Clay soil in France: the soil survey before selling

What is clay shrinkage and swelling?

Changes in the water content of clay soil are slow, but they cause ground movement. Detached houses are the most exposed, because their foundations are most often shallow. The ministry lists the following consequences:

• cracks in the façade;
• adjoining structures (garage, terrace, front steps) pulling away;
• distortion of doors and windows;
• break-up of floor slabs and partition walls, sometimes the rupture of buried pipes.

The cost of claims has risen from an average of €375 million a year over 1995-2015 to around €1.5 billion a year over 2018-2022. The average cost of an RGA claim is estimated at €16,500 (ministère de la Transition écologique, citing the CCR (state reinsurer) and the Cour des comptes (Court of Audit)).

Exposure zones and the 2026 map

The national map distinguishes four categories of zone (CCH, art. R132-3): high, medium and low exposure, and residual exposure where no clay ground has been identified. Only medium or high exposure zones trigger the obligations of articles L132-4 to L132-9.

Sources: arrêté of 9 January 2026; ministère de la Transition écologique. The update reflects the high recent claims rate: around 240,000 RGA claims between 2018 and 2022, i.e. 58% of all RGA claims since 1989.

Selling a plot: the preliminary geotechnical study

When a buildable undeveloped plot in a medium or high exposure zone is sold, the preliminary geotechnical study is provided by the seller (CCH, art. L132-5):

• it is attached to the promesse or, failing that, to the acte authentique (final deed of sale); in a public sale, to the cahier des charges (conditions of sale);
• it remains attached to the title deed and follows the plot through successive sales;
• it is valid for 30 years if the ground has not been reworked (art. R132-6).

Plots in areas where the planning rules do not allow detached houses to be built are not covered.

The study identifies the geotechnical risks of the site and sets out the general construction principles (art. R132-4). According to the ministry, it involves desk research, a site visit and, where appropriate, test drilling. A G1-type study complying with standard NF P 94-500 of November 2013 is presumed to comply. The land side is covered in detail in our guide on achat d’un terrain (buying a plot of land).

Building: the design study and the builder’s obligations

Before signing a building contract or a project management contract for a house (two homes at most), the maître d’ouvrage passes the preliminary study on to the builders. If it is not attached to the title deed, they must provide an equivalent study themselves or a geotechnical design study (CCH, art. L132-6). The contract states that the builder has received the study (see our guide on the contrat de construction de maison individuelle (house-building contract)).

The design study sets the construction requirements suited to the soil and the project; it is valid for that project only (art. R132-5 and R132-6). A G2-type study complying with standard NF P 94-500 is presumed to comply (ministère de la Transition écologique). The builder must then either follow its recommendations or apply specific construction techniques set by regulation (art. L132-7).

Some works fall outside these rules: those that affect neither the foundations, nor the structure, nor water drainage, nor heat exchange between the building and the adjacent ground, and extensions of less than 20 m² separated from the existing building (art. R132-7). When a new house in a medium or high zone is completed, the maître d’ouvrage attaches a certificate that the risk has been taken into account to their declaration of completion (CCH, art. R122-38, according to the ministry).

Guarantees and insurance: what protects the buyer

On resale, the studies carried out for the construction are attached to the promesse or, failing that, to the deed of sale (CCH, art. L132-8).

In new builds, RGA is the largest item of compensation under assurance dommages-ouvrage (building defects insurance) for detached houses (ministère de la Transition écologique): see our guide on the garantie décennale et la dommages-ouvrage (ten-year guarantee and building defects insurance).

In a natural disaster, the statutory excess is €1,520 where the damage results from ground movement following drought or soil rehydration, compared with €380 for other damage to a home (service-public.gouv.fr, checked on 10 April 2026).

In the état des risques (risk statement), a seller who holds the insurer’s expert report following a drought-related natural disaster must attach the list of works not carried out that would stop the damage, even if they have been compensated (service-public.gouv.fr, checked on 28 September 2026). For a defect discovered after the purchase, see our guide on vices cachés (hidden defects).

For existing homes, since October 2025 the State has been trialling, in 11 départements, a prevention fund that finances a vulnerability assessment and then works. Since June 2026, the works grant has been available for cracks up to 5 mm wide, compared with 1 mm previously (ministère de la Transition écologique, 9 June 2026).

Which professional to consult

The notaire (notary) checks that the preliminary geotechnical study is attached to the promesse and is less than 30 years old. The géomètre-expert (chartered land surveyor) defines the exact boundaries of the plot, which helps to place it on the exposure map. The agent immobilier (estate agent) can ask the seller for the study as soon as a plot is put up for sale. The diagnostiqueur (property surveyor) carries out the other surveys in the sale file.