Clays shrink in heat, swell in rain, and shear the foundations of ordinary houses.
Zones of medium or high exposure to clay shrink-swell cover 55% of French territory. The revised map has been applicable since July 1, 2026 to sales promises and deeds for buildable land as well as individual house construction contracts. The main political question is whether this prevention tool will worsen an already severe housing crisis.
The essentials
- Zones of medium or high exposure to clay shrink-swell cover 55% of French territory, according to the Ministry of Ecological Transition.
- Since July 1, 2026, the updated map applies to sales of buildable land and individual house construction contracts in affected areas.
- Clays lose water during droughts and absorb it when it rains, creating ground movements that crack structures.
- Climate disruption is accelerating the frequency and intensity of drought-rainfall cycles, worsening the risk across the entire territory.
- The political challenge is twofold: building better through the map, without prohibiting construction where housing is most scarce.
Clay beneath houses: a commonplace risk turned urgent
Cracks running across the walls of an individual house do not all tell the same story. Some are cosmetic. Others signal ground movement that can render a dwelling permanently unstable, even uninhabitable.
Clay shrink-swell belongs to the second category. Clay is a sedimentary rock that holds water in its structure. When soil dries out, during a hot summer or prolonged drought, the clay loses volume. It shrinks, sometimes by several centimeters. When rain returns, it swells again.
These alternating movements act directly on the foundations of buildings built on top. Individual houses with shallow foundations—the most common type in French suburban housing—are the most exposed.
The phenomenon is not new. It has been documented for decades as one of France’s costliest natural risks, second only to flooding. But it has become urgent for one simple reason: droughts are intensifying. According to Météo-France, the increase is estimated at +2.2°C over 2016-2025 since the early twentieth century, and soil drought episodes are lengthening. What was a statistically rare risk in certain regions is becoming a cyclical reality across much of the territory.
The map published by the Ministry of Ecological Transition reveals the geographic scale of the problem: practically all mainland departments are affected to varying degrees. Strong-hazard zones cover substantial portions of the Paris Basin, central France, the southwest, and Normandy. Medium-hazard zones extend even further.
Changes introduced by the July 2026 map
Prevention obligations arising from the ELAN law were already applicable before the 2026 map update. The 2026 map modifies the regulatory zoning that applies, without creating a new legal framework for prevention.
For an individual house in a medium or high hazard zone, the builder must comply with a geotechnical design study if one is provided, or apply the applicable regulatory techniques. This study specifies the nature of the soils and prescribes the appropriate foundation techniques: deep foundations, structure reinforcement, drainage. The additional cost is real—a geotechnical study costs between 1,500 and 4,000 euros depending on terrain complexity, and reinforced foundations can add several tens of thousands of euros to construction costs. But this is incomparable to the cost of repairs after damage, which frequently exceeds 100,000 euros for a severely cracked house.
The logic is one of prevention integrated into the project: it is better to adapt construction to the soil than to repair damage ten years later. This logic is sound. It runs up, however, against a practical problem: the map applies to new construction, but the existing stock—composed of millions of individual houses built on clay soils without particular precautions—remains exposed without a systematic mechanism for preventive renovation.
The article Cracked House, Collective Bill documented the insurance dimension of this problem: losses related to clays already cost several hundred million euros annually to the natural disaster insurance system, and this figure should grow as droughts multiply.
Building more or building better: the dilemma the map does not settle
The debate takes on political dimensions. In his 2026 book Abundance, Ezra Klein offers a diagnosis applicable to this situation: advanced liberal societies have developed a remarkable capacity to prevent things from happening in the name of precaution, without always measuring what it costs not to do them. The clay map illustrates this dilemma at ground level.
If the map is used primarily as a prohibition tool—no construction in high-hazard zones—it relieves insurers and protects future owners. It would also restrict the supply of buildable land in several zones where demand is significant, because clay soils do not concentrate in rural wastelands. The Paris Basin, the Lyon suburbs, the near ring around Bordeaux: these territories are precisely those where land pressure is most intense.
The answer to this tension is not to avoid mapping. Information is preferable to ignorance, and building houses destined to crack is not a solution to housing shortage. But the answer is not to decree white zones where families seek housing either. The British Flood Re model, cited by the OECD in its 2026 economic survey of France, offers an interesting counterpoint: this reinsurance fund explicitly excludes new constructions in flood-prone zones from pooled coverage, creating a powerful financial incentive not to build where risk is proven. Applied to clays, an analogous mechanism would orient builders toward adapted foundations rather than toward safer locations, which is to say the answer is technical before it is geographic.
Another reading, notably defended by economists close to the institutionalist current, emphasizes the role of local authorities: local urban plans must integrate geotechnical data to prescribe rather than prohibit. This shift in approach requires staff trained to read hazard maps, sufficient numbers of geotechnical engineering firms, and accessible financing for modest-income households wanting to build or renovate.
The existing stock, blind spot of the reform
The 2026 map looks to the future. It will protect new construction. It says almost nothing about the existing stock of individual houses built on clay soils without prior studies, constructed between the 1960s and 2000s, during the great period of mass suburban housing.
These houses are today in the hands of households that often do not know their soil is moving. Cracks appear gradually, interpreted as normal building deterioration until an expert inspection reveals their geotechnical origin. Natural disaster insurance compensation depends on an inter-ministerial order of recognition, issued after municipal application review. Some municipalities have waited several years for recognition of drought episodes that were nonetheless documented.
The existing stock problem calls for a different response than that required for new construction. It passes through systematic information to owners, on the model of energy performance certificates. It also passes through aid for preventive renovation. Since October 2025, the state has been experimenting in eleven departments a prevention fund financing diagnostics and certain water management, vegetation, and soil impermeabilization work on existing houses. Funding for these aids, in a context of budget constraints, remains a blind spot in climate adaptation policy.
Toward 2050: what the soil says about tomorrow’s housing
By 2030-2050, the frequency of complete drought-rewetting cycles on clay soils in a single summer is a central indicator. Today, the most exposed regions regularly experience complete drought-rewetting cycles per summer. According to Météo-France’s DRIAS-Water projections, the number of soil drought days would increase by approximately 24 days per year by 2050. Each additional cycle intensifies foundation fatigue, worsens existing cracks, and reduces building lifespan.
The question of insurability is directly linked to this trajectory. The Central Reinsurance Fund and major private reinsurers are closely monitoring clay claim trends. If claims multiply and insurance premiums for individual houses in high-hazard zones increase significantly, a growing portion of the stock risks becoming difficult to insure at an accessible price. This is the scenario where climate risks render insurance unaffordable or unavailable and reduce property values: households trapped in devalued houses, uninsurable or under-insured, without means to finance work or sell at a price sufficient to relocate.
The alternative scenario is one of organized adaptation. It requires several things not yet in place. A geotechnical engineering market capable of meeting demand for diagnostics and reinforcement. A subsidized preventive renovation policy, targeted at households unable to finance work alone. Urban planning rules that prescribe appropriate foundation techniques in hazard zones and orient new construction toward less sensitive soils when possible.
Information for buyers integrated into real estate transactions, on the model of what exists for flood risk in certain municipalities.
The signal to watch over the next five years is that of actual implementation of the 2026 map. If geotechnical studies become systematic in hazard zones, if municipal instruction services take hold of them, if appropriate foundation techniques enter the routine practices of individual house builders, prevention will work. If the map remains an administrative document no one consults, the stock will continue to be built on fragile ground.
The long-term challenge exceeds clay risk alone: it concerns the habitability of French territory in a warmer climate. Electrical grids are recruiting to finance the energy transition; clay soils, meanwhile, are gradually shifting, and buildings built on top bear the structural consequences. The 2026 map constitutes a useful response and a starting point.
Requirements of the reform for its actors
The map is published. Obligations are written into the texts. What remains is to bring them to life, which requires capable actors in sufficient numbers.
The first bottleneck is geotechnical engineering firms. France has several hundred, unevenly distributed across the territory. In rural and suburban areas—precisely those where suburban housing is most active—wait times for a study can exceed several months. If the 2026 reform creates an obligation without creating the capacity to meet it, it will produce bureaucratic effects, permit delays, workarounds, rather than preventive effects.
The second bottleneck concerns local authorities. Integrating geotechnical data into local urban plans requires trained staff and accessible tools. Small municipalities, which often process permits themselves, do not always have the necessary human resources. A national policy on training and supporting local urban planning services would be the logical follow-up to map publication; it has not yet been announced.
The third actor is the individual house construction sector, dominated by developers and builders who have long worked with standardized, inexpensive foundations adapted to “normal” soils. Adapting their practices—moving to deep foundations or reinforced slabs in hazard zones—represents a training effort, tooling, and revision of economic models. Some builders have already incorporated this; others resist because the additional cost weighs on their competitiveness in a market where the buyer first compares prices.
These three bottlenecks are not insurmountable. They are solvable, provided that public policy treats them as problems to solve rather than obstacles to circumvent. The 2026 map has posed the geographic diagnosis. The operational diagnosis remains to be made.
Sources
- Ministry of Ecological Transition, Our Environment, Clay shrink-swell beneath houses, almost all departments affected: https://www.notre-environnement.gouv.fr/actualites/breves/article/retrait-gonflement-des-argiles-sous-les-maisons-presque-tous-les-departements
- Ezra Klein, Abundance, Simon & Schuster, 2026: https://www.simonandschuster.com/books/Abundance/Ezra-Klein/9781668023976
- OECD, Economic Survey of France 2026: https://www.oecd.org/en/publications/oecd-economic-surveys-france-2026_e88a1716-en
- Météo-France / DRIAS, Regionalized climate scenarios for mainland France (platform for providing regionalized climate scenarios)
- Journal d’un Progressiste, Cracked House, Collective Bill: https://journaldunprogressiste.fr/maison-fissuree-facture-collective/
- Journal d’un Progressiste, Electrical Grids Are Recruiting!: https://journaldunprogressiste.fr/les-reseaux-electriques-recrutent/
