<Tr> <Th> Added to NRHP </Th> <Td> December 8, 1976 </Td> </Tr> <P> Philadelphia City Hall is the seat of government for the city of Philadelphia, Pennsylvania . The building was constructed from 1871 to 1901 within Penn Square, in the middle of Center City . John McArthur Jr. and Thomas Ustick Walter designed the building in the Second Empire style . City Hall is a masonry building whose weight is borne by granite and brick walls up to 22 ft (6.7 m) thick . The principal exterior materials are limestone, granite, and marble . The final construction cost was $24 million . </P> <P> At 548 ft (167 m), including the statue of city founder William Penn atop its tower, City Hall was the tallest habitable building in the world from 1894 to 1908 . It remained the tallest in Pennsylvania until it was surpassed in 1932 by the Gulf Tower in Pittsburgh . It was the tallest in Philadelphia until 1986 when the construction of One Liberty Place surpassed it, ending the informal gentlemen's agreement that had limited the height of buildings in the city to no higher than the Penn statue . </P> <P> In 1976, City Hall was designated a National Historic Landmark . </P>

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