The interest rate is not only based on credit score, of course is a very important factor. But the lender will also look at debt to income ratio as well as the loan to value, and other factors that in your particularly case, don't apply to you. I am assuming that his folks have had credit established for a long time, so you should be able to get a good rate. Specially if you are getting an adjustable rate mortgage. If you are concern about payments being too high, you can ask for a 2/28. The payment will be fixed for two years, and adjustable 28. With a 2 year prepay. Now in 2 years, your credit score will be great as long as you don't default on the loan. Then you will have to refinance to go into a possible fix, if you plan to stay in the house for a long period of time, and also so you can get your in-laws out of the loan and title. Fixed rates are higher than arms (.50%-.75%). A little bit over two months ago, I had a customer with great credit score 720+, we did the loan full doc, and I got him an interest rate, of believe it or not, 5.75% fixed for 30 years! Now I am not saying that is what you are going to get, but you should be able to get a good interest rate, specially if you are putting money down, and you can prove enough income.