How does the narrator describe the house? How do the words “colonial mansion” and “hereditary estate” factor in? Why would the narrator wish it to be a “haunted house” instead?

The narrator describes herself in the opening sentence as "ordinary." This is an important detail in relation to the way she then goes on to describe the house: as "ancestral halls," which suggests a grand house that would have been passed down through generations of a noble, aristocratic family. The narrator obviously feels that the house is not the kind of house she would ever have expected it to inhabit, and her descriptions lend it a Gothic quality, a sense that anything could happen there.
She even alludes to this explicitly by suggesting that to think of the house as haunted would be the height of "romantic felicity." This is a dream of hers, to inhabit a haunted house like the heroine of a novel. She describes the house using tropes: it is a "colonial mansion, a hereditary estate," the sort of house that might feature in a story. We know that the narrator feels confined by her own life, in which very little ever happens. She wishes to be allowed to work and explore in order to regain her health, but her husband and brother have insisted that she remain confined in the country air. There is a certain self-awareness, then, in the narrator's language: she knows that she herself is now the heroine of the sort of Gothic story in which the house plays a major part, and she is primed to expect something strange to happen there.