A Canadian couple snapped up the private island five years ago and built their dream home—now, a new owner is needed.

“Welcome to Vista Dhome, an extraordinary monolithic dome residence situated atop a breathtaking mountain in Yucaipa, California,” the listing on Sotheby’s International Realty says.

Back in 2014, spending $500,000 on a house was generally reserved for high earners. The typical U.S. home sold for $159,000, while starter homes averaged just $101,000. Fast-forward to today, and ...