Not much of the actual house is described, but it seems creepy. The mood and tone of the entire poem lends itself to making the reader think the house is unnerving.
The poem's speaker gives the reader some information regarding the kind of house that is in the poem. First, the house is not in a neighborhood. Readers are told the house stands alone in a forested area.
But only a host of phantom listeners
That dwelt in the lone house then
The reader knows the house is in a forested area because of lines three and four.
And his horse in the silence champed the grasses
Of the forest’s ferny floor.
I picture the house being at least two stories tall. I think that for two specific reasons. The first reason is because of the following lines:
Stood thronging the faint moonbeams on the dark stair,
That goes down to the empty hall.
I'm not sure how the man on the horse can see into the dark house, but he clearly sees stairs that empty out into a hallway. The second reason I think the house is a two-story house is because we are told the house has a "turret." I have never seen a one-story house with a turret.
I've attached a link to a picture of a house with a turret. A turret hearkens back to old castles, and the design gained popularity again during the Victorian era. A turret will make the house look quite imposing during the day, and—I would imagine—quite foreboding at night.
Lastly, the house is one of those houses that is covered in creeping vines. Readers know this because we are told the windows are "leaf-fringed."
No head from the leaf-fringed sill
Leaned over and looked into his grey eyes.
That means some kind of plant life is growing all around the windows. That image also gives the house the creepy feeling I spoke about at the beginning of my response because it's as if nature is taking back and enveloping the entire house.