Together with Ciril Metod Koch and Ivan Vancaš, he introduced the Vienna Secession style of architecture (a type of Art Nouveau) in Slovenia.

In 1917, Fabiani was named professor at the University of Vienna,[4] and in 1919 one of his pupils, Ivan Vurnik, offered him a teaching position at the newly established University of Ljubljana,[5] Fabiani however refused the offer, quit the teaching position in Vienna, and decided to settle in Gorizia, which had been annexed to the Kingdom of Italy, thus becoming an Italian citizen.

[11] A widely circulated but false story regarding Fabiani is that the young Adolf Hitler once briefly worked in his architecture firm in Vienna.

Fabiani won a competition against the more historicist architect Camillo Sitte, and was chosen by the Ljubljana Town Council as the main urban planner.

With the personal sponsorship of the Liberal nationalist mayor of Ljubljana Ivan Hribar, Fabiani designed several important buildings in the town, including the L-shaped secondary school for girls in the Mladika Complex facing Prešeren Street (Slovene: Prešernova cesta), which is now the seat of the Slovenian Foreign Ministry.

[16] During the 1920s, he coordinated a large scale reconstruction of villages and some historical monuments in the areas in the Julian March that had been devastated by the Battles of the Isonzo during World War I.