So you've gotten into writing this story idea, and the word counts are steadily climbing. You're not even a third of the way through your intended plot, and already you've hit the industry accepted word count for your genre. Before you rush off to turn your manuscript into a trilogy, think this through.
As I've discussed before in the post Length matters, but story matters more, agents and editors use word counts as a first-level indication of the maturity of the writing. Going dramatically over what is considered to be an acceptable word count length could be a sign of over writing — writing that is filled with too much backstory or unnecessary description. Yet, coming under the word count could be a sign that not enough attention was given to the details — that a story is all tell and no show.
But let's say that you are the most gifted writer on the planet, and that every word in your manuscript has its purpose. (We'll ignore the fact that a professional editor will have different ideas, but we'll carry on.) There is no way that you can do your story proper justice in a single book. Or maybe you set out from the beginning to actually write a series.
The place where a novel finishes is NOT when you hit 100,000 words, but rather when a story plot arc comes to a conclusion. This is where the dreaded synopsis can help in a big way.
Writers who are heading down the traditional publication path are likely about to scream at me. Synopses are scary things — whittling that 80,000+ words into only 500. But that's a submission synopsis. What I'm talking about is an editorial synopsis.