In 1870, just a few months before the fall of the Empire, Napoléon III apparently gave his approval for the installation of an astrophysical observatory at the Pavillon de Breteuil, naming a certain Dr Jules Janssen as Director (later to become Director of the Meudon Observatory).

During the siege of Paris the Pavillon de Breteuil was seriously damaged by shells aimed by the French at a Prussian battery installed on the hill just above the Pavillon. The stables and outbuildings in the courtyard were completely demolished but the servants quarters in the Petit Pavillon just to the south of the main building were untouched. This was the condition of the Pavillon when, in 1875, the French government offered the site to the Comité International des Poids et Mesures for the establishment of the Bureau International des Poids et Mesures (BIPM).

The foundation of the BIPM had been foreseen in the Convention of the Metre, signed in Paris on 20 May 1875 by seventeen nations. The International Committee agreed to accept the offer of the Pavillon de Breteuil as the site for the BIPM and work began to renovate the Pavillon and to construct a new laboratory building to house the instruments and equipment required by the BIPM.