I'm going through something similar right now. I've been friends with this girl for about 9 years now, but I never had any real feelings for her. That was probably because she's always had a boyfriend. But recently, she's single again, and I started having really strong feelings for her. I told her about them but she said she wasn't interested in a relationship at this time, but we both agreed to let whatever happens happen. For now, we're still friends, and actually spending more time with each other than before.
I suggest you just be open about your feelings if you are starting to be attracted to her. If you are friends I'm confident that neither of you will do anything that will jeopardize your friendship if you were to take it to the next level.