I'm not sure about Contra Costa County; however, typically the County Assessors Office does not update the characteristics of a remodel (i.e. number of bedrooms, baths, square footage, etc.) but they will certainly provide a Supplemental Tax Assessment for the increased market value created.
In Alameda County they have a form the Owner fills out to updated County records regarding updated characteristics which you can see here: http://docs.Steven-Anthony.com/AlamedaCo-PropCharacteristicChangeForm.pdf
My understanding is Appraisers cannot provide value for unpermitted work.