I have to agree with this statement here... Just depends on the type of friendship you have with the girl...
I know a lot of relationships that start out as just being friends. I mean how many relationships actually progress from meeting someone for the first time and then full on officially dating within a month? I mean do both happen, of course. But assuming you didn't ask the person you're dating out or hook up with them within the first 2 weeks of meeting them, you were prob just friends at the start.
As long as you don't make the girl think you're just another one of her girlfriends but instead just as her best friend it can develop into a actual relationship if the timing is right. All about you present yourself, and how she views you. Being actual best friends with someone, and I mean really best friends automatically encompasses a strong emotional connection. Assuming there is attraction between you two, it shouldn't be a surprise how this quickly can lead to a real relationship. Like I said the problem is the timing, but it really isn't that uncommon imo...
On another note, I have no idea how you could seriously date someone who isn't your best friend. Like that is the person you're going to be spending sooo much time with, if that person isn't your best friend it would prob just drive you mad.