Final answer:
The principle of value that is illustrated when two adjacent properties combined have a higher value than they do separately is the Principle of plottage.
Step-by-step explanation:
The principle of value that is illustrated when two adjacent properties combined have a higher value than they do separately is the Principle of plottage. This principle refers to the increase in value that comes from combining two smaller parcels of land into a larger, more valuable parcel. It often occurs in real estate development when multiple properties are purchased and merged together, resulting in increased value due to economies of scale or other factors.