Of equal importance, the other reasons for the growth of towns are population growth and the rural exodus. These have brought with them other needs, as well as housing: new jobs, leisure, sport.
Ferrara lies in a key area for a type of development based on accumulation, on a loss of the sense of what a town is. However, in a way it has remained partly immune. The building frenzy has taken place some tens of kilometres outside.
Ferrara represents an alternative model: whether for what has been achieved or for what might be achievable, bringing to completion the 'Addizione verde', twelve hundred hectares of agricultural land lying between the city walls and the River Po which are being taken into the urban environment, forming indeed a majority of its surface area, and are forming the growth zone of the city, a city which is developing differently from those around it, acquiring green space.
Ferrara has protected this measure throughout fifty years of growth.
It has suffered outrages and disfigurements from which it still bears the scars, but it has managed to retain a basic approach of social strength and quality which radiates outwards from the streets and piazzas of the historic centre to many of the areas of low-price buildings designed by Vieri Quilici which went up in the late 1960s and the early 1970s.
In 1978, while waiting for an architects' convention to begin, two friends sketched a design on a map of the city. They were Antonio Cederna and Paolo Ravenna. They had met in an internment camp in Trevano, Switzerland, at the end of 1943.
Their attention was drawn to the immense area on the map which extends as far as the Po, and which the Este dukes called the Barco. It is a strip of the Po valley where the city's former rulers built their country houses, hunted hares and wild boar and organised mounted games.
The area seems limitless, measuring twelve hundred hectares, furrowed in winter with rows of poplars veiled in a light mist, but coloured with the vivid yellow of wheat in summer.
In the view of Cederna and Ravenna, it ought to become a vast urban-agricultural park, the place for Ferrara's natural development, with well-defined boundaries, close by the city and available for the leisure activities of its residents.
The plan solidified over the course of time.
Meanwhile it acquired a name, the 'Addizione verde', recalling terms and ideas of Biagio Rossetti 'Addizione erculea'.
Soon after that the first steps were taken, pausing at an intermediate stage. That intermediate stage was the restoration of the Walls, a nine kilometre length built during the fifteenth century which, according to Bruno Zevi, is neither a simple fortification nor a fence: it is an inspired urban project, the framework for a Renaissance town planning scheme which encircles the buildings of the city with ramparts and embankments.
The condition of the walls was wretched, but a decade of skilful restoration in the years up to 1999 finally returned to the city a historical and artistic heritage, whose essential nature is also interwoven with tree-lined avenues, open spaces and cycle paths, all facilities which Ferrara needed.
Since then, the idea of the 'Addizione verde' has come a long way, sometimes taking a step backwards for every two steps forward. A project proposed by the Municipality, comprising a few sensible and simple things - cycle paths, a few restaurants, a campsite, rural tourist facilities - was left to languish, leading to the resignation of the external consultant.
There were, however, negotiations with the owners of the land with a view to taking advantage of European funding in exchange for switching to organic farming. The area was named after Giorgio Bassani.
Despite universal declarations of support, city policy is struggling to make the final jump, to recognise its own project in this plan perhaps because it is thought to be not very affordable in terms of image or the immediate economic benefits for the population.
It could be given a major impetus by another of the suggested schemes: a site for the National Shoah Museum, a work of contemporary architecture which could bring together the landscape and the history of Ferrara, in the park.