I think that many assessors do not understand the various types of RV-12s that are in use: ELSA, EAB, non-standard engine, factory-built SLSA, etc. The assessors likely lump all of the various RV-12 values together and come up with average valuations. The tax assessor people are not generally aware of such important market factors and may not be aviation savvy. When I submit the annual tax status form I endeavor to make clear the exact type of RV-12 I have as well as a clear description of the current used condition of the aircraft (e.g., engine and airframe times, exterior, interior, avionics, etc.). Sometimes it works, and sometimes it doesn't -- YMMV.
Ps -- Bob Y's suggestion above may be a useful addition.
David Heal - Windsor, CA (near Santa Rosa)
EAA #23982 (circa 1965) - EAA Technical Counselor and Flight Advisor; CFI - A&I
RV-12 E-LSA #120496 (SV w/ AP and ADS-B 2020) - N124DH flying since March 2014 - 1,060+ hours (as of Aug 2021)!
through June 2022.