In my experience, state property tax assessments do a decent job at trying to calculate relative values but a terrible job at defining actual property values. Meaning, they may pretty reliably value my house at 10 or 15% less than the house next door based on age or size, but the actual value they put on either house isn't even close to what it would sell for (I've always seen tax assessments come in much lower than market rate).

I don't know how that plays out with mansions though. Whether a mansion is worth $30M or $10M is often hard to predict with the pool of potential buyers being so low.

Property tax assessments are rarely fair market value. They are at best a very gross approximation.

But yes, a tax on "unrealized gains" basically amounts to a property tax, not anything related to an income tax.

> But yes, a tax on "unrealized gains" basically amounts to a property tax, not anything related to an income tax.

The main difference being that a property tax only takes into account the assessed value and ignores what you paid for it. They tax the value, not just unrealized gains.

Yeah, I just meant it is more similar to a property tax than an income tax. Of course the other difference is that you might be able to deduct the tax you paid if the value drops back down before the gain is realized... but I haven't heard enough of the proposed implementation details to sort that out.