Unlike buildings that may stay unchanged for long periods of time, the city never stops developing. In a sense a city that doesn't change is a dead city. The city is always time-based, it may grow, it may even shrink, but it never comes to a standstill. This makes designing and building of the city a difficult task, especially if one wants to change its course in a more sustainable direction. It is much easier to design an eco city from scratch and build it anew in a remote place, than it is to make an existing, functioning city more sustainable. It is like jumping on a running train trying to change its course in full speed. Changing the city has to be done 'on the fly'.
We cannot afford to keep on building new cities. We cannot afford to erase whole districts and rebuild either. We don't have the capital, but more important we don't have the natural resources anymore. Already half of the built stuff in the world is urban. It is there and it is and dynamic. So the dynamics of the city are to be treated as a given.
However, the dynamics of the city can also be seen as an advantage. The city is a living, breathing organism, with good parts and with bad parts, active in some places, passive in others. It's life-force can be used as an energy to make changes for the better. Most of these changes will be small, subtle, local and even invisible, like surgery or even acupuncture, but they will be necessary to keep the city alive. Functions of parts of the city may change, but this can also be done by improving and restructuring of the existing built mass. In that sense, like materials, the city itself must be reused again and again. One has to work within and with an existing situation. Seen like that (re)designing the city is less composition and much more improvisation. And in improvisation it is crucial to leave space for the other players.
Working on the city in this way is like gardening. Like the garden it changes over time, and a wise gardener doesn't cut down all his trees every time he is not completely happy with the layout. He may cut a bit here, plant a bit there, but most of his work is careful maintenance and let nature do its work.