This Halacha is an excerpt from our Sefer
Leaving part of one’s wall unpainted:
When the Temple was destroyed, the Sages of that generation instituted that one may never build a home that is painted and decorated like the palace of a king, and rather one is to cement the walls and paint the walls of his house with white paint, and then leave a space of 1×1 Ama [48cm x 48cm] unpainted. [Likewise, those that place curtains on their walls are to leave one Ama without a curtain.]
Non-white paint: Some Poskim rule the above obligation only applies towards pure white lime paint, however if one painted using lime which is mixed with sand, then it is not required to leave a 1×1 Ama area unpainted.
Leaving a black painted square area: Some Poskim rule it does not help to paint a 1×1 Ama space with black paint, and rather the space must remain unpainted. [It likewise does not suffice to leave a picture of the Temple on the wall, and the like.] Other Poskim however justify those who do so and so is the custom of many to paint a black square on which they write Zecher Lechruban.
A non-painted house: If one cements the interior walls of the house and does not paint it, it is not obligated to leave a square space without cement.
Where on the wall is one to leave the unpainted square? One is to leave the unpainted square opposite the entrance. [Some Poskim explain this to mean opposite the entrance of the house, in order so that when one enters through the main door he will immediately see and remember the unpainted area. Others however are accustomed to position it on top of the door in order so that when one sits facing the door he is able to see the unpainted area.]
One who purchases a house: One who purchases a painted house, or house with decorated walls, may remain in it with its painted state and is not obligated to scrape off the paint from a 1×1 Ama area. [This however only applies if the house was originally built and painted by a gentile. If, however, the house was originally built and painted by a Jew, and the Jew transgressed and did not leave a 1×1 Ama space unpainted, then he is obligated to scrape off the paint of a 1×1 area.]
The custom today: It is no longer accustomed today to leave an unpainted square on one’s wall for a variety of reasons. Nevertheless, some Poskim conclude that one is not to rely on the above custom. [Amongst Gedolei Yisrael, there were many that did not leave a 1×1 Ama space, while others were careful to do so. The Rebbe did not have a 1×1 Ama space in his home, although the Rebbe Rashab was careful to do so, and so was the custom of other Chabad Chassidim.]
If one paints the walls of his house with white paint, he is required to leave a space of 1×1 Ama [48cm x 48cm] unpainted. Some say this space should be made opposite the entrance of the house, in order so that when one enters through the main door, he will immediately see the unpainted area and remember the destruction. Others however are accustomed to position it on top of the door in order so that when one sits facing the door he is able to see the unpainted area. One who purchases a painted house, or house with decorated walls, may remain in its painted state and is not obligated to scrape off the paint from a 1×1 Ama area. Many people today are no longer accustomed to leave an unpainted square space in the home, although the custom of the Rebbe Rashab was to do so.
If one painted the home and forgot to leave an Ama space, must he peel off the paint?
Must one leave a 1×1 Ama space in every room of the house?
Some Poskim rule one is possibly obligated to leave an unpainted space in each room of the home. Other Poskim however rule it is only necessary to be done one time in the home. Practically, the custom today is to do so only in the main room of the home, opposite the entrance of the home.
Must one leave a 1×1 Ama space in a store or other building?
 Michaber 560:1; Braisa Bava Basra 60b
 Michaber ibid
 M”A 560:3 in name of Shelah
 Beis Yosef 560; Shulchan Gavoa 560:1; Mamar Mordechai 560:1; M”B 560:2; Kaf Hachaim 560:4
 M”A 560:3; Levush; Shelah p. 200; P”M 560 M”Z 1; Chayeh Adam 137:1
The reason: As this itself is considered a design. [ibid]
 Elya Raba 560:4; M”B 560:2; Kaf Hachaim 560:8
 M”B 560:2
 Michaber ibid
 1st explanation in P”M 560 A”A 1; Aruch Hashulchan 560:1; Main opinion of Shaareiy Teshuvah 560:1; See Igros Kodesh Rashab Halacha 24 [5 letter 1097] that implies he instructed it to be placed opposite the opening and not on top of it.
 Custom brought in P”M 560 A”A 1 and Shaareiy Teshuvah ibid; Final conclusion of P”M 560 M”Z 1
 Accordingly, if one’s sitting positions faces opposite the door, then according to all the unpainted square should be left opposite the door.
 Michaber ibid; Gemara ibid
 The reason: As we assume the house was painted by a gentile and was then purchased by a Jew. [M”B 560:4]
 M”A 560:4; M”B 560:4
 M”A 560:2
 P”M 560 A”A 2; Biur Halacha 560:1 “Sheiyn”
 Shaareiy Teshuvah 560:1; Kitzur SHU”A 126:1; M”B 560:2; Aruch Hashulchan 560:4; Hisorerus Teshuvah 1:164; Kinyan Torah 1:117
 The reason: Some explain that today we are lenient like the opinions which say that lime which is mixed with sand is not required to have a square left unpainted, and today all the lime paint has sand mixed in it. Others explain that the obligation only applied by perfectly white lime paint, in contrast to the lime paint of today which is slightly obscured. [Poskim ibid; M”B 560:2; Aruch Hashulchan 560:4]
 Shaareiy Teshuvah ibid “There is nothing to rely upon…”; Piskeiy Teshuvos 560:1; Nitei Gavriel 99:2; The M”B ibid and Kitzur SHU”A ibid conclude that the above reasons for the leniency are difficult to accept. However, the Aruch Hashulchan plainly rules that today there is no longer an obligation to leave an unpainted area being that all of our lime paint is mixed with sand.
 Heard from Rabbi Groner in a personal correspondence; Although to note that the Rebbe’s house may have been purchased from a gentile, in which case it is not required to have an Ama removed, as stated above, and hence no proof can be brought from the Rebbe’s custom.
 See Igros Kodesh Rashab Halacha 24 that the Rebbe Rashab wrote to his son the Rebbe Rayatz that he told the person in charge of arranging his home to leave a space opposite the entrance without any decoration.
 Rav Eliyahu Landa stated to me that his father, Rav Yaakov Landa, left an Ama unpainted by his house and that he never heard that one is not to do so. However, Rav Ashel Lemel Hakohen stated to me that the widespread custom is to be lenient and he and his father did not leave an Ama.
 Elya Raba 560:3; Kaf Hachaim 560:15; See M”A 560:4
 Elya Raba 560, brought in Shaariey Teshuvah 560:1
 Sheilas Yaavetz 169, brought in Shaariey Teshuvah 560:1
 Sdei Chemed Zayin 12; Kinyan Torah 1:117; Mishneh Halachos 5:72
 Sheilas Yaavetz 169; Piskeiy Teshuvos 560:7 footnote 37