Sounds like a case for swapping to land value tax if I've ever seen one. Can't hide land.
We have some such system for funding local government, and the values are, in general, an absurd fiction (in both directions) despite the legal theory that property values should be based on some notion of 'market value'.
One trouble is that when the property market declines, local governments don't take that into consideration, and refuse to reduce property valuations, because that would impact their revenue stream in the direction they dislike.
1) The government should commission an expert panel to investigate a system for assessing land value (or perhaps even crowd source it). A set of descriptors (size, location, if it's next to a sewer treatment plant, etc.) should be determined. Along with sale price records, a system should be designed to predict land values.
B) Every part of the system should be public information, including the set of descriptors, so that individuals can dispute them and have them updated in the records.
C) Per law, the system should be regularly reviewed and updated, so that it is as accurate as possible.
1) That panel exists in a variety of forms. The problem is, how do you nicely estimate land value in an economy were real estate transactions are few (which is happening for the last 10-12 years or so)? In many places it's impossible to estimate due to the above, so the alternative is to rely on very old values which are going to be out of date and don't reflect reality. And you can't put 0 for the land value as that would mean 0 taxes and the state financial situation does not allow that B) That is happening. Dispute is problematic as it takes years for the courts (yes, with the exception of blaring mistakes, the state has no incentive to accept that a land value estimation is wrong, so courts it is) to come up with a decision (never mind the fact that it will be appealed anyway so even more time and money spent on this), but it is happening. C) Again, this is problematic due to the same reason as 1). There aren't really that many real estate transactions to accurately estimate land values. Everything went into a pause for years in a row.