The United States is wrestling with a massive housing shortage — with more than 4 million housing units needed to meet demand. The lack of affordable housing has caused record-high rent burdens and soaring homelessness. More than 770,000 people were officially counted as homeless last year, the highest in modern history. President Donald Trump is even musing about whether to declare a national emergency to address it. At the same time, there are nearly 15 million vacant homes across the US, according to LendingTree, a platform that connects borrowers with banks offering loans. More than a third of these — over 5 million — are concentrated in just the country’s 50 largest metropolitan areas. Could already existing homes be a simple solution to the housing crisis? The idea sounds appealingly intuitive. Earlier this year, I wrote about the idea of seniors renting out their spare bedrooms. It seemed feasible that the same logic — easing pressure through using existing vacancies — could be applied to entire houses.