Answer: The Gemara (Baba Batra 60b) states that following the destruction of the Bet Hamikdash, the Sages of that generation instituted that one may not build a house plastered and adorned like a king’s residence. Rather, when one builds a home and plasters it, one should leave an area of one square Amma (48 centimeters x 48 centimeters) completely unplastered and unfinished opposite the doorway in commemoration of the destruction of the Bet Hamikdash, for if our holy Temple is in ruins, how can we build ourselves a complete home? The Rif, Rambam, and Rosh quote this Gemara as Halacha. The Sefer Sha’arei Teshuva writes that that in our generations, this law is treated very lightly, so much so that this Halacha has almost been forgotten, but there is indeed no room for leniency in this regard.
Nevertheless, Maran Rabbeinu Ovadia Yosef zt”l writes that there is indeed room for the customary leniency people attribute to this Halacha which is Maran Ha’Bet Yosef writes that according to the Rambam, this prohibition applies only when one plasters his home with plaster alone; however, if some sand or stubble is mixed into the plaster, this is not prohibited. Nowadays, all plaster has sand mixed inside and there is therefore room for leniency regarding not leaving over an unfinished area of one square Amma.
On the other hand, if one is covering the walls of the house with wallpaper as is customary in several places, one must leave over an unfinished area of one square Amma, for there is no reason for leniency regarding wallpaper.
If one purchases a home that is already completely plastered and adorned, one is not obligated to peel of an area of one square Amma of plaster if the house was purchased from a non-Jew. Nevertheless, if the home was purchased from a Jew who did so contrary to the Halacha, one is indeed obligated to peel of an area of one square Amma of plaster opposite the doorway.
Those who customarily paint this one square Amma black are incorrect in doing so, for our Sages enacted that this area be left completely bare of any plaster or paint.
An incident once occurred with the great Gaon, Harav Chaim Halberstam zt”l of Sanz, who requested from a close confidant of his to find a matter about which he was acting incorrectly and to bring it to his attention. The individual immediately replied that there was not a square Amma bare of plaster in the rabbi’s home, as per the law. The rabbi answered that since he purchased the home when it had already been completely plastered, he is not obligated to peel of the plaster of this area. Nevertheless, as an act of piety, Rav Chaim immediately climbed up a ladder and peeled off a square Amma of plaster in commemoration of the destruction of the Bet Hamikdash.