@fernando_scharnick thank you for your thoughtful comment.
The foundation of the calculations is this California State manual: https://boe.ca.gov/proptaxes/pdf/pub30.pdf
On the assessment accuracy, there are a few things I haven’t implemented yet. Right now I don’t do % adjustments on the comparable properties to account for minor differences. eg. a comparable property has 1/2 bathroom more, whats the impact on the value.
I implemented a scoring algorithm that matches the document from the link. I then take the $/sqft price of the comparable homes and use the median of that. This is the basis of multiplying your homes sqft to get to the assessed value.
From reading other appeals guides for the assessors office, it’s better to use a weighted algorithm that favors properties that are closer to the subject instead of the median.
Those two things are future improvements to be made.
But the good thing right now, it’s not a deal breaker as the appeal is the home owners “Opinion of Value” and is the basis for a negotiation with the assessor or the basis for the conversation in front of the appeals board (how does all this work anyway – https://www.appealseal.com/content/docs/how-the-appeals-board-works)
From my research with San Mateo county, the general opinion of value of the home owners is median -13%, with the actual reduction being 9-10% after. See: https://www.appealseal.com/content/blog/san-mateo-tax-appeal-in-numbers
Also 97% of the cases settle with a negotiation with the assessor and never go to a hearing. Thats what happened to me in the past two years. The assessor always called me the day before and offered me a number and asked if I’d be fine accepting that without going to the hearing.