Developer Nile Niami, currently building a spec house in Bel Air with a price tag of $500 million, rejects neighbors’ accusations of overkill. “‘Why would you not need four swimming pools?’ says Mr. Niami.”

In Los Angeles, where the median household income is $56,000, a $500 million house is rising.

If and when America's class war-driven revolution begins in earnest, there would certainly be worse targets for guerilla action than unoccupied $100 million mansions—monuments to pure greed and speculation.