The latter two are indeed difficult and that's why I would limit it to land.How much is your house worth if it hasn't been sold recently? The painting on your wall? Your share of the family business that isn't publicly listed?
Property value is already used for council tax purposes - and not even the current value but a notional historical value... we have websites such as zoopla that can estimate your current house price to a reasonable level of confidence.
Adopt one of those methodologies, let people challenge the valuation if they disagree (like with council tax), and if you refuse to pay because you think the valuation is too high, the government will purchase your property for that valuation.