As in your comment on PeterJansson's answer: there are two different issues, namely, the literal copyright business, and then also "ethics". The copyright aspect is quasi-objectively decideable.
So, suppose there's no copyright conflict. The "ethics" issue still needs clarification. First, there's the idea of accidentally or intentionally double-dipping in terms of getting two status-credit-points for just one thing. That's the main external objection to "self-plagiarism", as opposed to plagiarizing from others. A grant proposal scores no status points in my world, so that element of ethical problems seems not present.
It's true that an over-the-top notion of self-plagiarism would seem to dictate that one is never allowed to re-use anything one has written, no matter what. I can't agree with this, even at the level of work done for courses, where I have very mixed feelings about declaring students cheaters because they re-use their own work to varying degrees. One objection I have is that this only really makes sense if either the university or instructor declares that it "owns" all the work done by students, or, worse, that the only point of the enterprise is in-the-moment... no accumulated expertise is re-usable? Odd.
Similarly, it seems to me that too often our thinking about self-plagiarism in professional settings is exaggerated, due to thinking of things as mostly pointless except for status/money-scoring aspects.
Srsly, we're not allowed to ever re-use bits of things "of our own" that have been polished a bit, etc? Start over every time? A very artificial constraint. For that matter, isn't most progress fairly incremental? Obviously... so it is perverse to require that everything be retyped every time, etc. And be sure to use different words... even if one took considerable pains with the earlier wording? Old effort must be discarded? :)