Kyu-Yasuda-Kusuo-tei Garden (旧安田楠雄邸庭園)
Kyu-Yasuda-Kusuo-tei Garden, a small but elegant house museum, was constructed in 1919 as the residence of a business leader. It survived the destructive earthquake of 1923 and a member of Yasuda family (the owner family of the Yasuda Zaibatsu conglomerate) who lost his home in the quake moved here subsequently. It is now open to the public as a house museum (Japanese prefix "kyu" means "former").
They offer an explanation service of about an hour every 30 minutes in Japanese, and there seems to be an English guide also. Note that you cannot get down to the garden (unless on special occasions) so you cannot observe the house building from outside well except for the entrance part facing the gate. It's a 7-minute walk from Tokyo Metro Sendagi Station and is across the valley from Yanaka. (There used to flow a real river in this valley, though now it was undergrounded).
As aforementioned, the garden is usually off limits and therefore can be seen only from inside the house. But for several Saturdays in spring and fall, you are allowed to walk through the main garden, thus enabling you to observe the house building from outside as well. Below is what it looks like.