If you look at tax records, you should see a fairly consistent range of assessed land value as a proportion of total assessed value (land + improvements), for SFR property in a defined market area. In my area, it's typically 20-25%. Of course, assessed value is often a far cry from market value.
Now, here's a question. Should we expect those proportions to remain fairly constant over time, as market values change? In other words, if we value them separately, should the land's market value change at roughly the same rate as the improvement's? Why or why not? Which value is more volatile, land or improvements?