back house vs garden apartment

back house

noun
  • A garden apartment. 

garden apartment

noun
  • An urban apartment in a multistory building located at or below ground level. 

  • A detached mother-in-law apartment or cottage on the same parcel as the main house. 

  • One of a set of low-rise apartment buildings built among landscaped grounds and often arranged around courtyards that are open at one end. 

How often have the words back house and garden apartment occurred in a corpus of books? (source: Google Ngram Viewer )