Should I use the additional information section on the Common App?

Under 'Additional Information' in the Common App the following prompt appears:

"Please provide an answer below if you wish to provide details of circumstances or qualifications not reflected in the application. You may enter up to 650 words."

I have extenuating circumstances that I think colleges should know about, but would writing another 650 word essay damage my chances? It just seems like a lot for them to read in addition to the rest of the application. I don't want to devalue my other essays by making my application too long and cumbersome.

You should definitely use the additional information section if you feel it's warranted. This won't detract from the rest of your application. Many applicants use the additional information section for a variety of good reasons (additional achievements, explanation of discrepancies in high school transcript), and admissions officers treat it seriously. I'd recommend talking to your guidance counselor about this to verify that the information you want to include is relevant and important. You don't have to write an actual essay - it's fine to just write a couple sentences or use bullet points for the different things you want to address. It's not intended as another supplement.