PLANO, Texas—A half-hour drive straight north from downtown Dallas sits one of the fastest-growing counties in the country. Cotton fields have been replaced with Toyota’s new North American headquarters, a Dallas Cowboys training facility and a sand-colored shopping strip with a Tesla dealership and a three-story food hall.

Yet even with the booming growth, Dallas’s once vibrant housing market is sputtering. In the high-end subdivisions in the suburb of Frisco, builders are cutting prices on new homes by up to $150,000. On...