"McMansion is a slang architectural term which first came into use in the United States during the 1980s as a pejorative description. It describes a particular style of housing that—as its name suggests—is both large like a mansion and as generic and culturally ubiquitous as McDonald's fast food restaurants.
In addition to ubiquity, almost every reason to poke fun at McDonald's has been applied metaphorically to "McMansions". These criticisms include the deviation from traditional local or regional architectural style; a gaudy, sterile, mass-produced appearance; and perceived negative effects on nature and neighborhoods. . ."
That's the definition coalescing over on Wikipedia.