The term McMansion can sometimes be applied retroactively to eras where the moniker did not exist. For example, a description of Graceland in Memphis uses the term:
Graceland and the nearby newly opened tourist centre – clumsily titled Elvis Presley’s Memphis at Graceland – gets fans close to the King, but don’t dare touch anything. In bricks and mortar, the Georgian-inspired mansion is not really that big. These days, it’s more McMansion in scale than, well, a proper mansion.
According to Wikipedia, Graceland is over 17,000 square feet. The original part of the home was built in 1939 and only later did development encompass the large property (still over 13 acres).
This is still a very big house, even by today’s terms. I tend to apply the term McMansion when the size of the home is roughly between 3,000 and 10,000 square feet. Even then, homes of this size may not meet other traits of McMansions such as being too big for their lot (not a problem with Graceland), architecturally garish or poor quality (not a problem with Graceland), and associated with sprawl and luxury (maybe a bit applicable here). Perhaps Graceland might be McMansion in an interior related to pop culture and kitsch – but that is more likely a function of the home once belonging to a music superstar than it being a typical suburban McMansion.
Today, Graceland is still a mansion. Is it really that different than the large homes of entertainment stars and celebrities today?