Sagebiel was a sculptor's son, and after his Abitur in 1912, he began his studies in architecture at the Braunschweig University of Technology.
He eventually finished his studies in 1922, after they were interrupted by his participation in the First World War, which included a stint in a prisoner-of-war camp.
After Adolf Hitler and the Nazis seized power in 1933, Sagebiel became a member of the NSDAP and Sturmabteilung (SA).
As of 1934, he was being trusted with planning, as well as overseeing construction of, numerous Luftwaffe barracks (in Döberitz, Berlin-Gatow and Kladow, to name a few) as leader of the special works unit.
From 1938, he was directly subordinate to the Air Transport Minister, Hermann Göring, and as a result was considered among the Reich's most important architects.