[2] His architectural career began at the age of 16 when he worked for early Milwaukee architect, G. W. Mygatt.

He enlisted in the Civil War with the 24th Wisconsin Infantry as a private,[2] later becoming a draftsman on General Philip Sheridan's staff.

[3] After the war Koch returned to Milwaukee, where he formed a partnership with Mygatt until 1870, when he started his own firm.

[4] Koch worked in the Richardsonian Romanesque style, inspired by medieval architecture and popularized by Henry Hobson Richardson.

The style is characterized by semicircular arches, symmetry, round towers with pointed caps, copious use of stone, and generally simple facades.