The concept of urban unity is based on the continuity of the built environment and the number of inhabitants. Urban units are built in metropolitan France and in the French overseas departments according to the following definition: a commune or group of communes with a continuous built-up area (no more than 200 metres between two buildings) with at least 2,000 inhabitants.
If the urban unit is located in a single commune, it is referred to as an isolated town.
If the urban unit extends over several municipalities, and if each of these municipalities concentrates more than half of its population in the continuous built-up area, it is called a multi-municipal agglomeration.
If one of these municipalities concentrates less than half of its population in the continuous built-up area, but 2,000 or more inhabitants, then it is an isolated urban unit. The Paris conurbation refers to the multi-municipal conurbation containing Paris. Finally, the term "commune outside the urban unit" refers to communes not assigned to an urban unit.
These thresholds of 200 metres for the continuity of habitat and a population of 2,000 inhabitants are the result of internationally-adopted recommendations.
In France, the calculation of the space between two constructions is realized by the analysis of databases on the built of the National Institute for Geographic and Forest Information (IGN).
It takes into account cuts of the urban infrastructure such as waterways in absence of bridges, gravel pits, important difference in level. Since the cutting of 2010, certain public places (cemeteries, stadiums, airfields, car parks...), industrial or commercial land (factories, industrial estate, shopping malls...) are treated as built up with the rule of 200 meters to connect inhabited zones of construction, unlike the previous cuttings where these spaces were only cancelled in the calculation of the distances between built up.
The urban units are periodically redefined. The current zoning dated 2010 was established in reference to the population of the census of 2007 and on the territory geography on January 1st, 2010. A first demarcation of towns and agglomeration was realized whith the census of 1954. New urban units were then constituted during the censuses of 1962, 1968, 1975, on 1982, 1990 and 1999.
The urban units can extend over several departments, even cross the national borders (see : international urban unit).
The cutting in urban units concerns all the municipalities of Metropolitan France and French overseas departments. The cutting of 2010 integrates the municipalities of the new department of Mayotte.
These thresholds, 200 metres for the continuity of the built environment and 2,000 inhabitants for the population of built-up areas, result from recommendations adopted at the international level. For example, in the European regulation governing the population census, population statistics based on zoning in urban units are expected.
The calculation of the space between two buildings is carried out by analysing the databases on buildings of the National Institute for Geographic and Forestry Information (IGN). It takes into account the cuts in the urban fabric such as watercourses in the absence of bridges, gravel pits, major gradients.
Since the 2010 breakdown, certain public spaces (cemeteries, stadiums, airfields, car parks, etc.), industrial or commercial land (factories, business parks, shopping centres, etc.) have been treated as buildings with the 200-metre rule to link inhabited building zones, unlike in previous breakdowns where these spaces were only cancelled out in the calculation of distances between buildings.
Urban units are periodically redefined. The current zoning, dated 2020, is established with reference to the population known from the 2017 census and the administrative geography of the territory on 1 January 2020. The previous year, dated 2010, was based on the 2007 census and the administrative geography of the territory on 1 January 2010.
A first delimitation of towns and cities was carried out on the occasion of the 1954 census. New urban units were then formed in the 1962, 1968, 1975, 1982, 1990 and 1999 censuses.
Urban units may extend over several departments, or even cross national borders (see international urban unit).
The division into urban units concerns all the municipalities in metropolitan France and overseas departments.