“When we were building the house, I asked my father to draw the line on paper: Which is the land that we pay for, which is our common space and which is the land of their house,” Mrs. Melasniemi said. “I liked that he made it, as he knows the garden and he knows what he likes to think is their space.”

Mrs. Melasniemi’s grandmother, who was 91 when she moved in, has since died.

“My father got the opportunity to visit and talk to his mother every day,” said Mrs. Melasniemi, who has two children.

But the home is already being used for another phase in its planned long life.

“We discussed the kind of life span of the house,” Mr. Siitonen said. “We thought of how they could use the space when the kids move out, and then when the grandmother passes away. And of course, in the future the parents, one or both of them, might move in there. So it was kind of like we thought of things in five years and 10 years and 50 years.”