How to check natural-hazard risk before buying a house in Switzerland

The sales file for a Swiss house will tell you everything about the roof beams and nothing about the river. It is a strange blind spot: every canton publishes, free of charge and parcel by parcel, maps that grade the danger of flooding, landslide, rockfall and debris flow — and most buyers sign without ever opening them. When we ran the checks on our own house hunt, this layer took one evening to read. It changed the shortlist.

Here is how to run it yourself: where the maps live, how to read them, what they leave out — and the one question that turns a reading into a certainty.

What does a Swiss hazard map actually show?

Under a federal mandate, each canton maintains hazard maps for the gravitational processes: floods, landslides, rockfall, debris flows, and — in the mountains — avalanches. The map does not say “damage happened here.” It grades a combination of probability and intensity, established by specialist engineering bureaus from topography, event history and modelling.

That is the difference between the map and neighbourhood memory, which is the only other sensor a buyer usually has. The map includes the rare event that has not happened within living memory. “The village has never flooded” and “a blue zone runs through the village” can both be true at once — and it is the map, not the anecdote, that describes your exposure.

Where do you check the hazard map for a specific parcel?

Three doors, all free. Your canton’s geoportal publishes the hazard maps as layers you can read down to the parcel — the finest reading available. The extract from the cadastre of public-law restrictions (RDPPF) collects, for a given parcel, the official restrictions that bind it; depending on the canton, hazard zones appear there directly or by reference. And the federal map portal aggregates several of these layers nationally, which is enough for a first sort across candidate properties.

The habit that matters: read at parcel level, never at commune level. A commune “in a hazard zone” can have entire districts in white, and a commune with a quiet reputation can carry a debris-flow corridor across exactly three parcels — including, sometimes, the one in the listing.

What do the colours mean for a buyer?

Four grades, from serious to subtle. Red means high danger: new construction is in principle prohibited, and an existing building in a red zone is a question in itself — for its resale value as much as for its occupants. Blue means medium danger: building is permitted under conditions, with protective measures imposed, and an existing property there earns the question “which measures were carried out, and when?”. Yellow means low danger — typically possible material damage, rarely danger to people. Yellow-white hatching marks residual danger: the rare, heavy event that no protection fully neutralises.

And white? White does not mean “no danger.” It means “no danger identified at the current state of knowledge” — the maps are periodically revised, and today’s white can be the next revision’s yellow. That distinction is not one the seller will make for you.

Which hazards does the map not show?

The most important omission is surface runoff: rainwater travelling over slopes and streets before it ever reaches a watercourse. Federal estimates attribute a large share of Switzerland’s flood damage to it — on the order of half — and it has its own separate, nationwide, indicative map. Coarser than a hazard map, but sufficient to spot that a parcel sits below a slope that drains toward it. A cellar, a sunken garage or a driveway descending under a runoff path is a finding worth making before signature, not after the first storm.

Beyond that, several risks live in other registers entirely: earthquake, which is handled by construction norms rather than zoned danger maps; radon, mapped separately at federal level; and technological risk — the consultation perimeters around installations covered by the major-accident ordinance (OPAM) appear in the RDPPF extract, not on the hazard map. A complete location read crosses these layers; the hazard map is only the first.

Is a blue or yellow zone a deal-breaker?

No — which is exactly why it should be read early. A danger grade is not a veto; it is a parameter. It conditions what you may build or convert, the protective measures required and their cost, and sometimes how the building must be maintained. The difference between an informed buyer and a surprised one is that the first prices those measures into the offer and the second discovers them in a contractor’s quote.

Two checks complete the layer. Cantonal building insurance — mandatory in most cantons — covers natural-hazard damage, and the building’s claims history, where accessible, tells you what the map cannot: what has actually happened at this address. And for a property in a blue zone, the protective measures should exist somewhere other than the seller’s account of them: in a permit, an acceptance report, or an invoice.

What is the only confirmation that counts?

A rule we apply to our own deliverables belongs here: a desk read of a published map is an inference, not a certainty. Layers get revised, geoportals simplify, and a zone boundary can pass fifteen metres from the façade. Until the commune has confirmed in writing the danger grade and the restrictions applying to the parcel, you hold a reading — not an answer.

The good news: that confirmation costs one letter. The commune’s urbanism or planning service will answer, in writing, the question “which danger grade and which restrictions apply to parcel n° X?” — and while you are writing, ask what is currently at public inquiry in the sector, because the construction project that will transform the neighbourhood is already in the register. It is the highest-value one-letter question in the entire purchase process, and almost nobody asks it.

This reading is the exposure layer of a location assessment — one of five. The immediate environment, access and egress, dependencies and visibility are read with the same discipline, and where the documents stop, we go and look. If the decision in front of you deserves the full dossier, that is exactly what an Aegilo location assessment is for.