I stayed in the 6-tatami bedroom on the upper floor. The room has lots of storage and hanging space for clothes and a large balcony also.
The house is really nice and old fashioned with what I think are many of the original fixtures and fittings. There aren't many of these older houses left and it's nice to experience a bit of life from a bygone era.
The area is a quieter residential area close by the main shopping streets and near the station. It's super convenient for accessing Shinjuku (~25 mins from your front door by train). There's a nice little park nearby and Koenji has a huge range of restaurants and bars and supermarkets and vintage shops, and an interesting mix of young and old people.
The house is shaded by its neighbours and because of its age is a tiny bit draghty, which is probably ideal in summer and in winter you can use the heat pump in the bedroom and a floor heater in the kitchen.
Hiro has 2 cats who live in a room on the first floor. They can be quite vocal when they want attention but they are well cared for. At night you may hear them jumping around a bit, but I'm a very light sleeper and it didn't bother me much - I usually play white noise to drown out sounds anywhere I go, which helps.
Lastly the host is fantastic, super generous guy. I hope you have as good an experience as I did.