I’m in a completely different part of the country, but my house built in 1982 has an ensuite. no stairs, tho (thank goodness). So did my parents come to think of it and they bought maybe 10 years before 82? somewhere around there. Maybe Calif pioneered them first, tho? I am architecturally illiterate.
My house in California was built in 1978. Has an ensuite bathroom and a dressing room. No stairs here either. I also have a bathroom across from the other 2 bedrooms. Wow! Could’ve happened here! lol