Clay Shrinkage and Swelling: Impact onLuxury Real Estate
A new decree has just extended the risk zones to 55% of the French territory, effective from July 1, 2026. For properties of character - often old, made of stone, on vast plots - this risk deserves special attention.
What is clay shrinkage and swelling?
Clay soils have a particular characteristic: their volume varies greatly depending on their water content. During periods of drought, the clay shrinks; after heavy rains, it expands. This cyclical movement causes differential settlement of foundations, generating cracks, subsidence, and structural damage—a phenomenon that particularly affects old buildings with shallow foundations, typical of French architectural heritage.
This is not a marginal risk: the RGA ranks second among the most costly natural disasters in France, just after floods, with 16 billion euros of insured damages between 1989 and 2022 according to France Assureurs.
A new map that significantly expands the exposed areas
The decree of January 9, 2026, updates the national map of the RGA risk, replacing that of 2020. The result is significant: the areas of medium or high exposure, which until now covered 48% of the metropolitan territory, now cover.. 55%.
Exposed territory — 2020 map
Exposed territory — map 2026 (from July 1st)
In terms of the number of dwellings, the change is just as clear: the stock of detached houses affected increases from 10.3 million (52.5% of the total stock) to 12.1 million, representing 61.5% of the French stock of detached houses. A property previously classified as being in a low-risk zone can therefore now be reclassified as being in a medium- or high-risk zone—a change in status that has direct consequences for sales obligations and the property's insurability.
The obligations arising from the ELAN law
For any undeveloped building plot located in an area of medium or high exposure, the seller must provide a preliminary geotechnical study (G1), attached to the preliminary sales agreement. The notary will not sign the deed without this document.
A supplementary study (G2) and a risk assessment certificate, mandatory since January 1, 2024, must accompany any declaration of completion of work in an exposed area.
The purchaser has five years from the discovery of the damage to take action if problems related to the RGA appear after non-compliant construction.
The natural disaster (CatNat) scheme remains the only mechanism allowing a property owner affected by drought to be compensated by their insurance.
These obligations concern the sale of undeveloped land. For a property already built – castle, manor house, old villa – the RGA remains primarily a matter of preventive diagnosis and insurability, to be anticipated before the purchase rather than endured afterwards.
Why this risk particularly concerns old properties
Historic residences—castles, manor houses, longhouses, and bastides—often have shallow foundations, built using the construction techniques of their time, long before the phenomenon of geological regeneration was identified and regulated. These buildings are statistically more vulnerable to ground movement than recent constructions, whose foundations now incorporate geological regeneration prevention standards.
On properties with large parks or adjoining agricultural plots, the presence of trees in the immediate vicinity of the building also aggravates the phenomenon: the roots dry out the soil over a sometimes considerable radius, amplifying clay shrinkage in dry periods — a parameter to monitor particularly on estates with century-old trees close to the foundations.
Checks to be carried out
Before any purchase, verifying the exact exposure of the plot on the official Géorisques interactive map (clay layer) remains the first step — the new 2026 mapping is already available there. For an existing property, a structural assessment by a professional can identify any cracks or signs of differential settlement, often visible on facades or at the junctions between extensions and the original structure. Also check the home insurance coverage for the property, as the RGA risk can influence premiums or deductibles depending on the zone.
For a seller, anticipating this issue rather than discovering it during negotiations remains the best strategy: an up-to-date diagnostic report reassures the buyer and avoids last-minute discoveries that could cause an advanced transaction to fail.
Delving deeper into regulations and the market
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