You can do it @Turtle91's way if you like. Easier to just define an alternative paragraph style.
Your EPUB code may be compatible with more devices if you ALWAYS give a <p> tag an explicit style. So define a "p.normal" style as well as a "p.extra_space_above" one.
I agree with @JSWolf, white space is much more effective on the printed page than on screen. Don't fall into the trap of trying to make an eBook a fascimile of a well-designed printed page. It ain't going to happen!