Hello Shawn,
Even if it appears to be original square footage it may still have been an addition. It could even be a permitted addition that the county has not accounted for. Have you checked with the city to see if there was ever any permits pulled for an addition? They might even have the original plans as it is a fairly newer home. The county might be wrong, prove them they are so you can get the value from the sqft.
It has happened to us twice, where the home had a permitted addition and the county tax assessor did not match the new sqft. We went through the process to prove to the county that the square footage was legitimate.
The first it was pretty obvious that the added sqft was probably at one time a porch or patio. However it was a LEGAL addition. It was a short sale, the short sale lender most likely gave us a price based on what was on record for taxable sqft at the county. When we were ready to rehab, we did research and found the permits. We showed the assessor and they updated the property's legal sqft. We gained a little bit of value for the extra space.
The second had a full 2nd unattached unit, it was actually permitted by the county to be built in 1950's. The property was being marketed as a duplex with sqft by owner, and made sense as a rental. Therefore we purchased it cash with less regards to sqft and more into rooms/bathrooms. We received a code violation notice before starting the rehab stating we had an unpermitted structure! Wow did this scare us. We did research, finding records showing that it was all legal. LA county updated the property profiles to reflect the new found square footage. On this one, the benefit was not as great as we already paid cash for the property as a duplex. It will benefit a refi.
This has been my experience in Southern California - LA County and San Bernardino County.
Good Luck! Hope all the best!