Mishnà su Levitico 14:45

A house, one of whose walls was covered with marble, with rock, with bricks or with earth, is pure. A house that did not have in it stones, wood or earth, and a nega appeared in it and afterwards stones, wood and earth were brought into it, it remains clean. So also a garment in which there was no woven part that was three fingerbreadths square and a nega appeared in it and afterwards there was woven into it a piece of cloth three fingerbreadths square, it remains clean. A house does is not susceptible to negaim uncleanness unless there are in it stones, wood and earth.

He comes again at the end of the week and inspects the nega. If it has returned, "He shall break down the house, its stones, and its timber, and all the mortar of the house; and he shall carry them out of the city into an unclean place" (Leviticus 14:45). A spreading that adjoins [the original nega] is effective however small it may be; One that is distant must be no less than the size of a split bean. And a nega that returns in houses must be no less than the size of two split beans.

In the case of a stone in a corner, when the stone is taken out it, he must take it all out; But when [the house is] torn down he tears down his own [part] and leaves that which belongs to his neighbor. Thus it follows that there is a greater stringency for taking out than for tearing down. Rabbi Elazar says: if a house is built of rows of head stones and small stones, and a nega appears on a head stone, all of it must be taken out; but if it appeared on the small stones, he takes out his stones and leaves the others.