As mentioned above, a future-proof building is also one that does not become obsolete. Reed and Warren-Myers state that in the valuation of real estate, there are three traditional forms of obsolescence: physical, functional, and aesthetic. Physical obsolescence occurs when the physical material of the property deteriorates and needs to be replaced. Functional obsolescence occurs when the property is no longer capable of serving the intended use or function. Aesthetic obsolescence occurs when fashions change or when something is no longer in style. A potential fourth form, sustainable obsolescence, occurs when a property no longer meets one or more sustainable design goals (Reed and Warren-Myers 2012, 1). Obsolescence is an important characteristic of future-proofing a property because it emphasizes the need for the property to continue to be viable.