The issue here is defining what indoor plumbing is. It was a small majority in 1940 if you include heated water, a bath, and a flush toilet exclusive to that d

The issue here is defining what indoor plumbing is. It was a small majority in 1940 if you include heated water, a bath, and a flush toilet exclusive to that dwelling unit. If you include toilets alone and apartments with shared toilet facilities, the majority was reached a bit earlier-- around 1930, meaning that I judged indoor plumbing was rapidly becoming commonplace in the late 20s. In 1923, a huge chunk of households still used kerosene lamps as a primary means of lighting (it was a decent percentage in 1940, even).