Hatoyama Kaikan (鳩山会館), or the Hatoyama Hall, a Western-style building and garden completed in 1924, used to be the residence of the Hatoyama family, a prominent political dynasty in modern Japan.
The residence is now converted into a memorial hall and is open to the public. There is an English-style residential building with an English-style garden (which means that this is not a place to expect a "traditional Japan").It's a 6-minute walk from Edogawabashi Station or an 8-minute walk from Gokokuji Station, both of wich are stations along the Tokyo Metro Yurakucho Line.
The entrance for visitors is on the west side. You have to walk up the slope to get to the mansion.
The house building, constructed in 1924, is in English-country-house-style.