Koch has emerged as one of the brightest young talents in Germany this season, which has included a national team call.
Fussball Transfers says RB Leipzig see Koch as a replacement for wantaway defender Dayot Upamecano. However, they'll face competition from England, where the Hammers are keen.
It's also emerged intermediaries from a top six Premier League club has been in touch.
Just last month, Koch, tied to Freiburg until 2021, said: "It is clear that I want to take the next step."