The two-household dwelling is designed for two closely related families, such as those of elderly parents, and their grown children and grandchildren to live independently under the same roof. They usually have separate entranceways and separate kitchens. This type of dwelling, allowing two families to live together and yet maintain a degree of independence, has been popular in Japan for some time. In 1980, the Housing Loan Corporation established a special loan category for the two-household home.
Reference -- Chiezo, published by the Asahi Shimbun Sha.