You start where it matters the most, to you. That might be the lounge or the kitchen. What would drive you mad every day?
First rooms we did were the children's bedrooms because we wanted them to have a space in the new house that felt like "theirs." Then our bedroom, and slowly worked our way round.
The most dramatic was the kitchen extension but that took a while to save up for it. It was worth the wait.
So it comes down to necessity - like a rewire, then what room would you feel needs to be first.