. It is likely that the addition was not updated by the county. Oftentimes, the city will have the permits and documentation and then it is not properly communicated to the county. What I would recommend is for you to contact the county and see what they will need to make the correction. They will probably have you provide the file from the City of San Mateo in order to make the correction. I hope this is helpful!
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