"We paused before a House that seemed A Swelling of the Ground –" What image does the language in these lines create?

The image created by these lines is of a grave that has been recently dug, with the coffin placed inside and the dirt heaped back over on top of it. The result is like a mound rather than a smooth, flat replacement of the soil, since the coffin took up some of the space inside the hole. It's like the ground has swollen up to create a little bump.

If you were reading these lines literally, out of context, you may have imagined instead an actual house that's just a bit squat and earthy-looking, like it grew up out of the ground. (Perhaps a Hobbit house came to mind!)

But consider the lines in the context of the poem. Up until this point, Death has been riding in the carriage with the speaker; they drove around town until the sun went down and it got chilly. Now they're pulling up to a "House," where centuries will fly by. It's pretty clear that this "House" is where the speaker will remain; it's where Death is bringing her. So it's a grave.

You might ask, well, why didn't speaker say "grave" or "tombstone" instead of "House"? She probably wants us to envision this final resting place as more of a civilized kind of home. You know, the place Death would bring you if he arrived like a gentleman in a carriage, and not like a spooky cloaked figure with a scythe.

Anyway, the next few lines will make it more clear that we should imagine a grave and not an actual house:

The Roof was scarcely visible –
The Cornice – in the Ground –

This means that it's a "house" on which you can barely see the roof, meaning it's just poking out of the ground a little, and the "cornice," or the decorative part of a building that pokes out of the very top of it, is actually under the ground.