In 1901 Šen returned to Zagreb and started to work as a servant in the City council Civil engineering office.

In 1905 Šen co-founded the Croatian architects club (Klub hrvatskih arhitekta).

His early eclecticism is characterized by the idiosyncratic elements on the main facade of the Croatia osiguranje palace in Tomáš Garrigue Masaryk street, Zagreb.

Representative public buildings, such as the Institute of Forestry (1926) in Božidar Adžija street, Zagreb, is massed in large cubic forms expressing restrained monumentality.

Other Sen's realization of that era are apartment buildings "Mervar" (former "Society humanity home") in Petrinjska street and residential building "Bezuk" in Boškovićeva street, all in Zagreb.