Sefer HaChinuch ספר החינוך

The commandment of the matter of the impurity of a house that has an ailment: To render an ailing house impure; meaning to say that we do with a house that has tsaraat according to the statute that is written in the section of the Torah, as it is stated (Leviticus 14:35), “And the one who the house is his shall come, etc.,” as it is written there; and that we assume one who entered the house [to be] impure, as it is written (Leviticus 14:46), “And the one who entered the house [...] shall be impure, etc.” And this commandment includes all of the matters of the impurity of houses — which one requires quarantine or destruction of some of the walls or all of them. And I have heard (see Ramban on Leviticus 14:35) that this matter of tsaraat existing in stones is not a natural matter, but rather a miraculous matter, [and] it sometimes comes to the houses of Israel to rebuke them. As it is from God’s loving them that He afflicts them with something outside of their bodies, so that they will repent before they are liable for [Him] to punish them with their bodies. And so was the matter, that if one did not repent, the aliment would also spread to his clothes; if he still did not sense it, it would also spread to his body. And they, may their memory be blessed, said (Vayikra Rabbah 17:6) that at the beginning when they conquered the Land, God, may He be blessed, brought the ailment of tsaraat to some of their houses for their good, in order to destroy the house and reveal the buried treasure that the Amorites buried there. And even though it was possible to inform them [of] this through a prophet without an ailment, it is known that God will do miracles for people in a hidden way, as we wrote above.

From the laws of the commandment is that which they, may their memory be blessed, said (Mishnah Negaim 14:3), that the measure of the ailments of houses is like two gris (split beans), one next to the other. And there are three signs of impurity in the house: deep green; deep red; and spreading, and all of them are explicit in Scripture. And two appearances combine, one with the other, for the measure of two gris. And all measures are a law of Moshe from Sinai. And when one sees an appearance in a house — even if he is a sage who knows that it is an ailment — he does not decree and say, “An ailment has appeared to me,” but rather, “[Something] like an ailment has appeared to me.” As it is for a priest to render ailments impure and pure, and not for another man. And a dark house cannot become impure from an ailment, as we do not open up [new] windows with which to see the ailment, as it is stated (Leviticus 14:35), “appeared to me” — and our Rabbis, may their memory be blessed, explained (Sifra, Metzora, Section 5:11), “to me,” and not “to my light.” And lest you say, he should see the ailment with a candle, sight with a candle is not complete. And therefore a dark house is never rendered impure by an ailment.

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.)

And it is practiced in the house of a male or female in the Land [of Israel], at any time that there are priests fitting for the thing. And one who transgresses it has violated a positive commandment.