BOISE, Idaho—Laura Johansen has always loved the easiness of life in her hometown: The first-rate hiking and rafting a short drive from her front door. The quick commute to her job at a local hospital. The affordability.

But when Ms. Johansen, who was renting her home, decided to buy a house earlier this year, the Idaho native was shocked. The median price of a single-family home in Boise’s Ada County had shot up to almost $300,000, well-above the $200,000 homes she was eyeing when she first considered homeownership two years...