We bought a new house in a new housing development. This was Phase I. The house was $300K when we bought it in 2018. Taxes were paid based on that value. In 2021, the county has said that they were retroactively raising the value to reflect current values and that we owed back taxes. How is this legal? The house was sold in 2021 and we moved out of California. They are calling the alleged back taxes an unsecured debt that is collectible for ten years. I truly have no intention of paying as they cannot go after my current property in another state. But I would like to know how it is legal to go back in time and raise the value based on current day values.
If a question is "is it legal" then you probably want to talk to an attorney. If the amounts are significant to you then that's what I'd advise you do.
Generally, in California reassessments are triggered by a change of ownership and some other events, other than that the value for property tax cannot rise by more than 2% a year (thanks to Proposition 13). That said, at the change of ownership event they can reassess as they see fit, and you can argue against their valuation. It does sometimes take a long time for them to actually go through the process.
If they think that you purchased the property below market values then the situation you're describing is possible. You'll need to show that your purchase was an arms-length transaction and bring some additional evidence from independent appraisers.
Talk to a local (to that county) real estate attorney who specializes in property tax disputes.