The average rate on a 15-year fixed-rate mortgage went up 17 basis points to 5.268% APR, and the average rate on a 5-year adjustable-rate mortgage fell three basis points to 5.24% APR, according to rates provided to NerdWallet by Zillow. The rate on the 30-year fixed went up almost three-quarters of a percentage point from the end of July to the end of August. But with a 5.875% interest rate, the buyer could afford to borrow $338,100.