“So this guy with the three car garage is actually making a business of turning his neighbors cellars into “Luxury Basements” for their children (who have no hope of ever leaving) to live in.”
In California houses aren’t built with basements. This is the land of the concrete slab foundation. Simply having a basement is a novelty. I do know someone who added a ‘basement’, in reality a large temperature controlled wine cellar.
He could be building safe rooms, hidden storage rooms or bomb shelters and use the luxury basements as a cover.