Improvement value is commonly estimated, eg by insurance companies. Eg, you insure your building for 100k even though the market price was 300k, because it would only take 100k to rebuild it. So you then know that the 'unimproved' value was 200k. (This ignores dilapidations etc that you'd also have to account for).
This often undervalues the land as a side note - because an empty lot is more valuable than the same land under an “undesirable” structure (because demolition comes at a cost) - even if it’s perfectly serviceable.
