River and coastal flood risk when buying in Spain

On 29 October 2024, a DANA caused 230 deaths, damaged 69,000 homes and led to more than 13 billion euros in direct losses in the province of Valencia alone. Many of those buildings had stood for decades on flood-prone land. Their owners either did not know, or had chosen not to know.

The information exists. It is published by the Ministry for the Ecological Transition, it is public and it is free. The problem is that almost nobody checks it before signing a purchase.

At Hogarfax, we do it for you. But first, it helps to understand what we are actually looking at.

Two types of flooding every buyer should understand

Fluvial flooding
This is caused by rivers, streams, ramblas and barrancos overflowing when rainfall exceeds the channel's capacity. The water rises from the land itself and can travel kilometres downstream from where the rain actually fell. Valencia's DANA is the most recent and devastating example, but far from the only one: Murcia, Seville, Girona, Valladolid and many other cities have suffered serious episodes in recent years.

Marine flooding
This is what happens along the coast when the sea, driven by storms, spring tides or extreme weather, pushes inland. It particularly affects low-lying coastal areas, marshes, deltas and built-up seafronts. According to Sociedad de Tasacion's Map of Risks in Spain's Housing Stock, 1.3% of homes, more than 330,000 units, lie in areas with some level of exposure to coastal flooding, with Cantabria being the most affected autonomous community.

Why it affects more homes than most people think

Almost 9% of homes in Spain, 2.3 million properties, are built in areas affected by fluvial flooding. That is one in every eleven homes. Even so, most buyers never verify this.

The reason is that the risk is invisible at a glance. A dry barranco, a rambla with a few puddles, a plot far from the nearest river can all look safe. The fact that our parents may never have seen an area flood does not mean water will not run through it again within a few years. These channels form part of flood-prone areas with technically calculated return periods, regardless of what we have or have not witnessed in our own lifetime.

The 2024 DANA proved this brutally: at least 16,000 flooded hectares lay outside the area covered even by the 500-year flood scenario. Water does not respect the lines on a map.

What return period means: the explanation nobody gives you

Here comes the key concept every buyer should understand. Without technical jargon.

Imagine rolling a loaded die. You know that one particular number will come up, on average, once every hundred throws. That does not mean you must wait exactly one hundred throws for it to appear. It could come up twice in a row, or not appear in 200 throws. What it tells you is the long-term statistical probability.

Flooding works the same way. A T100 return period means a flood of that magnitude has a 1% chance of occurring in any given year. Over 100 years, it is likely to happen once. But it could happen twice in ten years, or not at all in 150. It is not a prediction of when; it is a measure of how likely.

Hazard maps are drawn for three scenarios: high probability, with a 10-year return period; medium probability, with a 100-year return period; and low probability or extreme events, with a 500-year return period.

Why we use T100 at Hogarfax, not T10 or T500

Three options, three very different logics:

• T10 is too conservative a basis for a buying decision. It shows only areas that flood fairly often, roughly once every tenth of your adult life on average. It covers little territory and leaves out many areas with real risk. If you looked only at T10, you would feel falsely and dangerously reassured.
• T500 is the extreme catastrophic scenario. It is useful for critical infrastructure engineering, but excessive for assessing an ordinary home: it covers such a broad area that it tells you little that is practical about whether that specific property carries a reasonable level of risk.
• T100 is the balance point. It is the standard scenario used by the European Floods Directive, implemented in Spain through Real Decreto 903/2010, for territorial planning and housing risk assessment. A 1% annual probability is serious enough to matter and technical enough to distinguish accurately between properties facing real danger and those that are not. It is the benchmark used by river basin authorities, insurers and planning instruments.

Why we talk about "hazard", not just "flood zone" or "risk"

These three terms sound similar, but they mean different things:

• Flood zone is a broad geographic concept: the area that could be covered by water under a given scenario. It does not quantify how likely it is or what damage it would cause. It is the broadest denominator.
• Hazard goes a step further: it describes the probability and intensity of the phenomenon for a specific scenario, in our case T100. It includes the extent of the water footprint and the expected depth. It is more precise and more useful in practice. Hazard maps are what define the flood zone and form the basis of risk-management plans under Real Decreto 903/2010.
• Risk adds the human dimension: it combines hazard with what is actually in that area, how many people, which economic activities and which infrastructures. It is the most complete concept, but also the hardest to interpret at the level of an individual property.

At Hogarfax, we use T100 hazard because it is the most precise and directly applicable data point for a specific home: it tells you whether water would reach your plot in the official reference scenario, and to what approximate depth.

How we do it at Hogarfax: real geometry against official layers

Checking a map by eye is not enough. Scale is misleading, boundaries are fuzzy and it is easy to assume your home sits outside the flood extent when in reality it is right on the edge.

At Hogarfax, we cross-reference the cadastral plot's real geometry, its exact coordinates, shape and area, against MITECO's official fluvial and marine flood-hazard layers. The result is not a visual impression. It is a precise geometric intersection that determines whether the plot lies inside, partly inside or outside the water footprint for the T100 scenario.

That allows us to tell you, for any home in Spain, whether it is affected by fluvial flooding, marine flooding or both, not at the level of the municipality or neighbourhood, but at the level of your specific plot.

Want to know whether the home you like is affected?

Search for any home on Hogarfax by address or cadastral reference and check in seconds whether it falls within a flood-hazard zone.

What the DANA taught us about unseen risk

The Mediterranean is warming at a rate 20% above the global average, multiplying both the frequency and intensity of torrential rainfall. Episodes that used to occur every 50 years are beginning to occur every 15. Hazard maps are updated, but the existing housing stock does not change.

More than one million people were living in 2021 in areas of medium flood risk within Spain's major urban areas, and the vast majority did not know whether they lived in a flood-prone area or not.

That is going to change. Banks are already starting to request climate-risk reports before granting mortgages. Insurers are already adjusting premiums. Information that is optional today will be required tomorrow.

Better to know before you sign.