Home prices are on the up, but that may lead to trouble in the future.

The median cost of a single-family home jumped 12 percent in the third quarter to $313,500, marking the biggest year-over-year increase since 2013, according to data from the National Association of Realtors that was first reported by Bloomberg.

“Favorable mortgage rates will continue to bring fresh buyers to the market,” said Lawrence Yun, NAR’s chief economist. “However, the affordability situation will not improve even ...