What is external obsolescence?

In the cost-depreciation approach to valuation, external obsolescence (sometimes referred to as economic obsolescence) is a loss in value caused by some factor beyond the boundaries of the property

An example of external obsolescence is a situation where a highway is built close to a home; the home becomes less attractive than it was originally due to its proximity to the newly constructed highway

External obsolescence is incurable, since it exists beyond the boundaries of the property and therefore the control of the property owner