I want to write a chapter that would benefit a sub-plot I have in the book I'm writing,but doesn't have any of the major main plot points in it. Obviously I have them all coming together towards the end, I was just wondering if it would be risky writing another chapter just to contribute to a sub-plot?
I wrote a single chapter in my first story that was originally intended to be at the beginning Although pretty much unrelated to the main plot, it introduced a character who became essential to the story later on.
In the end, because it took place years later than the first chapters I put it halfway through the book and, since it was quite long (8.5k words), made it a whole section, writing a new prologue including this character for the beginning. Luckily I found a well-known battle he took part in the same year the main story began.
So I'd say so long as it's easy for the reader to understand what's happening, go with Adrian's comments.
While you're on your early drafts, write whatever you want and enjoy it.
If you do intend to add another chapter make sure that it earns its keep. There is the danger that an additional chapter may appear to be stuck-on. Skilfully written sub-plots usually evolve over several chapters and are seamlessly woven alongside the main storyline and plot. Minor crises that advance the main storyline, and serve to characterise and individualise various characters
Good novels have sub-plots that help to support and advance the storyline. Side-stories that involve one or more characters that are not the protagonist or antagonist. It's important the sub-plot events do not have a greater impact than those of the main plot. The focus should always be on the protagonists struggle against adversity, how he/she adapts to their rapidly changing circumstances, learns from their experiences as they try to achieve their aims.