One morning this past June, Claire Segeren and Cal Hunter received legal permission to live in their own home. It had only taken five years. The couple had spent that time renovating their house in the Scottish village of Sandbank, about 35 miles west of Glasgow, after buying it in 2018. With limited funds, they became their own contractors, painstakingly rebuilding the 120-year-old red-sandstone house nearly from the ground up in an effort to restore its turn-of-the-century glory. “When we got the sign-off” from local building authorities, Mr. Hunter said, “it did feel like a real milestone. We both just…