It's described as a viewing platform at https://www.capelmanorgardens.co.uk/whats-here/ 

But it's doubtful if it qualifies for tourism=viewpoint if the view it gives is primarily that of the maze.

Zooming in on the info board at https://www.tripadvisor.com/Attraction_Review-g580413-d2297743-Reviews-Capel_Manor_Gardens-Enfield_Greater_London_England.html#/media/2297743/601786590:p/?albumid=-160&type=0&category=-160 suggests a structure with a roof, so it might qualify as a building but not really possible to tell without a visit.
