Halakhah zu Wajikra 14:45

And likewise, a house cannot be rendered impure until there are four square ells or more in it; as less than this is not called a house - and the Torah stated, "house." And so [too,] it must be in a way that it has four walls and is built on the ground, with stones, wood and dirt, as it is stated (Leviticus 14:45), "its stones, its wood, and all of its dirt." And bricks and marble are not considered [to be] like stones. And Jerusalem, and outside the Land [of Israel] as well, do not have the law of impurity of houses, as it is stated (Leviticus 14:34), "upon a house in the land of your holding." And outside the Land is not a holding; and Jerusalem was also never divided for the tribes - and therefore it is not included in "your holding." And the rest of its details are in Negaim (see Mishneh Torah, Laws of Defilement by Leprosy 14).