When I did my remodel (4 years ago), the county assessor's office asked a series of questions in order to determine the "market value" of our remodel. They computed the increase in my tax base on the "market value" of our remodel. It turned out that the increase in our tax base was almost identical to the cost of our remodeled times 1.2 percent. For example, if our remodel was 250K, the increase in tax base would be $3,000 a year.
However, please check with the County Assessor's office for the official explanation based on your job.