Jens Casper was a co-founder and director of the Berlin based architectural practice Realarchitektur between 2003-2008. 

In 2003, Jens Casper, Petra Petersson and Andrew Strickland, formed Realarchitektur. Their most important collaborative project, Boros Collection Residence, attracted international recognition. With a focus on timber construction, Casper, Petersson and Strickland participated in a number of competitions. This includes Swooosh!!! in collaboration with ifau - institut für angewandte urbanistik, a competition entry for a collective housing scheme in Tensta, Stockholm, awarded first prize in 2005, and a low rise housing scheme in Flottiljen, Stockholm, honoured as 'the most beautiful proposal' by the jury in 2007. In 2007, the Wallpaper* Architects Directory ranked Realarchitektur among „101 of the world´s most exiting new architects“. 

Andrew Strickland left Realarchitektur in 2005. Petra and Jens decided to go their own way by the end of 2007. Since 2008, Petra Petersson runs Realarchitektur on her own (external link). In 2010 Jens Casper formed Casper Mueller Kneer based in Berlin and London, and since 2021 he summarizes his practice as C,.