Actually, for the middle class (which was admittedly somewhat smaller than what we may think of as "middle class" now) domestic help was pretty normal before the 20th century. http://www.bradford-delong.com/2013/01/agatha-christie-i-nev... is somewhat telling; Agatha Kristie's Wikipedia entry says her family was "upper middle-class", but it's not that common for families like that to have live-in help nowadays. Except maybe au pair arrangements.

Having occasional domestic help has been typical of the middle class in capitalist societies for a long time; in early capitalist societies, often full-time rather than occasional, but with smaller staff than the upper class.