1) Your question is on the Rambam, Hilchos Tum'as Tzara'as 16:10, who writes that if someone speaks Lashon ha'Ra, the walls of his house change. If he continues in his bad ways until the house is destroyed, then his clothes change, and if he still does not do Teshuvah, he himself will be afflicted with Tzara'as. How does this fit with the Gemara in Sanhedrin 71a that says that the Bayis ha'Menuga never occurred and never will occur?

2) I want to try to answer that the Gemara in Sanhedrin 71a states that it is only according to Rebbi Eliezer b'Rebbi Shimon that one must say that the plagued house could never happen. However, the Bartenura on Nega'im 12:3 writes that the Halachah does not follow Rebbi Eliezer. The Halachah follows Rebbi Akiva that one requires two Grisin on two stones (but does not require them to be on two walls in the corner, as Rebbi Eliezer requires). Therefore, we can say that, according to Rebbi Akiva, the Bayis Muchlat did actually happen, and we rule like Rebbi Akiva.

3) I would like to add, bs'd, a way of understanding how the Teshuvah, repentance, from speaking Lashon ha'Ra that the Torah encourages by means of the Nega'im can apply according to Rebbi Eliezer b'Rebbi Shimon as well.

a. The idea is that Rebbi Eliezer says that a person should not wait until his house is actually demolished before he starts doing Teshuvah. Rebbi Eliezer says that the conditions for the house being liable for destruction are so exacting that they never in fact happened, but he agrees that if a person's house receives such a bad plague, that according to Rebbi Yishmael and Rebbi Akiva (who, in the Mishnah Nega'im 12:3, maintain slightly less exacting conditions) it must be destroyed, then he should stop speaking Lashon ha'Ra if he has not done so yet.

b. The Rambam (16:10) writes: "If he remained in his bad ways until his house is demolished, the leather vessels in his house start to change." The Rambam writes, "until his house is demolished," because he does not rule like Rebbi Eliezer, as we find in Hilchos Tum'as Tzara'as 14:7. However, the same general idea that the Rambam writes in 16:10 can apply also according to Rebbi Eliezer, and according to him we would just change slightly the wording and say, "If he remained in his bad ways until the house is full of plague, the leather vessels start to change."

c. Therefore, according to Rebbi Eliezer, it is not necessary for the house actually to be demolished for the clothes and then the person's body to become affected. If there are four stones affected according to Rebbi Yishmael, or eight stones according to Rebbi Akiva, then Rebbi Eliezer agrees that this is a sufficient warning, and the process can continue to the clothes and to the body.

4) A Talmid Chacham added another insight along the lines of what I wrote above in my second answer.

a. We find in Vayikra 14:34 that the Torah states "When you shall come to the Land of Canaan, which I give you for a holding, and I will put the plague of Tzara'as in a house of the land of your homestead." The Mishnah (Nega'im 12:4) learns from this that Chutz la'Aretz does not become Tamei through Nega'im.

b. The question is, what does Rebbi Eliezer do with this verse, that tells us clearly that Tzara'as in a house applies only in Eretz Yisrael? According to Rebbi Eliezer, even in Eretz Yisrael the plagued house is never demolished, so why is it necessary to have a verse to tell us that it does not apply in Chutz la'Aretz?

c. The answer is similar to what I wrote above. Rebbi Eliezer agrees that the plague in the house serves as a warning to the owner that he should do Teshuvah. It cannot make him destroy the house, but when he sees these strange stains and decay in the walls of his house, he should realize that it is a miracle and a wonder, and it should serve as a reminder to him to mend his ways. In Chutz la'Aretz, one does not even have this reminder, since this miracle does not happen outside of Eretz Yisrael, so in Chutz la'Aretz he does not have the privilege to be warned to do Teshuvah.

5) Another Talmid Chacham provided a different proof for the above ideas.

a. The Mishnah in Nega'im 2:3 (cited in Sanhedrin 92a and Chulin 10b) states that the windows of a dark house are not opened for the inspection of the Nega. This is derived from a verse (Vayikra 14:35) which says that the house owner says to the Kohen, "Something like a plague has shown itself to me in the house." We learn that it is required that the Nega show itself "to me" and it is not sufficient that it showed itself only "to my candle."

b. It seems that the Nega may very possibly be a Nega which could lead to the house being demolished, but because of a side problem -- of not being visible to the unaided eye -- the Kohen cannot make any use of this Nega. The question is, why did Hash-m send this Nega, especially as we know that it is anyway a miraculous, supernatural occurence? The answer is that the Nega that the owner sees in his house is meant to be a warning to him that he must do Teshuvah and stop speaking Lashon ha'Ra. We learn that it is not only according to Rebbi Eliezer b'Rebbi Shimon that a Nega can be a sign for Teshuvah even though the house cannot be destroyed, but in fact everyone agrees that the main purpose of the Nega'im is to awaken us to Teshuvah.

Dovid Bloom