Gemara

(Mishnah): If one buys a Chatzer with ledges extending into the Reshus ha'Rabim, he has a Chazakah that these were built properly.

(Rav Huna): If the ledges fell, he may rebuild them.

Question (Beraisa #1): Nowadays (after the Churban), we may not plaster, do Kiyur or Piyu'ach (apply very white plaster, or pictures) or make pictures on the walls. If one bought a plastered or pictured Chatzer, he may keep it that way. If it fell, he may not rebuild it like it was.

Answer: That is an Isur; it is different.

(Beraisa #2): One may not plaster his house. If he mixed in sand or straw, it is permitted.

R. Yehudah says, if he mixed in sand, this is Trachsid (cement), and it is forbidden. If he mixed in straw, it is permitted.

(Beraisa #3): After the Churban, many Perushim (pious people) abstained from meat and wine. R. Yehoshua dissuaded them.

R. Yehoshua: We must mourn, but we should not mourn too much. "Ba'Me'erah... Kove'im ha'Goy Kulo" - we do not make decrees that the congregation cannot endure. Chachamim said that one may plaster his house, but he leaves a small amount unplastered.

(Rav Yosef): He leaves a square Amah.

(Rav Chisda): He leaves it opposite the entrance.

Rishonim

The Rif and Rosh (Ta'anis 10b and 4:36) bring from our Gemara the obligation to leave an Amah without plaster.

Rambam (Hilchos Ta'anis 5:12): After the Churban, Chachamim of that generation decreed never to build a plastered building with pictures on the walls like kings build. Rather, he plasters his house, but he leaves unplastered a square Amah opposite the entrance.

Question (Tur OC 560): R. Yehudah taught that if he mixed in sand, this is cement, and it is forbidden. If he mixed in straw, it is permitted. We conclude that any plaster is permitted, as long as he leaves a square Amah unplastered opposite the entrance!

Answer (Beis Yosef DH v'Tamahni): Beraisa #3 is not the conclusion of the Sugya. It is an independent Beraisa! Even if you will say that it argues with Beraisa #3, perhaps the Halachah follows Beraisa #2. The fact that Beraisa #3 was brought last does not mandate that the Halachah follows it. Also, perhaps the Beraisos do not argue. Beraisa #2 discusses a Heter through mixing in sand or straw, and Beraisa #3 discusses a Heter through leaving a place without plaster! According to the Tur, which plaster is like kings use and the Rambam forbids it, and which does the Rambam permit for one's house? Rather, the Rambam does not distinguish between kinds of plaster. Beraisa #1 forbids pictures made on plaster. Without plaster, they are not beautiful, and it is permitted. Alternatively, commoners put mud, and then plaster. Kings use only plaster. The Rambam says 'one may never build... like kings build', for there is no Heter for this, even through leaving a remnant. He holds that Beraisos 2 and 3 discuss Heterim through mixing in sand or straw, and leaving a remnant. In the Tur's text of the Rambam, the Rambam rules like R. Yehudah. In our text, he rules like Chachamim, and he did not need to mention sand or straw, for cement is always permitted! The Rif and Rosh omitted Beraisa #1. Perhaps they hold that since normal plaster requires a remnant, surely plaster like kings is forbidden even with a remnant! Or they hold that a remnant permits even this. The Ran (10b DH Garsinan) brings Beraisa #1, and connotes that the Rif rules like it, like my first explanation. Also the Ramban brings it.

Taz (560:1): The Tur holds that the three Beraisos do not argue. The Rambam explains that Beraisa #1 forbids plaster, unless there is mud underneath. People will know that there is mud underneath only if he leaves a remnant. A remnant without mud does not permit, for people will think that he will finish it later. The Tur argues, for just like Beraisa #2 gives a Heter without mud, also Beraisa #3! Rather, Beraisa #1 forbids plastering the entire house. It does not discuss Heterim.

Rambam (ibid): If one bought a plastered or pictured Chatzer, it has its Chazakah. He need not peel the plaster off the walls.

Poskim

Shulchan Aruch (OC 560:1): After the Churban, Chachamim of that general decreed not to build a plastered building with pictures on the walls like kings build. Rather, he plasters his house, but he leaves unplastered a square Amah opposite the entrance.

Magen Avraham (3): Some people make the square Amah black. This is improper. Also this is a picture! Those who hang embroidered garments around the walls should leave a remnant of a square Amah.

Mishnah Berurah (3): The remnant should be visible immediately when one enters. Some put it above the door opening, and one who enters does not see it. Perhaps this is so the Ba'al ha'Bayis, who sits near the wall opposite the opening, will always see it. Some make the square black and write on it 'a remembrance of the Churban.'

Kaf ha'Chayim (8): Blackening an Amah is improper. This beautifies the house! It does not help to write on it 'Zecher l'Churban.' Perhaps only black paint is forbidden, but one may simply blacken it.

Kaf ha'Chayim (10): Mishbetzos Zahav requires that the remnant be square. He says that it is near the doorway. Sha'arei Teshuvah says that it is opposite the doorway. People have become very lenient. This law is almost forgotten; there is no justification for this. Every house that fulfills this will last forever, and also those who live in it

Mishnah Berurah (1): The custom is like the Tur, that leaving a remnant of a square Amah permits even if there is no mud.

Mishnah Berurah (2): If sand was mixed with the plaster, this permits through a remnant of a square Amah. Some say that it permits without a remnant. Nowadays people do not leave a remnant. Perhaps they rely on the latter opinion. However, people use pure plaster (without sand)! Perhaps our plaster is permitted because it is not so white. This is difficult. What do people rely on? If one used only mud, all agree that no remnant is needed.

Shulchan Aruch (ibid): If one bought a plastered or pictured Chatzer, it has its Chazakah. He need not peel the plaster off the walls.

Taz (1): The Beis Yosef says that the Rambam did not need to teach that if one bought a plastered or pictured Chatzer, and it fell, he may not rebuild it, for this is obvious. This is astounding. The Tana held that it must be taught! The Gemara challenged another opinion from this clause! Rather, the Rambam's final words ('it has its Chazakah. He need not peel off the plaster') are redundant, i.e. to teach that he is exempt only from the toil of peeling off the plaster, but if it fell, he may not plaster it again. The Tur did not bring the Heter of one who bought a plastered Chatzer, for the Chazakah is that one may assume that it was built b'Heter before the Churban. The Tur brings only laws that often apply nowadays; the Rambam brings even what does not apply nowadays.

Magen Avraham (4): One may assume that it was built b'Heter, i.e. before the Churban or through Nochrim. If one knows that it was built b'Isur, he must peel it off.

Kaf ha'Chayim (2): One may totally plaster a Beis ha'Keneses or Beis Medrash. It seems that the same applies to a Sukah during Sukos. If he sits in it at other times, he should be stringent.

Kaf ha'Chayim (5): Some say that one must leave a remnant in every room. Others exempt; one who is stringent need not leave a full Amah.