Ann Arbor's residential neighborhoods contain architectural styles ranging from classic 19th-century and early-20th-century designs to ranch-style houses. Among these homes are a number of kit houses built in the early 20th century. Contemporary-style houses are farther from the downtown district. Surrounding the University of Michigan campus are houses and apartment complexes occupied primarily by student renters. Tower Plaza, a 26-story condominium building located between the University of Michigan campus and downtown, is the tallest building in Ann Arbor. The 19th-century buildings and streetscape of the Old West Side neighborhood have been preserved virtually intact; in 1972, the district was listed on the National Register of Historic Places, and it is further protected by city ordinances and a nonprofit preservation group.