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 lime paint, and then leave a space of 1×1 Ama [48cm x 48cm] unpainted.
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 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 it must remain unpainted. 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 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 lime paint, he is required to leave a space of 1×1 Ama [48cm x 48cm] unpainted. Some 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 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.
If one painted the home and forgot to leave an Ama space, must he peel off the paint?
 Michaber 560/1; Braisa Bab Basra 60b
 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; Chayeh Adam 137/1; P”M 560 M”Z 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
 Custom brouht 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 face opposite the door, then according to all the unpainted square should likewise 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
Custom of Rebbe Rashab: The Rebbe Rashab was careful to leave a space unpainted as is required from the letter of the law. [Igros Kodesh Rashab Halacha 24]
 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 stringent and he and his father did not leave an Ama.
 See M”A 560/4