Geotechnical survey: why it is essential before you build

A house is only as solid as the ground that carries it. In clay areas, droughts make the soil swell and then shrink, and cracks appear. Since the ELAN law, a soil survey has been compulsory in exposed areas.

Clay shrinkage and swelling

Clay soils swell when they become saturated with water and shrink when they dry out. These movements can crack the walls and distort the foundations of a house that is not adapted to them. With drier summers, the phenomenon has spread: a large part of France is now affected, particularly Nouvelle-Aquitaine.

What the law requires

Since 1 October 2020, under the ELAN law, the sale of a building plot located in an area of medium or high exposure must be accompanied by a preliminary geotechnical survey (G1), provided by the seller and appended to the preliminary sale agreement or the deed. Before building a house in these areas, the builder must either rely on a design survey (G2) or follow construction techniques prescribed by the regulations.

G1, G2: what is the difference?

• The G1 survey characterises the land and its general risks. It informs the buyer.
• The G2 survey starts from the specific project: siting, loads, type of foundations. It is used to design the building.

These assignments are defined by the French standard NF P94-500, which classifies geotechnical surveys from G1 to G5. The preliminary survey provided at the sale remains valid for thirty years, as long as the ground has not been disturbed (order of 22 July 2020). A builder without a G2 survey must apply the specific construction techniques set out in that same order.

A map updated in 2026

The areas of medium or high exposure were redrawn by an order of 9 January 2026, published in the Journal officiel on 31 January. The new map applies to preliminary sale agreements for building plots and to individual house construction contracts concluded since 1 July 2026, and it extends the areas concerned.

And for an extension?

An extension raises the same questions: foundations that differ from those of the existing house may move differently and cause cracks at the junction. Even when the law does not require it, a soil survey is a modest investment compared with the risk.

Good practice on clay soil

The guides from the French Ministry for Ecological Transition set out a few principles, with or without a survey:

• Sufficiently deep foundations, uniform, and with no partial basement that would create different bearing levels.
• A rigid structure, with horizontal and vertical tie beams.
• Trees at a good distance from the house: their roots dry out the soil in summer.
• Controlled water: connected gutters, rainwater drained well away from the foundations, and a watertight path around the house to limit variations in moisture.

Mistakes to avoid

• Relying on how the land looks: clay soil is hard to recognise by eye, especially in winter.
• Confusing G1 and G2: the seller's survey informs; it does not size the foundations of your project.
• Planting a hedge or a tree right up against the walls after construction.
• Letting downpipes discharge at the foot of the façade.

Finding out more

The official Géorisques website shows the exposure of an address to clay and other natural hazards. It is the first check to make before buying a plot or starting an extension.

The stages, from buying the plot to the foundations

1. Consult Géorisques using the address or the plot reference.
2. In a medium or high exposure area, check that the G1 survey is appended to the preliminary sale agreement, and read it.
3. Pass this survey on to the builder or contractor from the very first discussions.
4. Have the G2 survey carried out on the specific project, or make sure the specific construction techniques are applied.
5. Check that the foundations in the quote follow the survey's conclusions.

Our method

On our construction and extension projects, the soil survey is part of the preparation: the foundations are sized on the basis of its conclusions, before the first spadeful of earth is dug.

Sources : Loi n° 2018-1021 du 23 novembre 2018 « ELAN », article 68 (étude géotechnique en zone argileuse) · Décret n° 2019-495 du 22 mai 2019 (zones exposées au retrait-gonflement des argiles) · Arrêté du 22 juillet 2020 définissant le contenu des études géotechniques (validité de trente ans de l’étude préalable) · Arrêté du 22 juillet 2020 relatif aux techniques particulières de construction dans les zones exposées au retrait-gonflement des argiles · Arrêté du 9 janvier 2026 modifiant l’arrêté du 22 juillet 2020 définissant les zones exposées (Journal officiel du 31 janvier 2026, applicable au 1er juillet 2026) · Norme NF P94-500 (missions d’ingénierie géotechnique G1 à G5) · ecologie.gouv.fr, sols argileux, sécheresse et construction · georisques.gouv.fr