According to a post above, the melech hamashiach will build the beis hamikdash. The question is, who is mashiach? How will he know he is mashiach and how will we know he is mashiach? Perhaps building the beis hamikdash is what makes him mashiach.
I think what the OP is asking is:
If it were possible to build the beis hamikdash today, and I take the initiative to go do it, would that make me mashiach? Or do I have to wait for some sort of nevuah or shofar blast before I am allowed to build it?