Reading Spain's official flood maps (zonas inundables) for an address

Terms used

• SNCZI — The SNCZI is Spain's national free online viewer of flood-zone maps, run by the Ministry for Ecological Transition with the river-basin authorities.
• Zona de flujo preferente — The zona de flujo preferente is the part of a flood plain that carries the flood, where water would be deep or fast.
• Zona de policía — The zona de policía is a 100 m strip each side of a river channel where works need basin authority approval.
• ARPSI — ARPSI are river or coast stretches officially found to have significant potential flood risk, and have the most detailed maps.
• Confederaciones hidrográficas — The confederaciones hidrográficas are the river-basin authorities that manage rivers and flood mapping across several regions.

Key rules

• Frequent floods: A home inside the T=10 or T=50 zone has frequent exposure and a serious risk.
• Long-term odds: A 1% yearly chance (T=100) is roughly 26% over a 30-year mortgage.
• Outside the map: Not being on the map does not mean no risk, because small streams and heavy-rain flooding are often missing.
• Second check: Repeat the check on the regional viewer and ask the town hall's planning office about the plot.

What the maps give you

Spain has a free national flood viewer, the SNCZI, and several regional viewers. Together they are the best free evidence of whether a home sits in a flood zone. They are a good start, not a guarantee. You need to read them correctly and know what they miss.

You reach the SNCZI through the website of the Ministry for Ecological Transition (search "SNCZI" or "cartografía de zonas inundables"). No registration is needed. The checklist below walks you through the clicks.

Reading the return periods

A return period (T) is how often a flood of a given size is expected, on average. It is also a yearly chance:

• T=10: about a 10% chance in any year. This is a frequent flood.
• T=50: about 2%.
• T=100: about 1%.
• T=500: about 0.2%. This is rare, but some of the worst events happen here.

These are averages, not schedules. A T=100 flood can happen in two years running.

What it means for a home:

• Inside T=10 or T=50: you face frequent exposure and a serious risk.
• Inside T=100: a 1% yearly chance is roughly 26% over a 30-year mortgage. Treat it as significant.
• Inside T=500 only: the risk is low but real.

Climate change may make past return periods understate today's risk.

Flow zones and strips along rivers

The zona de flujo preferente is the part of the flood plain that carries the flood, where water would be deep or fast. New housing, garages and basements there are generally not allowed. An existing home inside it is highly exposed.

Along a river channel, the zona de policía is a 100 m strip on each side where works and uses need authorisation from the river-basin authority. Its inner 5 m is the zona de servidumbre. If the property is in either strip, ask whether the works were authorised. Rules for new building became stricter after a 2023 change.

Where the viewer shows depth and speed layers, read them. They show how dangerous a flood would be.

The ARPSI layer

ARPSI areas are river or coast stretches officially judged to have significant flood risk. They have the most detailed maps. Switch the layer on and see whether your address is in or near one. Then read the basin's flood risk management plan for the measures planned there, such as defences, works and early warning. You get it from the basin authority. Make sure you read the latest version.

A home outside an ARPSI is not automatically safe. Only selected stretches are included.

Regional viewers

Repeat the check on the regional viewer. The right one depends on the municipality and its basin. Search "[region] visor zonas inundables", or start from the SNCZI, which links to the basin authorities.

• Most of Spain is covered by the river-basin authorities (confederaciones hidrográficas), which publish their own viewers.
• Catalonia, the Basque Country, Andalusia, Galicia, the Balearic Islands and the Canary Islands have regional water bodies with their own information.
• Valencia has its own regional flood-risk plan and viewer.

The town hall's planning office (urbanismo) can also confirm the risk for a specific plot.

What the maps miss

The maps have limits, listed in the warning below. In short, check local history and visit the site in heavy rain if you can. The October 2024 floods around the Poyo ramblas in Valencia showed that an extreme event can go beyond the mapped zones. A coarse map scale can also place a house on the edge of a zone wrongly, so zoom in and check the edges.

Keep a record

Save dated screenshots of each map view and give them to your lawyer or agent. If the result is close to a zone or you are unsure, ask the basin authority or the town hall's planning office before you commit. The decision block below helps you weigh what you found.

Choosing between options

How seriously should you take the flood zone your address falls in?

• Walk away or get expert advice: The home is in the 10-year or 50-year zone, or in the zona de flujo preferente. Exposure is frequent and new housing there is generally not allowed.

• Proceed only after further checks: The home is in the 100-year zone. The risk over a mortgage is significant, so check depth and velocity layers and ask the basin authority.

• Accept a low but real risk: The home is only in the 500-year zone. Exceptional events still happen here, so check local history.

• Keep checking: The home is outside every mapped zone. Small streams and rain flooding may not be mapped, so visit the site and ask the ayuntamiento.

Go by the most frequent flood zone the building touches: the more often it floods, the more you should walk away or take advice. The odds for each period are set out in Reading Spain's official flood maps (zonas inundables) for an address.

Checklist

• Open the free national SNCZI flood-zone viewer from the website of the Ministry for Ecological Transition (MITECO) and accept the conditions of use.

• Enter the address, place name or coordinates in the search box, or zoom to the location.

• Switch on the flood layers for 10, 50, 100 and 500 years, the zona de flujo preferente and the ARPSI layer if there is one.

• Set an aerial image as the background so you can find the building.

• Click on the building with the identify tool and note whether it sits inside or close to a flood polygon.

• Read the depth and velocity layers where they exist.

• Find your region's viewer by searching '[region] visor zonas inundables' and repeat the check there.

• Ask the basin authority or the ayuntamiento's planning office whether the plot is in the zona de policía or the zona de flujo preferente.

What to watch out for

• Small streams are often missing: Small ramblas, barrancos and urban streams are often not on the maps, because mapping mainly covers selected river stretches. Being outside the coloured zones does not mean no risk. Ask neighbours and the ayuntamiento about past floods and visit the site. Ask neighbours and the ayuntamiento about past floods, and visit the site in person.

• Rain flooding is mostly not shown: Water running over streets and overloaded drains in heavy rain is mostly not mapped, and neither is flooding from drainage failures. A house can flood even when the river maps show it clear. Look for low ground, underground garages and ground-floor rooms. Look for low ground, basements and garages when you visit.

• Maps can be out of date or too rough: Some maps date from before new building, changes to the river channel or changed rainfall. At a coarse scale a house on the edge of a zone can be shown on the wrong side. Zoom in fully and ask the basin authority how recent the study is. Zoom in fully and ask the basin authority how recent the study is.

• Extreme floods can go beyond the map: The October 2024 floods in Valencia reached beyond the mapped zones. Maps use standard return periods and do not show dam releases or debris, and climate change may make past figures too low. Treat the 500-year zone as a real risk, and treat a house just outside it with care. Treat the 500-year zone as a real risk and combine the maps with local history.

• Frequent flood zones mean serious exposure: A home in the 10-year or 50-year zone floods often. The 100-year zone adds up to roughly 26% over a 30-year mortgage. Think hard before buying there, and ask the basin authority about the zona de flujo preferente. Ask the basin authority about the zona de flujo preferente before going further.

Frequently asked questions

Does a house outside the flood zone have no flood risk?

No, a house outside the mapped flood zones can still flood, because small ramblas, barrancos and street-level runoff in heavy rain are often not mapped. The October 2024 floods in Valencia went beyond the mapped zones. Add local history and a site visit to the map check.

What does the ARPSI layer tell me about my address?

The ARPSI layer shows whether your address is on or near a river or coast stretch with a significant flood risk, and those stretches have the most detailed maps. Being outside an ARPSI does not make a home safe, since only selected stretches are included.

Can I keep a record of what the flood viewer showed for my address?

Yes, you can export or print the map view, take a dated screenshot, or download the layer data. Do this on both the national and the regional viewer. Keep the files to show your lawyer or agent.

Who can confirm the flood risk of a specific plot?

The planning office (urbanismo) of the ayuntamiento can confirm the risk for a plot, and the river-basin authority can say whether works near a river were authorised. Bring the address and your screenshots.