It's perfectly all right to fall in love with a good friend. I've had friends whom I wasn't attracted to initially and then our friendship became deeper, or something changed and I started feeling attracted to her.
It's perfectly ok, and it does happen to some of us. When you notice the change, it's best to let the other person know. Otherwise, the friendship becomes a lie imo if you aren't upfront about how you feel toward your friend. Whether the other person reciprocate their feeling or not, it's better to be upfront when you have notice this shift in you and you'll feel relieved. Here's a good way to do this that Steve mentions:
Initiating Relationships with Openness, Honesty, and Directness