I'm in Yolo county and last year I received a tax statement that almost doubled the value of the aircraft. I just made a phone call to the assesors office explaining why I thought there assesment was incorrect. I found an ad for the most similar plane I could find, and informed them I would send an email.
I had to pay the invoice that they originally sent to not incur late penalties. They reviewed the previous tax statments, the ad I send with comparisions to my plane, and issued a new statement with a refund of the difference.
I did not get an appraisal. Also, it seems the value of aiplanes have skyrocked in the last year; it may indeed be worth 8k more.
So... start with a phone call.
(started: Feb 2002 --> Completed: May 2016)