Profiting from the building boom of Greater Romania, and the rising popularity of functionalism, Janco's Birou was in much demand. Compared with mainstream functionalist architects like Horia Creangă, Arghir Culina, Rudolf Fränkel or Marcel Locar, the Jancos received commissions that were sparse and small-scale, but they had a decisive role in popularizing the functionalist versions of Constructivism or Cubism. Heralding the change of architectural tastes with his articles in Contimporanul, Marcel Janco described Romania's capital as a chaotic, inharmonious, backward town, in which the traffic was hampered by carts and trams.
A major breakthrough was his Villa Jean Fuchs, built in 1926 on Negustori Street. Its cosmopolitan owner allowed the artist complete freedom in designing the building, purportedly the first Constructivist structure in Bucharest, and a budget of 1 million lei. The result caused a stir in the neighborhood, while the press found it to be reminiscent of a "morgue" and a "crematorium". The architect and his patrons were undeterred by such reactions, and the Janco firm received commissions to build similar villas, as well as the Philippe Suchard pavilion at the Obor fair of 1926.
Until 1934, when Marcel Janco finally received his certification, his designs continued to be officially recorded under different names, most usually attributed to a Constantin Simionescu. This had little effect on the Birou's output: before 1937, Janco and his brother designed some 40 permanent or temporary structures in Bucharest, all of them located in the northern and central areas (the "Yellow" and "Black" sectors, as they were known at the time). These and other projects also involved the 1924 exhibit's Miliţa Petraşcu, who is herself better known as a modernist sculptor.
Several other Bucharest homes result from this creative collaboration: the Maria Lambru Villa of 1928, on Popa Savu Street; the Florica Chihăescu house on Şoseaua Kiseleff (1930); the Jean Juster and Paul Wexler Villas, on Silvestru and Grigore Mora streets, respectively (1931). Janco also designed a house for his Simbolul friend Poldi Chapier. Located on Ipătescu Alley and finished in 1929, this was occasionally described as "Bucharest's first Cubist lodging". These projects are joined by a private sanatorium of Predeal, which is the principal of Janco's Constructivist designs outside of Bucharest.
Janco had one daughter from his marriage to Lily Ackermann, who signed her name Josine Ianco-Starrels (b. 1926), and was raised a Catholic. Her sister Claude-Simone had died in infancy. By the mid 1920s, Marcel and Lily Janco were estranged: already by the time of their divorce (1930), she was living by herself in a Braşov home designed by Janco. The artist remarried to Clara "Medi" Goldschlager, the sister of his old friend Jacques G. Costin. The couple had a girl, Deborah Theodora ("Dadi" for short).
With his new family, Janco lived a comfortable life, traveling throughout Europe and spending his summer vacations in the resort town of Balcic. In 1931, Janco built himself a new family home, the blockhouse known as "Clara Iancu Building", on Caimatei. The Jancos and the Costins also shared ownership of a country estate: known as Jacquesmara, it was located in Budeni-Comana, Giurgiu County. The house is especially known for hosting Clara Haskil during one of her triumphant returns to Romania.