As has been outlined here, putting 20% down is considered a "compensating factors", so if other parts of your borrowing profile fall below standards (such as having a debt ratio higher than 45%, less than 2 months PITI in reserves after down payment/closing, or imperfect credit) it can compensate for those areas. For example, a gal I was helping refinance back in 2007 had 550 scores, but had a good debt ratio (36%) and plenty of reserves (about 8 months PITI), the kicker was she had 50% equity in her home... so she was able to qualify for the lowest interest rates available at the time. I fiddled around with the numbers in automated underwriting and just for kicks I put she only had 10% equity, and it got a lower level approval, thus the equity was the deciding factor in getting her those lowest interest rates.
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