Following that, he worked as a site manager for Christian Friedrich von Leins, who was engaged in restoring rural churches.

From 1870, he was in Hamburg where he was employed by an engineer named Schmetzer; until 1873, when he and an old friend from Karlsruhe, Wilhelm Emil Meerwein, began their own architectural practice.

Their best known project is, perhaps, the Kaispeicher B [de] (Quay Warehouse B, 1878–79), a large structure on Hamburg's waterfront, which was incorporated into the Speicherstadt development.

At that time, he joined the "Rathausbaumeisterbund", a group of architects, organized in 1885 by Martin Haller, who had been appointed to create the new Hamburg City Hall; a project that lasted from 1886 to 1897.

In 1901, Meerwein was elected to the Hamburg Parliament and devoted most of his time to it so, when Hanssen's health began to decline in 1905, he retired and closed their firm.