To a visitor strolling along Fifth Avenue the substantial limestone buildings may seem some of the most permanent things imaginable To a real-estate developer the view is quite different. With a map of the underlying zoning in his head he knows that many sites along the Avenue are soft that is the zoning would permit a far larger building than is there right now Although Fifth Avenue is Midtown ‘s most expensive land the demand for office space makes it economic to redevelop which had not been anticipated when the zoning was drawn in 1961.