Haus Carstanjen, Historical villa in Plittersdorf, Germany.

Haus Carstanjen is a three-story stone villa in Bad Godesberg featuring steep slate roofs and prominent towers. Long colonnaded galleries overlook the Rhine River and shape the building's distinctive appearance.

An original structure from 1716 was transformed by Wilhelm Adolf von Carstanjen into a classical castle in 1881. This conversion reflects the rising wealth of merchant families in the region.

The estate's name recalls the Cologne banker Wilhelm Adolf von Carstanjen, who bought the former Auerhof in 1881 and had it rebuilt as a castle; he was ennobled the same year. His son Robert had the villa officially called a Schloss in 1907, and the family kept the estate until 1941.

The building now houses United Nations organizations, particularly the UNFCCC secretariat, following extensive renovation by the federal government. Access is therefore limited and depends on the operating hours of these institutions.

South of the house, facing the Rhine, stands the Carstanjen family mausoleum, a large domed rotunda modelled on a small Roman pantheon, with several floors of internal and external decoration. Wilhelm Adolf von Carstanjen commissioned it between 1894 and 1896 and was interred there after his death on 24 June 1900.