Nahonsan Seonghwan personally explains the rumor

According to the community, the landlords of the house where Koo Seong-hwan lives are said to be his parents. When asked if that was true, he waved his hand. "That's already causing me a lot of trouble. They've made me seem like a building owner and a rich person, but that's not true. I also pay rent and live there. Since it's not true, I'm not really responding, but there's so much detailed information about the people living there on the Internet that's wrong and it's embarrassing. I hope those rumors don't spread any further," he said, showing that he's paying for his own fame.

https://www.imbc.com/broad/enews/view.html?idx=419726

I've also heard the story about the building owner and the building owner's son, but they say it's not true.