They are all in a Conservation Area, but not all the buildings are grade listed, this explains the difference and which are which:
http://74.125.39.104/search?q=cache:28wcnmqASmQJ:www.barnet.gov.uk/bishops-avenue-characterappr.pdf+hampstead+garden+suburb+outside+conservation+area&hl=en&ct=clnk&cd=1&gl=uk&ie=UTF-8