As people who love renovations and spine-tingling viral moments in equal measure, we've seen a lot of spooky surprises hidden in homes. (Creepy crawl spaces? Secret messages? Entire ancient cities?

When Gannon learned of the single-stair movement a few years ago, he said, everything clicked. "I was like, 'Oh, my god,'" Gannon told me. "A light went on, like, 'Oh, this is why apartments in ...