A Smial

Smials, also called Hobbit-holes, were the preferred dwellings of Hobbits in the Shire.
"In a hole in the ground there lived a hobbit. Not a nasty, dirty, wet hole, filled with the ends of worms and an oozy smell, nor yet a dry, bare, sandy hole with nothing in it to sit down on or to eat: it was a hobbit-hole and that meant comfort."
—Description of a Hobbit-hole from The Hobbit, "An Unexpected Party"