A 6 GeV multi turn energy recovery linac ERL as a diffraction limited source of synchrotron radiation has been investigated at HZB. The results of a feasibility study are presented in the form of a Conceptual Design Report. Particular emphasis is paid to dedicated particle tracking simulations and the tools used to suppress unwanted beam degradation. For the first time in such a study, comprehensive start to end simulations are undertaken to track the beam throughout the 8 km long, split linac multi turn ERL facility. The Report highlights the advantages of ERL based facilities as a candidate for 4th generation light sources
