It’s the depreciated value so for businesses that’s easy - the same way as they do it now. For households the main asset is the house - we have market values, RVs and insured values. So long as councils get their act together then RVs will become more tractable and so between the 3 alternatives you can triangulate to get a value. Or you could just opt for the RV which I’m sure will have overs and unders over time compared to realised sales figures.