The highly-rated 21-year-old central defender made his Premier League debut at the end of last season, starting in Chelsea's 2-0 win at Liverpool in April, but Jose Mourinho is ready to sanction a loan deal.
Cologne are understood to be looking to clinch a two-year loan for Kalas, similar to a spell he had with Dutch outfit Vitesse Arnhem from 2011 to 2013.
Cologne general manager Jorg Schmadtke confirmed that talks are ongoing.
Schmadtke told BILD: "Nothing has been signed just yet. But I am expecting the (loan) deal (of Tomas Kalas) to go through."