WARNING: amateur advice belowJohn 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.