Thus began the present existence of the Pavillon de Breteuil. In 1884 the laboratory building called the Observatoire was opened, and in 1889 the 1st Conférence Générale des Poids et Mesures sanctioned the new international prototypes of the metre and kilogram and formally required that they be deposited at the Pavillon de Breteuil. The Observatoire was extended in 1929 thanks to a gift by the Rockerfeller Foundation. Since then, extensions of the activities of the BIPM have required the construction of additional buildings: in 1964 two laboratory buildings were opened for work on ionizing radiation (these required an extension to the site, bringing it to about 4 hectares); in 1984 a building for laser work was opened; in 1988 a new library and office building was inaugurated; and, most recently, the Pavillon du Mail was inaugurated in 2001.
Since 1875 there have been thirteen directors of the BIPM and they have all, with the exception of Govi, during whose time the Pavillon de Breteuil was being repaired, resided in the Pavillon de Breteuil.
The status of the BIPM vis-à-vis the French Government was formalized in an agreement signed on 25 April 1969 between the Comité International des Poids et Mesures and the French government. The site of the Pavillon de Breteuil is now considered international territory and the BIPM has all the rights and privileges accorded to an intergovernmental organization.
The Baron de Breteuil would, we hope, have been pleased to know that, after its chequered early career, the pavillon that bears his name in the Parc de Saint-Cloud has become the permanent home of an international scientific organization, known the world over for the precision of its work.