Upon returning to Lucerne, he worked as an assistant at Armin Meili's firm from 1924 to 1927 before establishing his own office.
In 1958, he partnered with Hans Käppeli, who continued to run the office after Dreyer's death.
[4] He then built the Catholic church of St. Theodul in Littau (1938) and, one of his major works, St. Josef in Lucerne's Maihof district (1940).
During the Second World War, he built the Catholic church Guthirt in Aarburg (1941–1942) and in the 1950s the Bruderklauskirche in Kriens (1952–1953).
His main work is the Lucerne Central Library, originally designed for a building site next to the Jesuit Church, the current location in the Hirschmatt quarter was found after a petition to keep the side façade free.