The Davidsons built a new porch on their property. The porch added $2,000 to the property's market value. This is an example of the principle of value.

contribution

Explanation:

Based on the scenario being described within the question it can be said that this is an example of the contribution principle of value. This principle states that the value of a specific element is measured in comparison to what it adds to the value of the property as a whole. Which in this case would be $2000 as that is what the porch adds (contributes) to the value of the entire property.