Baroque premises were not sufficient for a number of offices of the ministry headquarter. Thus the architecture competition requested to design an annex of the Czernin Palace in the guidelines too. After the reconstruction of the Czernin Palace in 1929 - 1934, Janák built an annex in 1937 – 1940. Janák Annex was completed in 1940 when the complex of the Czernin Palace and its annex was used as the Headquarter of the Reichsprotektor Office.

Pavel Janák was ordered to redesign the project of reconstruction of the palace and the annex several times. The reconstruction of the baroque palace was not completed until 1929 - 1934. Construction of the annex was started in 1937 according to Janák´s tenth project. Thus the so called Janák Annex was completed during the Nazi occupation of Czechoslovakia in 1940.

Years after 1925 are marked as a late period of Janak´s work. At that time Janak left cubism and turned to purism. Janák Annex was inspired by bare brick style of the Dutch modernism.

Janák Annex is an austere building, completely conformed to its function of an office building. A plain facade is structured by horizontal lines of windows. Axes of the main and traverse wings of Janák Annex are formed by corridors with adjacent office rooms. There is a paternoster lift in the building.

Janák successfully linked the annex with the Czernin Palace proper. Contrary to projects of other participants of the architecture competition for the palace reconstruction (Bedřich Bendelmayer, Bohumil Hypšman, Kamil Roškot) Janák subordinated the annex to the original building. The Czernin Palace retained its position as a dominant of the complex of the Ministry of Foreign Affairs as well as of the whole Loretánské náměstí square. Janák hid the annex behind the palace to be visible only from a narrow view point at a border of the Prague Castle District. Although the architect had thought about linking the main staircase to the annex at first, he left the idea. Both buildings have been connected by narrow passageways in side wings. Different floor levels was a further issue, a ceiling of the palace ground floor being in the same height as a ceiling of the first floor of the annex. Janák solved the problem smartly having linked both ground floors and having closed a corridor in the first floor of the annex by a wall and a circle window. The circle window seemingly elongates an upper part of the corridor in the ground floor of the palace.