Use time machine in order to go back and future to teach people on how to build better buildings with experiences from the future as well as learn from past!

In short, get access to previous energy bills or simulate existing situation (in case of absence of old bills) as well as monitor building performance through tests and surveys! Then apply improvements: adding insulation, improving airtightness, changing appliances for more efficient ones, changing windows and doors if they are not meeting targeted requirements. Perform after retrofit monitoring.

As an answer for the question, apply double efforts because what is good today will be worse tomorrow! Because, people, who lived in 1950th thought their houses are perfect, while caveman had only rocks on top and fur/leather as only means of thermal protection. Who knew that Passivhaus Standard will be the best solution tomorrow...so double your efforts and requirements and you will be on safer side.