You could probably find several idioms in Julio Cortázar’s short story “House Taken Over.” Remember, an idiom is a common expression that mostly only makes sense within a specific language or culture. For example, someone not familiar with English or American culture might not understand the idiom “raining cats and dogs.” They might not get that it's a popular way to talk about heavy rain.
Your search for idioms in Cortázar’s short story might reveal how idioms can sometimes transcend culture and language. Remember, Cortázar wrote the story in Spanish. Yet it appears as if Spanish idioms can translate to English idioms.
One idiom you could note happens when the brother mentions his and his sister’s financial situation. He says, “We didn’t have to earn our living.” This could be considered an idiom because, literally, people don’t earn a living. Humans don’t buy their way into existence. They don’t have to pay to be born. What the brother means by this idiom is that he and his sister don’t have to work or acquire extra money to afford the things they want and need in life.
Another idiom you could discuss occurs when the enigmatic beings begin their conquest of the house. The brother says, “I was a little lost without my books.” This could be considered an idiom because the brother isn’t actually lost. He knows where he is. He’s in his family home. When he says “lost,” he means something else. He probably means he’s having a hard time figuring out what to do since the mysterious things have deprived him of his reading materials.