The retaining
wall.

The Ministry of Housing restated its function in a reply published in the Journal officiel on 2 September 2025: a retaining wall prevents the soil or buildings of a property from sliding or collapsing onto the land below. The planning code gives no legal definition of it; case law and practice draw its outlines.

In practice, the structure constantly resists a horizontal thrust: that of the soil, increased by the water it contains and by everything bearing down at the top – a driveway, a terrace, a building. The higher the wall, the greater the thrust, and the more the foundation has to balance it. A boundary wall, by contrast, holds nothing back: it separates two plots.

On a sloping plot in Savoie, one is built in three cases: to create a platform (terrace, parking, swimming pool), to stabilise a crumbling bank, or to replace an old stone wall that is bulging or cracking. Each time, the stakes go beyond appearance: an undersized wall can tip over or slide, taking with it whatever it supports.

The choice depends on the height to be retained, the space available at the foot of the bank, the ground and the look you want. It is made on drawings, after calculation, not from a catalogue.

Cast-in-place reinforced concrete. A reinforced wall stem tied into a footing, often shaped like an L or an inverted T: the footing extends under the backfill, and the very weight of the soil contributes to stability. It is the answer for great heights and heavy loads, for example along an access road.

Formwork blocks. Hollow blocks laid, then filled with concrete and reinforced with vertical and horizontal steel bars: the wall works like a reinforced concrete wall, without full formwork. Not to be confused with ordinary hollow concrete blocks left empty, which are made for a fence, not for holding back a bank.

Mortared stone or dry stone. These are the walls of the villages and vineyard hillsides of Savoie. Their mass gives them their stability, and dry stone lets water through. Repairing them is a mason's job: dismantling the bulging section, restoring the batter of the face towards the bank, rebuilding with interlocking stones. For old façades, see our article on stone walls and lime renders.

Gabions and rock armour. Wire cages filled with stones, or large boulders placed against one another. These gravity structures are permeable, which settles part of the water question; on the other hand, they need ground space and access for machines.

Timber. Sleepers and logs suit the small level changes of a garden; in permanent contact with damp soil, their lifespan remains limited.

Waterlogged soil pushes much harder than dry soil, and water that cannot drain away eventually gets through: via the joints, under the foundation or over the coping. When a wall bulges or cracks, water built up behind it is one of the first causes to examine.

Behind the face, a well-designed wall therefore includes a drain at the base, laid to fall towards an outlet; gravel backfill separated from the soil by a geotextile, so that it does not clog; weep holes, the small through-openings that let water out if the drain is saturated; and protection of the buried face against damp.

The remaining question is where this water goes. Article 640 of the Civil Code requires the lower plot to receive the water that flows naturally from the upper plot, but forbids the upper owner from making this burden worse. Collecting the water of an entire bank in a drain and discharging it onto the neighbour's land is precisely making it worse: the outlet is decided at the design stage — a rainwater network if the regulations allow it, a soakaway or a swale.

The stability of a retaining wall is checked from three angles: it must not tip over downhill, slide on its base, or sink into the ground that carries it. These checks depend on the ground, the height, the presence of water and the loads at the top. They require a calculation note drawn up by a structural engineering firm, preceded by a soil survey when the structure is tall or the ground is clayey, disturbed or unstable.

The footing must also go below frost depth, which varies with altitude: a wall founded by the lake and a wall founded in a ski resort are not anchored in the same way.

In the mountains and on hillsides, one question comes even before the calculation: is the plot exposed to ground movement? The risks recorded for each commune and the natural risk prevention plans can be consulted on georisques.gouv.fr; an approved plan may regulate earthworks and construction within the zones it defines.

Our masons then build the structure on that basis: earthworks, footing, reinforcement, formwork or block laying, drainage and backfill. The calculation sets the sections and the steel; on site, we stick to it.

In principle, no. Article R.421-3 of the planning code exempts retaining walls from any formality, except when they are located within a remarkable heritage site or near a historic monument. The ministerial reply of 2 September 2025 confirms this, and states that the government does not intend to change these rules.

Three situations do, however, change things.

The wall also carries a fence. When a fence sits on top of a retaining wall, each structure follows its own rules; for a structure combining both functions, the stricter rule applies. A low wall topped with wire mesh remains a fence, with the formalities the commune imposes on fences.

The project moves a lot of soil. Independently of the wall, article R.421-23 requires a prior declaration (déclaration préalable) for excavation and raising of ground levels of more than two metres in height or depth, over an area of one hundred square metres or more, unless they are needed to carry out a building permit. A wall intended to create a platform on fill may therefore require a declaration for the accompanying earthworks.

The wall is part of a building. The retaining structure of a new house or an extension is then dealt with in the building permit or declaration file for the project itself.

In all cases, the local plan (PLU) may add its own rules: height, appearance, materials, setback, limits on fill. Reading the zone regulations before designing the wall avoids having to rework it afterwards. When a declaration is needed, our guide to the prior declaration sets out the procedure.

Between two plots at different levels, the question often comes up when a crack appears. In a ruling of 12 October 2022 (3^rd civil chamber, appeal no. 21-21.841), the Court of Cassation upheld the rule that a retaining wall is presumed to belong exclusively to the owner whose soil is held back by the structure — most often, the owner of the upper plot.

This presumption gives way to a title stating otherwise: a deed of sale, a division plan or an agreement between neighbours may provide differently. Before building on the boundary or repairing an existing wall, reread your deeds, have the boundary surveyed if it is uncertain, and put in writing what has been agreed with the neighbour.

A new wall on the boundary also almost always means going onto the neighbour's land to dig, set formwork or lay the drain: this access is negotiated before work starts, not during it.

We do not give prices per linear metre: no official source publishes any, and two walls of the same length may have nothing in common. A serious quote does, however, itemise the following, which lets you compare two offers line by line.

Studies: soil survey if needed, calculation note. Set-up and access: a track for machines on a sloping plot, protection of the surroundings. Earthworks: excavation, storage, removal of surplus soil. Foundation: blinding concrete, footing and its steel. The wall itself: cast wall, filled blocks, stone or gabions. Drainage and backfill: drain, geotextile, gravel, weep holes, connection to the outlet. Finishes: facing, coping, and any fence.

What makes the budget vary: height above all, since the thrust — and therefore the foundation — increases with it; machine access; the type of ground found when the trenches are opened; the load at the top; the chosen facing. When an old wall has collapsed, demolition and making the bank safe are added to the estimate.

A retaining wall with foundations is a construction work: the builder's 10-year structural guarantee (garantie décennale) covers, for ten years from the day after handover, serious defects that threaten its soundness or make it unfit for use. Service-public.fr in fact lists works with foundations among those it covers.

Before signing, check that the company's insurance certificate does mention masonry and structural work, since only the work declared in the policy is covered. For a major structure, consider dommages-ouvrage (building damage) insurance: taken out by the client, it pays for repairs without waiting for liability to be established.