Very close to Shinjuku (about 10 minutes walk) and right next to Higashi-Shinjuku station.
Do note that it's not a quiet neighbourhood and it can get loud especially during weekends. We've stayed in small accommodation (which is normal for Japan
We had some issues which we raised with Ohkubo and he was kind enough to secure us separate (and really good) accommodation, but this place felt constricted.