Building in a
flood zone.

The Plaine de Chautagne PPRi was approved by prefectoral order on 28 August 2015. It revises an older document that did not account for flooding from the lake itself — a major gap in a plain where the two phenomena combine.

Seven municipalities are covered: Motz, Serrières-en-Chautagne, Ruffieux, Vions, Chanaz, Conjux and Chindrieux. Saint-Pierre-de-Curtille is not included. The risk is not theoretical: Rhône floods have already cut off and flooded hamlets in Chautagne.

The area is in fact an alluvial plain with some 1,800 hectares of wetland, historically drained and embanked. Hydroelectric development of the Rhône has changed the flood pattern, but has not removed the risk: it has shifted and contained it.

The most decisive consequence is technical and often misunderstood: in areas covered by the PPRi, building height is measured from the reference level set in the risk document, not from natural ground level. A project drawn without this data is a project that will have to be redrawn.

This has knock-on effects: the lowest floor level, whether there is a crawl space and how high it is, the type of foundations, where plant rooms go and the choice of materials on lower levels. On a renovation, it can also determine whether a habitable room can be created on the ground floor.

Other tools also come into play, which we check case by case: hazard maps, R111-3 maps and Z-indexing plans (PIZ). The PLUi regulations, for example, detail a PIZ in Vions, with no-build zones due to rockfall and areas where buildings are “limited to their existing footprint”.

This is the most common misunderstanding. A PPRi does not freeze an area: it classifies zones according to hazard intensity and sets out, for each, what is prohibited, what is permitted subject to conditions, and what is unrestricted. Many plots in Chautagne remain buildable subject to specific requirements.

What costs money is not the constraint: it is discovering it late. A mandatory crawl space, a floor level raised by forty centimetres or adapted foundations can be costed without drama if they are built in from the initial sketch. Discovered after the permit application, they blow the budget or kill the project.

Our in-house design office reads the PPRi regulations applicable to your plot before costing. That is what allows us to honour a commitment on price and timescale, including as general contractor.

For the vast majority of our clients in Chautagne, the question is not about a new build but an existing property: a village house, a farmhouse, a waterside second home. Here the PPRi works differently: it mainly governs extensions, the creation of habitable floor area on lower levels and certain alterations.

An interior renovation, a roof overhaul or a thermal upgrade generally raises no difficulty. On the other hand, turning a flood-prone ground floor into a bedroom, or adding a single-storey extension, often runs up against the regulations.

We tell you what is possible before you become attached to a plan. And when the regulations close one door, they almost always open another: building upwards rather than outwards, converting the loft, reorganising the existing levels.