The subject is a local apartment complex built during the $1960$s$.$ The units originally were laid out with $1$ bedroom, $1$ bath, and a large windowless open area marketed as a den. Over the years, a previous owner decided these dens were not appealing and enclosed the rooms marketing them as second bedrooms, but because of their location within the building, windows could not be added.

What type of depreciation would this be categorized in the Cost Approach?