Very nice and cosy place hidden in a lovely inner courtyard in Shinjuku City. Great location. The host answered our questions quickly and gave us everything we needed.
The thermal insulation was not good (in winter) but it seems to be the norm in japanese buildings. Once you turn the heating system on, the apartment gets warm fairly quickly, eventhough some specific spots in the main room still had cold air drafts passing through. Make sure to keep the doors to the entrance corridor closed, that's where most of the cold air comes from.
The japanese bed sheets are very good at retaining heat though, so the weak insulation didn't keep us from sleeping comfortably.
We found what we believe to be 2 small rat or mouse feces coming from the storage compartments probably, and one person from my group heard rodents/small animals moving within the walls one night, but overall the place was very clean and this didn't hinder our stay.