Dickenson is alluding to the narrator's grave -- a "house" that seemed a swelling of the ground. Since it is a slight swelling, the "roof" is scarcely visible. A cornice is a molding at the top of a wall; since it would be no higher than the roof it would be in the ground. Some say the roof is a reference to the top of the coffin, and the cornice the molding around the coffin.
There are different interpretations of this great poem. Some say it suggests a peaceful attitude about death, which is described as kindly and civil. Others say it draws a parallel between being tricked by death and being seduced by a gentleman only to find that marriage is an empty “house”.
Remember that this whole poem is an extended metaphor. Dickinson uses a ride in a carriage (or a journey) to symbolize death. However, she does not maintain the dark, gruesome tone associated with most death poems. Her whimsical attitude toward death enables her to portray her ride past everyday, cheerful aspects of human life. When she writes,
We paused before a House that seemed
A Swelling of the Ground--
The Roof was scarcely visible--
The Cornice--in the Ground--
her whimsical tone continues. The "House" that swells in the ground is the mound from a grave; the roof is the headstone, and the description of the cornice being in the ground means that the grave is the actual house of the entombed; so only the roof (tombstone) is visible above the earth.