Accurately estimate the cost of building land

I am not looking for a property and still see such plots on many walks, waiting for the grandchildren because the children ended up building elsewhere. Or where the children got divorced before they could build. Now the owners are in their mid-seventies or older and just don't feel like advertising the plot – or would rather have a meadow next to them for the rest of their days than a house belonging to someone else. That's why they are often not approachable for real estate agents, but quite open to sympathetic buyers with their own building plans.