Thanks to the free World Heritage Shuttle from the Congress Centre/Darmstadtium stop, you don't have to worry about getting from the parking facility to Mathildenhöhe Darmstadt.
The free World Heritage Shuttle runs daily between the city centre and Mathildenhöhe Darmstadt. Between 11.16 a.m. and 4.46 p.m., the HEAG mobiBus electric bus departs every 30 minutes from the Kongresszentrum/Darmstadtium stop. On the way, it stops at the Jugendstilbad and the Ostbahnhof.
The return journey is also every 30 minutes between 11.32 a.m. and 5.02 p.m. from the Olbrichweg stop (Museum Künstlerkolonie), also with stops at the Ostbahnhof and the Jugendstilbad and ending at the Schloss stop. The timetable of the World Heritage Shuttle Mathildenhöhe Darmstadt can be accessed
here.