ZEKHER LAMIQDASH: leaving an unfinished wall

Although our official mourning for the destruction of the Bet haMiqdash ends the day after Tish’a beAb, our rabbis established some traditions of mourning for the Bet haMiqdash that are maintained throughout the year, as a permanent reminder that our Bet haMiqdash has not yet been rebuilt.
The Gemara in Baba Batra 60b mentions that when a Jewish family builds a house it should not cover the house walls with ceramics, carpets or other eccentric coverings or coatings. So says Maimonides משחרב בית המקדש, תיקנו חכמים שהיו באותו הדור, שאין בונין לעולם בניין מסוייד ומכוייר, כבניין המלכים; אלא טח ביתו בטיט, וסד בסיד, ומשייר מקום אמה על אמה כנגד הפתח, בלא סיד. “When our Bet haMiqdash was destroyed, the rabbis of the generation established that we should not build a building covered as the residences of the Kings. Rather, when plastering the walls with clay and lime, one should leave a space of one ama by one ama without lime.”
As we see, there were two elements to bear in mind when finishing our house: 1. not coating the walls of the house with luxurious materials, and 2. leaving a space of the wall unfinished (Penine Halakha,  Ha’am veha-Arets, p 186, according to the Beit Yosef Sh. A. 560).
Other rabbis following the opinion of the Tur (Rabbenu Asher ben Yehiel) were more lenient and did not limit the type of coating a private Jewish house can have. They said that all what is required is that when building a private house, one should leave at the entrance of the house, facing the front door, a square of wall unfinished and unpainted, to remember, every time we walked into our house, that the house of haShem still remains in ruins.
Most contemporary rabbis follow this second opinion.

The size of this uncoated wall is a square of roughly   1.5 ft by 1.5 ft. (אמה על אמה). That segment of the wall should be left unplastered and unpainted. Similarly, if a person covers his walls with paper instead of paint, he must leave a square of the wall without paper.

If possible, this segment of unfinished wall should be left on the wall opposite the entrance, or as close as possible to the front door, so everyone who comes into the house can see it. Among Ashkenazim there are those who leave the square of unfinished wall above the main door.
When one does not build his own house but buys a house where someone lived there before, does he need to remove the plaster and uncover that segment of wall?
It depends. If the person who built and lived in that house was a Jew, since he was obliged to leave that piece of unfinished wall, and he did not, the obligation to leaving an unfinished square falls now on the new owner.  However, if the original owner was not Jewish, and thus, he was not obligated to leave an unfinished wall, the new Jewish owner is under no obligation to do so now (Shulhan ‘Arukh OH 560: 1), although if he wants to, obviously, he can do it.
In this case and in any other case where technically there is no obligation to leave a square wall unfinished (a rented house, for example), one could hang on a wall a picture or a decorative painting of Yerushalayim, its walls or with the words  אם אשכחך ירושלים   “If I forget thee, Jerusalem …” to educate ourselves and our children to remember the Bet haMiqdash