What drives change in the urban tissues? What matters in that question is how diverse the answer can be; there are changes from the bottom up (individual answers to problems that seem personal, but whose sum can produce some kind of “emergent intelligence”), as well as defined from the top of the pyramid (actions defined from the public administration which can lead to direct actions or to mandates to individuals to act in a given manner). Here is a quite concise catalogue of the possible situations, a kind of “table of contents” of what will follow:

• Bottom-up
  • Changes in the dimension of the use unit (households, firms, public facilities…) that mean a change in the physical dimensions of the associated spaces, either through reduction or through extension. For instance, if the number and median size of households rises and urban growth is not possible, what you get are either overcrowded homes or extensions of current homes, either in width or in height.
  • Technological changes that imply a larger choice of locations, be it for homes or for other uses. For instance, cars have allowed urban sprawl.
  • An evolution of social demand that triggers an evolution in the way the functions of spaces in a typology are arranged. If the demand for balconies increases, home façades will change accordingly, transforming the urban landscape.
  • Changes in ownership conditions: a neighborhood in which the residents are mainly owners of their homes can look different from one in which rental is the main condition.
• Top- down
  • Political or technological factors determine how the land can be used: if city walls become obsolete, either as a result of the evolution of military technology or of the disappearance of the risk of war, cities can demolish them and extend.
  • Fiscal conditions: when some architectural types are subject to higher taxes, this can change the real estate market and the building production.
  • Urban planning: plan define mandatory conditions. Paris, Manhattan or Kyoto have particular urban landscapes as in a given moment the decision was taken to organize streets in a certain way and to define how buildings should look.

What is often said to be “the charm of the city” is the combination of all those elements, with varying degrees of poetry and bureaucratic zeal.