In 2016 he joined English side Saracens.[5] During his time at Saracens he has won two Premiership titles in 2018 and 2019.[6][7] He also helped Saracens win the European Champions Cup in 2017 and 2019.[8][9]
On 9 December 2021, Koch announced he would leave Saracens after five seasons at the club to join Premiership rivals Wasps on a long term deal ahead of the 2022-23 season.[10]
Wasps entered administration on 17 October 2022 and Koch was made redundant along with all other players and coaching staff.[11]
On 27 October 2022 it was confirmed that Koch had signed for French Top 14 side Stade Français.[12]
In 2016, Koch was included in a South Africa 'A' squad that played a two-match series against a touring England Saxons team.[17] He was named in the starting line-up for their first match in Bloemfontein,[18] but ended on the losing side as the visitors ran out 32–24 winners.[19]
Koch was named in South Africa's squad for the 2019 Rugby World Cup.[20] South Africa won the tournament, defeating England in the final.[21]