Returning to Courtauld the same year, Timmerman began his PhD in History of Art and started working on his dissertation Staging the Eucharist: Late Gothic Sacrament Houses in Swabia and the Upper Rhine, with the assistance of his supervisor Paul Crossley.
He first became a lecturer in 2002 at University of California, Berkeley, where he taught for one year until moving to Berlin as a member of the faculty at European College of Liberal Arts until 2004.
Architect and historian Steven J. Schloeder noted Timmermann's intervention that the politics of the medieval church may be seen in the design of sacrament houses.
In his review of Timmermann's work, he summarises his arguments that "anti-Semitism, the Hussite Utraquist controversy (that the Eucharist must be administered under both species), and the later Protestant challenges shaped the display of the Sacrament into grand statements of orthodoxy and ecclesiastical unity".
[4] Timmermann's book, Real Presence: Sacrament Houses and the Body of Christ, c. 1270–1600 (2009) discusses microarchitecture, introducing his intervention "pointing out the close stylistic relation between micro- and macroarchitecture in the thirteenth and fourteenth centuries that distinguished the Gothic phenomenon from previous uses".