The Association of German Pfandbrief Banks (Verband deutscher Pfandbriefbanken, or vdp) have started a new (welcomed) initiative publishing a German office building rent index. The first result of this index are available on the vdp website (you will find the press release
http://aox.ag/mfaQxW , and the index itself
http://aox.ag/kkFoxR - theses links are for the English version but the same documents are available on vdp site in German as well).
