For years, there was an 80/20 rule that was a good approximation for most of the country. That was when RE prices grew at a rate of 1% above the rate of inflation. However, beginning in the 1980's with land conservancy trusts, which took a lot of buildable land off the market. the added expense of environmental impast studies, and then the easy financing policies from the 90's on, price growth became skewed particularly in the East and West coastal areas. On the one hand there were actions decreasing supply and increasing costs and financial and lending policies increasing demand.
Now, it is not possible to make estimates of the Building/Land ratios because they are all over the place. So, now, as Catherine suggests, it is best to use property tax statements to get the right ratio.
A good example of both methods is found in a return I prepared recently. The client had lived on the East Coast and was transferred to SF, in 2015, where he bought a rental property. He had his usual preparer in CT prepare the 2015 return and this preparer used the 80/20 rule. However, his property tax statement for 2016 was nearly the reverse of this. It was 30% Building and 70% Land.
There is an interesting interactive chart that shows the relative price changes, from 1980 to 2016, for 3 US cities. You can have a little fun with this clicking on the other cities to the right of the chart . The east and west coast cities showed the largest bubbles; mid-western cities not so much, but Pittsburgh was a straight line, meaning it was barely affected by the housing chaos.
https://www.economist.com/blogs/graphicdetail/2016/08/daily-chart-20