The California Tax Nazis (AKA State Board or Equalization) main goal is to "equalize" everyone by removing their money. My experience was that  shortly after registering my airplane I received a notice of assessment for $450,000 for the value of the plane. Apparently someone at the tax office called the factory posing as a kit buyer and asked how much a finished IV-P may be worth and they went with the high end of the range. I explained to the "english as a second language" lady at the tax office that  I built the plane myself and that I had spent only a fraction of that on the parts and materials. 

This was back in '95  when there were only two dozen IV's flying and only four Ps so I asked for guidance on assessing the value of my plane. She confirmed that the value of the plane was equal to the value of the parts at the time the plane was registered. I collected up the receipts that predated the registration, made copies and sent them in with a letter document what the tax lady said. I paid taxes on that (low) amount until two years later when I was sent a reassessment (much higher) after I had been interviewed for a newspaper article where I made the unfortunate mistake of  boasting about the value of the IVs. It never occurred to me that anyone in the assessors office read newspapers. Ah, the cost of pride.

At that time the CA assessor determines a plane's residency in CA by where it is hangered on January 1 of that year.  Since we were planning to move to Idaho in March and I didn't want to give CA ANOTHER stack of Benjamins, I rented a hanger at COE in December and used that as my home base. CA still tried to send me a tax bill because I was still renting  a hanger in CA. I resolved this by sending in a copy of a sublease agreement showing I had rented the CA hanger to a friend of mine.

Now my airplane lives in Idaho where it is taxed at a penny a pound gross ($33.00 per year).

Good luck.

Brent Regan