WARNING: amateur advice below
John Gardner spent 16-18 hours/day working on his novels and several of them took him years to write. He seemed to be a compulsive re-writer, going over each scene again and again until the perfectly birthed scene was born, each revision bringing him closer to the "true" meaning.I didn't get the impression that it was an effort for Mr. Gardner to "bulk up" his own novels, they were often rejected for publication because of their bulk!
However, I would like to say that during my own revising process, most scenes (the ones that aren't cut entirely) actually GROW in length because I "see" the details that were there all along ---- so, perhaps, if you work this way too, your re-writes/edits will give your story the bulk it needs. I wouldn't even begin to think about the word count until you are done with your first revision. I am writing by hand and have no idea how many words I have (although I estimate it's currently at 45K).
Another thought --- you might have yourself a beautiful best-selling novella. These things do happen. I wouldn't add details irrelevant to your story.
The Art of Fiction: Notes on Craft for Young Writers
On Becoming a Novelist
I think these are or were in the Public Library Catalog, so many public libraries will have at least one of them.


Happy writing!
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