Perhaps there is a statute that allows non-permanent homes (such as mobile homes, motor homes, caravans) on a different permit basis than "stick built" homes. Here that is the case...if a building is smaller than a certain size or if it is portable in some way it need not comply with all the planning and zoning laws. Having land and living on it are different things, unfortunately. Perhaps the person who sold them that land did them a dis-service, and may have known the
green belt status.