The sustainable city is not simply the city that resists the wearing effects of time, but the city that transmits to future generations a preserved, if not increased capital. A city that does not produce its own resources will not be able to make the investments necessary for its sustainability. This is the basic thesis of this study in which a new urban social contract between the civic small and medium sized enterprizes (SMEs) and its cities are proposed. They have to evolve in the framework of co-existence and functional mix, of urbanity and citizenship in order that the city becomes once more the "big collective establishment".