The other answer gives a succinct description of Thoreau's cabin. I have seen its reproduction at Walden Pond, and a main takeaway is how tiny it is.
Thoreau, however, talks expansively about the building of it, the materials he used, and his philosophy about a dwelling place. Anticipating the tiny house movement by a century and a half, he finds his new home's size a feature rather than a flaw. He has paid cash for his house: unlike the majority of the civilized world, he is not paying rent or toiling twenty to forty years to work off a mortgage. He is snug and content in a tiny home he owns outright.
Thoreau fells his own trees and gather his own stones for the house, and digs a cellar. However, he purchases most of the home's materials by buying the small shanty of an Irishman, who lived there with his wife and child. Thoreau disassembles this house, lets the shingles from it dry in the sun so they will no longer be warped, and with the help of his friends, raises his new home. He notes it has a garret, a closet, a lean-to, two windows, two trapdoors and a front door. He adds the fireplace later, cooking over an outdoor fire until the weather gets cold.
While Thoreau lived very simply, the refashioned cottage seems to have been clearer and brighter than when the Irish family lived in it--partially because Thoreau lived alone, not with a wife and baby, and partially because, unlike them, he didn't insulate it with dirt five feet around so that it looked like a compost pile.