This section is from the book "Practical Real Estate Methods For Broker, Operator & Owner", by Thirty Experts. Also available from Amazon: Practical Real Estate Methods for Broker, Operator, Owner.
If it is a plot 25x100 up to 50x100 and has no improvements in the line of buildings that, if occupied, will yield an income sufficient as a carrying value, 5% of the market value is added as plottage, and the building or improvement loses its identity. If the same conditions prevail with a plot 50 feet front by 100 feet in depth, or more, and the building or buildings are not an adequate improvement, the buildings entirely lose their value, and 10% of the market value of the entire plot is added as plottage. But if the buildings are an adequate improvement, then he proves the value of the plot of ground, either one lot or more, and the buildings add so much to the value of the land.