Over the last decade, growth in housing stock in the Paris (Île-de-France) region has occurred primarily through the densification of already urbanised areas.  In 62% of the 926 municipalities in the region that experienced densification between 1999 and 2008, housing stock increased without a concomitant spread of the existing habitat area. This is the result of policies that have explicitly sought to densify the existing urban fabric (Darley and Touati 2011) and reconfigure existing housing: one in every four new dwellings registered between 2001 and 2011 was obtained via the restructuring of extant properties (Davy and Mertiny 2013).
While the processes that lead to the “spontaneous” adaptation of housing stock to market requirements are of great importance, the dynamics involved are varied and sometimes contradictory, depending on the context: large dwellings are created by merging and expanding housing in the urban fabric of the city of Paris proper and the Hauts de-Seine département (immediately to the west, containing many well-off inner suburbs), while a stock of small housing units for rental is growing through the division of detached houses in lower-income areas that benefit from good transport links within the Paris conurbation (Richard and Driant 2013).
The division of individual dwellings – not to be confused with the division of plots  – is a lever for the production of housing that has been underestimated to date. And yet this form of densification through the internal division of housing, produced by processes disconnected from any local political will or controlled public action, remains relatively unknown and difficult to measure.
From large family homes to small rental properties
From an analysis of Filocom  files from 2001 to 2011, it is estimated that the division of 770 detached houses can result in a total nearly 2,000 housing units each year in Île-de-France.
This phenomenon primarily concerns municipalities close to the centre of the conurbation with good transport links. These towns have two characteristics that mark them out as areas of high growth potential for investors: (i) properties in these areas have low valuations (for now) on the first-time-buyer market, but have strong potential to increase in value as a result of their good location and transport links; (ii) furthermore, their central location is due to become even more central – it is in the département of Seine-Saint-Denis (immediately to the north-east of Paris) that the phenomenon is most significant: 1,500 detached houses were subdivided between 2001 and 2011 (20% of all changes observed in Île-de-France).
The main consequence of this process is a change in the types of property on offer, and more specifically the replacement of large, single-family, owner-occupied houses (51% of cases) by private rental apartments (68%) that are generally much smaller (with just one bedroom for the most part) and of low quality. While 90% of homes created in this way possess all the features one might expect, the overall quality of these features tends to be on the low side.
The social effects of this reconfiguration are significant. The occupants of housing resulting from such divisions are households with limited resources: 81% have incomes lower than the thresholds for private rental properties used as social housing, while 49% have income levels below the thresholds for the most basic social housing (known officially as “PLA-I”) – a figure that rises to 62% in Seine-Saint-Denis. These occupants are often young people at the beginning of their residential careers (52% of all tenants aged 25 to 40) or single-parent families (in 18% of cases). Half of such tenants have less than 23 m² (248 sq. ft) per person and 18% of homes are overcrowded  (30% in Seine-Saint-Denis).
The emergence of new, dispersed social housing stock
Four decades after the explosion of suburban development in the Paris region, the prospect of obsolescence looms large for a considerable number of houses, calling for extensive renovation works. These refurbishment needs are similar to those faced by collective housing built during the same period, but are much more dispersed, more “invisible”, and more difficult to deal with as part of major urban regeneration projects. In some areas, a significant proportion of the owners of these homes are no longer in a position to fund the work necessary to renovate their homes and bring them into line with current standards and regulations, either because the original owners have been replaced by less well-off households, or because access to loans is more difficult owing to their age (over 70 in many cases). The shortage of homes on the market in central parts of the Paris urban area therefore opens the door to all sorts of strategies to adapt properties and make them more profitable.
The owners of divided properties are mainly (61%) individuals (as opposed to rental agencies), who do not generally occupy (80%) the dwelling they own. However, they tend to know the area, with three quarters of owners living in the same département. Despite this relative homogeneity, a variety of approaches to the subdivision of suburban homes can be observed.
On the one hand, there are subsistence-related approaches, typically implemented by owner-occupiers in order to:
- facilitate family cohabitation by reorganising the dwelling to accommodate elderly relatives or to enable young adults to continue living there;
- generate a supplementary income by maximising the profit that can be made from renting out their property (in the case of retired homeowners or households in difficulty seeking a means of easing financial problems);
- simply buy property, where the restructuring of the existing property (to generate additional income from rental) is a necessary condition for the purchase of another property.
What all these profiles have in common is the fact that owners remain stakeholders in terms of the maintenance of the restructured building, which they still inhabit and which generally complies with certain minimum building regulations and standards of repair.
On the other hand, there are investment-driven approaches, typically implemented by non-resident landlords, including:
- asset-enhancement strategies, implemented by inheritors of family homes who, in the absence of any pressing financial constraints, decide to transform an inheritance into a rental investment; in some cases, this may also resolve the problem of how to share an inheritance between several heirs;
- approaches amounting to financial exploitation, implemented by unscrupulous owners – sometimes practically property-development professionals – who seek to divide properties in certain neighbourhoods in order to create regulation-free “pensions” and obtain the highest possible returns per square metre by conducting work of questionable quality and organising rental channels intended for vulnerable populations who are excluded from the mainstream housing market.
These practices, once established, sometimes have a “contagion effect” that spills over to the rest of the street or neighbourhood. Tenants, as the victims of unscrupulous investors, often face issues such as the informal management of communal areas and problems related to the sharing of utility bills in cases where individual electricity, gas and water meters have not been installed, in housing stock that is often of poor energy quality to begin with.
Tighter controls to ensure better housing quality?
The suburban fabric of our cities has long been considered by politicians as a planned, finished space that is almost unchangeable, whereas they are today seeing its previously unsuspected potential for urban development, with some even seeing it as a “hidden” resource for achieving certain housing objectives of the Greater Paris project. The division of suburban homes – already at work “under the radar” – fills a void left by public action. As Anastasia Touati has observed, “private actors (developers, builders and individuals) are managing to implement densification in places where municipalities have proposed no clear action in favour of densification. In these towns, this is often permitted by the local development plan, but is sometimes also the result of illegal practices (that is to say, which do not comply with planning regulations and which are in principle not approved politically)” (Touati 2012). This analysis refers collectively to the phenomena of subdividing plots and subdividing houses, both of which can be described as urban‑planning “stowaways” (Petitet 2013), producing a housing supply that is disconnected from both local planning processes and analyses of the local infrastructure’s ability to absorb the needs of the new residents housed in these properties.
Municipalities therefore not only face difficulties identifying and anticipating such developments, but also suffer from the lack of an appropriate legal framework. In the absence of any obligation to obtain a building permit or any relevant planning permission procedure,  they have the means neither to verify the quality of what is produced nor to anticipate the consequences of such densification.  Often, local authorities only become aware of the phenomenon of spontaneous densification when they come face to face with its effects: soaring demand for parking in certain streets, poor housing quality leading to greater numbers of energy support applications and rehousing requests, the saturation of public facilities and services, school overcrowding, etc.
They are thus caught between two equally undesirable alternatives: attempting to block any possibility of changing the built environment by ignoring the needs that can only be met by these kinds of dwellings, at the risk of developing a supply of informal housing of poor quality, on the one hand; and attempting to support the process of densification by regulating it, thus creating an added difficulty in the form of increased demand for public services and facilities without being able to mobilise the funding channels that come with new urban development operations, on the other.
This “spontaneous” densification of the suburban fabric is one of the dynamics at play at metropolitan level. It is the result of a real-estate market characterised by a shortage of properties. In the short term, this phenomenon would appear to be especially difficult to contain given that it offers a rapid response to the need for housing in the Paris region. This is particularly true in the inner suburbs, where the urban fabric is due to become increasingly integrated into the regional transport network and is set to accommodate a significant share of the housing growth objectives laid down in the SDRIF (Schéma Directeur de la Région Île-de-France – Master Plan for the Île-de-France Region). Low-level but altogether real densification in these suburban areas shows the limits of the “preservationist” desires that continue to be enshrined in many (restrictive) local development plans. Furthermore, it calls for public authorities to seriously rethink the future of these spaces, along with the operational tools used to control the social and environmental impact of their densification.
- Darley, Amélie and Touati, Anastasia. 2011. “Processus et politiques de densification de l’habitat en Île-de-France”, Note Rapide, no. 567, Institut d’Aménagement et d’Urbanisme – Île-de-France (IAU‑ÎdF), September.
- Davy, Anne-Claire and Mertiny, Peggy. 2013. “2011‑2011 : un logement nouveau sur quatre a été créé dans le bâti existant”, Note Rapide, no. 613, IAU‑ÎdF, January.
- Petitet, Sylvain. 2013. “Densifier l’habitat pavillonnaire : des démarches individuelles aux projets collectifs”, Métropolitiques, 20 March.
- Richard, Mélanie and Driant, Jean‑Claude. 2013. “Nouveaux logements issus de la restructuration : étude dans le Val‑de‑Marne”, Note Rapide, no. 612, IAU‑ÎdF, January.
- Touati, Anastasia. 2012. “L’habitant maître d’ouvrage, au cœur de la densification pavillonnaire”, Études Foncières, no. 157, June.