Would it ruin a good friendship if i told a girl that i liked her as more than a friend?

Ok well as far as I know I am this girl's best guy friend and she is my best girl friend (not like relationship gf but like a best friend kinda thing) so would it be bad to tell her that I like her because I'm not sure how she would react to me liking her. idk if you guys would kinda subtly telling her i like her, but i was talking to her friend a lot about trying to get the girl i liked to go to prom with me. weve knowneach other since august 07, in january we got to TA a cl together so weve gotten to know each other a lot better. since then ive started to like her a lot and i know she likes me as a friend but i cant tell if she like me more than that. also, if i told her that i liked her as more than a friend do you think that would ruin our friendship if she didnt like me as more than a friend?