When Poe speaks of the "house of Usher" in "The Fall of the House of Usher," he refers to both the physical home Roderick and Madeline live in and the Usher's genealogical lineage: the house of Usher also means the family line that goes back many generations. As the narrator notes as he approaches the house, the words "house of Usher"

seemed to include, in the minds of the peasantry who used it, both the family and the family mansion.

A problem, however, has occurred to the family lineage: at this point, Roderick and Madeline, brother and sister, are the last alive in the Usher family line. As Madeline is dying and Roderick seems far too afflicted with a nervous disorder to think about marrying and having children, it looks as if the Usher family will end with their deaths.

In highly convoluted language, the narrator suggests that this identification of house and family as one is due to no "branch" in the family tree: a family normally has more than one child, and the second, third, fourth, and fifth children (and so on) marry and start alternate family "branches" that break off from the patriarchal descent of eldest son to eldest son. The younger children usually would leave the ancestral home and live elsewhere. In this family, however, the parents seem to have always had either only one child, a son who inherits the estate from the father—or if there is a daughter, the implication is that she perhaps marries her brother. That there is incest is not altogether clear, but the family line seems to be vitiated and unhealthy—as if there were too much inbreeding. In any case the narrator explains that the lack of family "branches" has meant all of the Usher family has always lived in the Usher house, which is why the two are so closely associated.

The story adds a twist to the identification of the house with the family. When Madeline and Roderick die at the end of the story, ending the Usher family line, the physical house "dies" too, fissuring and collapsing into the tarn, as if its existence supernaturally depended on the existence of the family.

Poe's narrator comments, at the beginning of the story, that the term "House of Usher" has come to mean both the house and the family. Both are impossibly antique and given to extreme melancholy. The fact that Roderick Usher is a direct descendant of the ancient Ushers makes him a kind of living artifact, and it is true that the forlorn aspect of the house and its rooms seems to be the outward representation of Usher's mental state. Usher himself expresses the idea that his morbid mental state is connected to the house, which he has not left for many years.

Some details that support this idea include

• Usher's studio, as it is described as vast and dim; the decaying furniture and scattered books and musical instruments "failed to give any vitality to the scene" and reflect Usher's unordered mind.
• Usher's song, "The Haunted Palace." It is a version of Usher's own history. The "evil things" that assail the monarch in the song transform the palace into something ghastly.
• Usher's conviction of the sentience of "vegetable things," and in particular his belief that the arrangement of the stone of the house and the moss and lichen that grows there create an "atmosphere" that exerts a "terrible influence" over his family for "centuries."
• Usher's placing Madeline in a tomb far beneath the house as both a literal and figurative burial. The house is a kind of grave; like Madeline, Usher himself is "buried" in the house.
• Madeline's appearance, as if risen from the dead, as a kind of culmination of the Usher's morbid fears. When both he and Madeline die, the house collapses, suggesting that the house cannot exist without the family.