Is this also in California? Can’t imagine they’re very many places in the world where people behave this way. That is, people with enough wealth and interest in doing this in particular location.

Can’t imagine they’re very many places in the world where people behave this way.

Really? Because it happens everywhere. I've seen it from Chicago to Seattle to South Carolina. Start going to the zoning board meetings of any town with enough people, and you'll run into it.

In London, they tend to expand down, rather than out, but it happens so often there's a term for it there: Iceberg homes.