This Halacha is an excerpt from our Sefer
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If a wall fell in one’s house and created a mound of heavy rocks, then if the mound is three Tefach in height one is not required to remove the mound and search there for Chametz and is rather to rely on his future nullification. Furthermore, even if one knows for certain that there is Chametz under the mound, if there is three Tefach of mound over the Chametz, he is not required to undo the mound, and rather it suffices to nullify the Chametz before Pesach. This, however, only applies if one does not plan to undo the mound on Pesach. If the mound is less than three Tefachim [24 centimeters] in height, or he is unsure if there is three Tefachim of rocks over the Chametz, then he must undo the mound to search if there is any Chametz under it, even if he plans to nullify his Chametz before Pesach. See here for the full article on this subject!
Practical application-Must one search for Chametz under furniture or appliances?
Based on the above law regarding a mound, the following is the ruling regarding searching and cleaning for Chametz under furniture and appliances:
Furniture or appliances which have not been moved throughout the year, and will not be moved during Pesach, and are a height of 24 cm [such as closets, book cases, ovens, and items of the like] do not have to be moved before Pesach and have the Chametz that is under them cleaned out. This applies even if one sees Chametz under them. If, however, the Chametz is reachable with one’s hands, one must destroy all the Chametz that can be reached. Furthermore, if the furniture is commonly moved, then one is required to move the items and clean under them even if they have a height of 24 cm.
If one had already moved the furniture before Pesach: The above law only refers to a case that throughout the year, since the previous Pesach, the appliance or furniture was not moved. If, however, the furniture had been moved anytime since the previous Pesach, and was not checked and cleaned for Chametz prior to being replaced, then it is subject to the law explained in E. [There it is explained that initially throughout the year, one must clean and check certain areas prior to replacing the furniture. However, Bedieved if one did not do so he is not required to move the item and check under it, unless the item was moved within thirty days before Pesach, in which case it must be moved again before Pesach to check for Chametz.]
An appliance that is on wheels: An appliance which rests on wheels is to be moved and have its Chametz cleaned from under it before Pesach.
Checking for Chametz before placing an appliance or furniture on one’s floor:
Throughout the year, prior to moving in a new appliance or piece of furniture into one’s home, in an area where one frequently eats, one is to clean and check the area for Chametz, just as he would do on the night of the 14th. Within thirty days before Pesach, one is to do so even when moving a new appliance or piece of furniture into areas that one does not frequently eat in, but is Rabbinically obligated in Bedika. Thus, prior to installing a new oven into the kitchen or a new set of book cases into the dining room, one is to clean and check for Chametz in that area. The above law is not limited only to a case where a new piece of furniture is being entered into the home, but applies likewise to a case that one is moving a piece of furniture or appliance for whatever reason, and then plans to replace it in that area. Anytime this is done throughout the year, one is to first clean and check the area for Chametz, and only then move the item back.
Based on the above law, when moving homes, prior to moving furniture and belongings into a new home, one is to clean and check the floor for Chametz. This applies throughout the year, anytime one moves to a new home.
The Pesach room:
Many people designate a Pesach room to place all the Pesach items some while before Pesach. Based on the above law, it is imperative for one to check the room for Chametz using a candle, the night prior to placing the items into it. This applies even though one has already cleaned it thoroughly from Chametz, nevertheless, a night checking is required. It once occurred by the Rebbe Rashab, that the Pesach room was cleaned and stored with Pesach items prior to a Bedika being done and the Rebbe Rashab instructed for everything to be removed from the room and have it checked for Chametz as required.
 If however its height does not reach 24 cm, then we suspect it may become revealed by a dog just as we suspect by a mound less than 24 cm. This is not similar to Chametz which is stuck between cracks, as explained in Kuntrus Acharon 433:4.
 The reason: As they have the same law as Chametz found under a mound, and Chametz found under a floor panel of which nullification suffices. Just like in those cases we do not suspect one will come to undo the tile, similarly here we do not suspect one will move the furniture or appliances and a dog will not be able to remove it. As with regards to a Mapoles less than three Tefachim of which we suspect it may become removed
 Must one use a broom and the like to reach the Chametz under an appliance? In all the above cases in which one is not required to remove the item sitting on the Chametz, he does not need to use a broom to remove the Chametz that is beyond his hands reach. [So is implied from all Poskim who never mention a need to use an item to help one reach the Chametz.]
 If the item is commonly moved, then it falls under the category of a mound that one plans to undo on Pesach. If, however, one plans to not move the item throughout the entire Pesach, then seemingly there is no requirement to clean under it if it reaches a height of 24 cm. As in such a case it is exactly similar to a mound. [See Sefer of Rav Blumenkrantz in 3-42]
 The reason: As a) The wheels greatly elevate the appliance and thus it is not directly resting on the Chametz as is the case with a mound. B) It is common, and easy, to be moved.