The information you may find on this site or others like them is only as good as the source, and the source is sometimes inaccurate from inception. This question brings me back to a similar situation I was obliged to work on many years ago when the property owner who hired me to list and sell his 3- unit building in Daly City, and he was surprised to discover (through public record information), that the property (APN - "Assessors Parcel Number") still showed a single family dwelling on the site!
The situation didn't cause me much concern as it was abundantly obvious that the property, which several years back had been razed and converted into a 3-plex, WITH PERMITS, had for some reason not been updated in every possible listing. This notwithstanding, the updating of information is important, but, if you have the correct information, all the required paperwork, and permits, etc., it is an administrative concern to be corrected.
In your case, however, it is important for a variety of reason -- less square footage will mean a potential reduction to the fair market value as compared to other properties. On the other hand, if you have this item adjusted now, it could also increase your property taxes, as these are based on the same information.
You'll have to research the county and have them open a case to review the permits, building records, and any other pertinent information to correct this error, (if, in fact, it is).... more