How do we remember the Churban on our homes?
When I did construction on my home I asked about this and was told that there is no need to leave an unfinished spot in America. The difference between American and Israeli construction methods should explain this. In Israel, everything is poured concrete, and plaster is put over it - thus it would make sense to continue to practice of leaving one square amah unfinished. In America, by contrast, we frame our walls with wood, and then nail sheetrock over it. In other words, we never plaster and it does not make sense to cut a hole in the sheetrock, thus exposing wires and pipes. Leaving a square amah unpainted is not what the halacha requires of us.
The Chayei Adom(137:1) says a person who has a home should leave a part of the wall without plaster or paint. The person who does it is guaranteed that his house will stand forever ,and nothing will happen to it (Kaf Ha’chaim 560:11). There is a weak Heter today as people homes are made of less permanant material,sand mixed into the plaster (Rivevos Ephraim 4:136) The Kitzur Shulchan Aruch (126:1) still wonders why people are lenient.It should be made opposite the door of the house (Kitzur Shulchan Aruch 126:1) it should be visable and not be blocked by a curtain or anything else(Sefer Habayis 19:footnote 7).If you can not make it on the wall opposite the entrance you should make it as close to that wall as possible. Although highly unadvisable some people make a zecher l’churban by painting a part of the wall black and writing “zecher l’churban” after on the area painted black. the person who buys from a gentile need not be concerned with making it as it was made by a gentile. If somone bought a home from another Jew and it does not have a "zecher l'churban" unfinished wall area, the owner must make a "zecher l'churban".(Mishnah Berurah 560:1, Piskei Tshuvos 560:3).