Mattapan is a neighborhoo in Boston, Massachusetts, United States. Historically a section of neighboring Dorchester, Mattapan became a part of Boston when Dorchester was annexed in 1870. As of the 2000 census, it had a population of 28,145. Like other neighborhoods of the late 19th and early 20th century, Mattapan developed, residentially and commercially, as the railroads and streetcars made downtown Boston increasingly accessible. Predominantly residential, Mattapan is a mix of public housing, small apartment buildings, single-family houses, and two and three family houses. Blue Hill Avenue and Mattapan Square, where Blue Hill Avenue, River Street, and Cummins Highway meet, are the commercial heart of the neighborhood, home to banks, law offices, restaurants, and retail shops. In 2008, Mattapan is scheduled to open its new Library Branch site. The new library is projected to cost over 4 million dollars.