For starters, she is most likely not the head of the household.
Her parents understand that she is a grown woman, but you have to understand that she is also their little girl.
When you date a girl who lives with her parents, you have to court her mother and father too, if you want to be successful.
Not only will your girlfriend judge you based on how you treat the most special people in her life, but having the parents on your side can be a big plus in moving your relationship ahead.
When you pick her up, make sure you are cordial to her parents.