The City of Yesterday (top diagram) was unsustainable. It required large inputs of water, energy and food and produced a horrendous torrent of wastes.
The City of Tomorrow, we must pray, will be more sustainable. The second diagram shows a fully vegetated city with the roof space devoted to food production, water management, recreation and gardens, partly public and partly private. Rooflights and courtyards are used to allow daylighting of internal space. Walls are used partly for glazing, partly for vegetation and partly for photovoltaic cells. Soil and vegetation make an enormous difference to the insulation value of walls and roofs.
Twentieth century cities were unsustainable. Except for glazing, the roofspace and wallspace is wasted.
Sustainable cities will make better use of roofs and walls. See detail (above) and overview of a sustainable city (below).