If you don't tell your agent this until after he/she begins sending your work out, you could put him/her in a really awkward position.
Okay, imagine this. I've just sent out a manuscript. The author writes, "GK, GK! I have a brilliant idea! This is a series! This is only a series! It cannot be anything but a series! Go tell the editors!"
I will then probably:
- Consider whether the author is just kidding, like the "And my next book is about vampires! Just kidding!" kind of way/if I think it would be better for the work to be a stand-alone or a series. If I think the author just wants a multi-book deal but that the work will not be as good editorially in multiple parts, that's a separate discussion. But, assuming it would work as a series, I'd...
- Get mad at author.
- Consider the best way of contacting editors.