Fannie Mae has a matrix (PDF) that shows what risk factors will increase your mortgage interest rate and by how much. Of note to real estate investors: investment properties will add at least 1.75%. (But then, we all knew mortgages for investment properties cost more.) The matrix also shows how much your rate will increase based on your credit score. They also give the adjustments for condos, manufactured homes, 40 year loans (as opposed to the normal 30 year) and some other scenarios. Interesting reading.