Born 23 December 1932 in the Prussian city of Breslau (now Wrocław in western Poland), Michael Richard Ernest Feldhagen was the son of Kurt Feldhagen and his wife Getrud, nee Cochius.  By the early 1950s, Feldhagen was studying architecture at the Universität der Künste Berlin (Berlin University of Arts), then under the direction of Professor Paul Baumgarten (1900-1984).  In 1952, Feldhagen was part of the three-person team that won first prize in a student competition for the design of a children’s playground.  After graduation, Feldhagen worked in an architect’s office in Berlin, which was then divided into two halves and landlocked within communist-controlled East Germany.  Disturbed by this political instability (which would bring about the erection of the Berlin Wall in 1961), Feldhagen and his wife Helga decided to migrate to Australia with their young daughter Alexandra.  They arrived in Melbourne on 13 September 1958.

On the strength of prior experience  with the design of high-rise apartment blocks in Berlin, Feldhagen secured a position with the Housing Commission of Victoria, which, at that time, was starting its own regime of multi-storey public housing.  By 1960, however, he had left to take a position in the office of fellow European émigré architect Ernest Fooks (1906-1985).  Migrating from Austria (via Canada) in 1939, Fooks had also once worked for the Housing Commission before opening his own office in 1949.  By the time that Feldhagen joined him, Fooks was well established as one of the leading architects to Melbourne's thriving post-war Jewish migrant community.  A number of Fooks’ clients, impressed by Feldhagen’s speed and efficiency, urged him to start his own practice.  Perhaps discouraged by reports from other migrant architects (perhaps even Fooks himself) that their overseas qualifications were not instantly recognized by the Architects’ Registration Board of Victoria, Feldhagen did not attempt to become registered himself.  Instead, with a client base already developing, he simply opted to style himself, for legal purposes, as an ‘architectural designer’.

Established by 1963, Feldhagen’s practice was initially based in Acland Street, St Kilda, but later moved to Darling Street, South Yarra and later still to Shakespeare Grove, St Kilda.  In 1967, he re-configured his business as a public company, M Feldhagen & Associates Pty Ltd.  His early output included a number of small factories, but he soon established a reputation as a designer of large houses, including the private residences of a number of Jewish builders with whom he regularly worked.  Feldhagen frequently collaborated with fellow European émigré professionals, including engineers David Shalit and Pento Stojanoff, while his own office staff included young architectural students of Asian and Turkish descent.  

From the late 1960s, when changes to strata title legislation prompted a boom in apartment construction, Feldhagen became one of the leading exponents of the type.  During the years 1968-69 alone, eight examples of his multiple-dwelling projects were published in the property columns of the Herald and Age newspapers.  Seeking to diversify his business, Feldhagen decided to expand into building works and, in May 1970, formed a second company, M Feldhagen Constructions Pty Ltd.  Around the same time, he acquired a fire-damaged house on Mount Dandenong Road, Kilsyth (formerly owned by the sculptor Norma Redpath), which he extensively rebuilt and renovated as his own family residence. 

During the 1970s, Feldhagen continue to thrive as both a designer and a builder, maintaining earlier associations with migrant property developers. However, as the decade wore on, commissions from those sources began to dwindle.  Feldhagen declared bankruptcy in 1975, and his two public companies were subsequently de-listed.  Nevertheless, he resumed practice as an architectural designer, with his teenage son as his draftsman and sole employee.  The practice continued into the 1980s under the name of Michol Designs (ie combination of the forenames of Feldhagen and his son).  One of Feldhagen’s most notable projects from this later period was a church in Chirnside Park, which opened in 1986.

Feldhagen retired to Queensland in the 1990s and died there on 21 January 2013, aged 80 years.