Many buyers first hear the words estudio de inundabilidad from their architect, after they have bought. The plot sits near a stream or a dry rambla, the town hall or the river basin authority wants proof of where the water goes in a big flood, and nobody can submit the building application until an engineer has produced it. The study costs money, takes time, and can end with a result that changes what you can build.
This guide explains what a Spanish flood risk study is, when it is required, what it calculates and with which thresholds, who prepares and approves it, how to read the national flood map yourself before you sign anything, and what to do if the result is bad. For the 5-metre and 100-metre strips along watercourses and the permit they trigger, see building near a river or rambla. For a tour of the flood map layers, see flood zones in Spain; for the rest of the purchase checks, buying land in Spain.
A real study, published by a city
In May 2024 the City of Toledo published, as an annex to a planning file, the hydrological and hydraulic study of the Arroyo de la Rosa where it crosses development unit PAU UA-100 Oeste of the municipal plan. A private company commissioned it, an engineering firm wrote it, and the document analyses flood flows up to the 500-year return period and designs works to rebuild and protect the channel. It also says plainly who decides: "this study will be evaluated by the Tagus River Basin Authority" (Confederación Hidrográfica del Tajo).
Toledo had already commissioned a flood study of the Tagus through the city in 2004 for its municipal plan, built around a hydraulic model of the urban stretch. In 2025 the Official State Gazette (BOE) announced another application, file O-0264/2025, for flood-risk reduction works in the east of the city, naming both the Tagus and the Arroyo de la Rosa.
What this tells a buyer: the official flood map is where you start. The study, approved by the river basin authority, is what your permit is decided on.
Map versus study
Spain's National Flood Zone Mapping System (Sistema Nacional de Cartografía de Zonas Inundables, SNCZI), run by the Ministry for the Ecological Transition (MITECO), publishes flood zones for studied river stretches, mainly the "areas of potentially significant flood risk" (ARPSI). It shows the areas reached by the 10-, 100- and 500-year floods, depth (calado) layers for those floods, and in many stretches the preferential flow zone.
A flood risk study is an engineering calculation for one stretch and one project. It estimates the flows in the watercourse that affects your plot, models how it overflows on a detailed survey of the ground, and gives the water depth and velocity at each point. It is used where the SNCZI has no study, to refine the SNCZI with better topography, and to show that your project meets the land-use limits. MITECO notes that the protection of mapped zones is precautionary until more detailed technical studies exist: a study can widen or narrow what the map shows.
When you will be asked for one
There is no single article of law that says "build here, bring a flood study". The requirement comes from three places.
1. The river basin authority. The Public Water Domain Regulation (Reglamento del Dominio Público Hidráulico, RDPH, Royal Decree 849/1986) controls building within 100 m of a watercourse (Article 9, with prior authorisation under Article 78), in the preferential flow zone (Articles 9.2, 9 bis and 9 ter) and in the 500-year flood zone (Article 14 bis). Since the 2016 reform (Royal Decree 638/2016), the authority may only authorise, in the preferential flow zone, activities that are not vulnerable to floods and do not significantly reduce the capacity to carry floodwater. To apply, you must show where the water is in the reference flood and at what level your building sits. That is the study. The authority has six months to decide and silence means refusal.
2. The planning process. Plans and plan changes near watercourses carry flood studies, as Toledo's did. In the Valencian Community the regional flood risk plan (PATRICOVA, Decree 201/2015) requires specific studies to develop land in hazard zones; Callosa de Segura, Torrevieja, Benidorm, Burriana and Monòver have published theirs. If you buy a building plot in an undeveloped sector next to a gully, ask whether the sector's flood study exists and whether the river basin authority has reported on it.
3. The municipal licence. Many local planning rules make a licence in a flood zone conditional on a study and on a favourable report from the river basin authority. Look in the plan's chapter on watercourses or natural hazards, and in the planning certificate for your plot.
The flood risk sworn statement
On top of the study, anyone carrying out works in a preferential flow zone or flood zone must, in the cases set out in Articles 9, 9 bis, 9 ter, 9 quáter and 14 bis RDPH, sign a sworn statement (declaración responsable) to the water authority stating that they know and accept the flood risk, as the Júcar River Basin Authority's information sheet explains. The Balearic model adds that you accept the risk regardless of any compensation from the state flood insurer, the Consorcio de Compensación de Seguros. Read the study before you sign: it tells you what risk you are accepting.
What the study calculates
How likely the floods are
Floods are described by return period. The 100-year flood has a 1 % chance of happening in any given year; the 500-year flood, 0.2 %. Over the life of a 25-year mortgage the odds add up:
- 100-year flood at least once in 25 years: 1 − 0.99²⁵ ≈ 22 %.
- 500-year flood at least once in 25 years: 1 − 0.998²⁵ ≈ 5 %.
Roughly one in five for a house in the 100-year zone.
How deep and how fast
The hydraulic model gives depth and velocity at each point, and from those the zones that matter for your plot.
Preferential flow zone (zona de flujo preferente, Article 9.2 RDPH) is the union of:
- the main floodway (vía de intenso desagüe): the strip that would carry the 100-year flood without raising the water level by more than 0.3 m;
- the zone of serious damage in the 100-year flood, where at least one of these is true: depth over 1 m, velocity over 1 m/s, or depth × velocity over 0.5 m²/s.
Flood zone (zona inundable): the area reached by the 500-year flood.
The calculation that surprises buyers
Three points on the same plot in the 100-year flood:
| Point | Depth | Velocity | Depth × velocity | Serious damage? |
|---|---|---|---|---|
| A, by the stream | 1.20 m | 0.8 m/s | 0.96 m²/s | Yes (depth over 1 m) |
| B, mid-plot | 0.60 m | 0.9 m/s | 0.54 m²/s | Yes (product over 0.5) |
| C, at the back | 0.40 m | 0.7 m/s | 0.28 m²/s | Not on these criteria |
At point B neither depth nor velocity reaches 1, yet the product is over 0.5 m²/s: it falls in the preferential flow zone with water below knee height. If the house only fits at C, the study must also show that C is outside the main floodway and give the 500-year flood level there.
Floor level
Say the ground at point C is 210.40 m above sea level and the study puts the 500-year flood at 211.15 m. Depth: 211.15 − 210.40 = 0.75 m. Under Article 14 bis, as MITECO explains it, homes must be at a level the flood does not reach, so the habitable floor goes above 211.15 m. With a 0.50 m safety margin, 211.65 m: 1.25 m above the ground. Your single-storey farmhouse has become a raised house on a tall plinth or piles.
What you should receive
- A report with the catchment, the flows and the calculation method.
- A topographic survey of the stretch and the plot.
- Depth and velocity maps for each return period.
- The boundaries of the preferential flow zone and the flood zone.
- In the Valencian Community, the PATRICOVA hazard analysis, which includes geomorphological hazard.
- A conclusion on whether your project complies and, if not, what would make it comply.
Who prepares it, how long, how much
A qualified technical professional hired by you prepares it (in Toledo, an engineering firm) and the water authority evaluates it. Professional fees in Spain are free: since the 2009 Omnibus Act, Article 14 of the Professional Associations Act (Law 2/1974) bars professional bodies from publishing fee scales. There is no official price to quote. Ask at least two firms for a quote on the same scope, in writing, and check whether it includes the topographic survey, whether an existing SNCZI study for the stretch can be reused, and whether it covers answering the authority's follow-up questions.
On timing: the study has to be finished before you apply, the authority then has six months, and the municipal licence comes after that. See building permit for a house: cost and time.
Regional rules worth knowing
Valencian Community. PATRICOVA grades hazard into levels and its Annex I sets building conditions, with extra requirements at levels 3, 4 and 6. After the October 2024 floods (DANA), the regional government passed urgent planning measures by Decree-law 20/2024 (regional gazette no. 10017, 2 January 2025), later replaced by Law 2/2025 of 15 April. As announced by the regional government and reported in the press, the headline measure was a suspension of licences on non-developable land in affected areas until the Júcar River Basin Authority had reported on runoff. If you are buying in an affected municipality, ask the town hall whether your plot is in the suspended area.
Catalonia. Article 6 of the regional planning regulation (Decree 305/2006) prohibited building where flooding is serious, with the same 1 m, 1 m/s and 0.5 m²/s thresholds. Ask the Catalan Water Agency (Agència Catalana de l'Aigua, ACA) which criteria it applies to your stretch today.
Andalusia. Decree 189/2002 approved the plan for preventing floods in Andalusian urban watercourses, described in a 2012 resolution as the general framework; under it, the 500-year flood defines the extraordinary flood zone.
Check the flood map yourself, step by step
- Open the SNCZI viewer on the MITECO website and find your plot (search by municipality or coordinates; the cadastral reference gives you the location in the Cadastre).
- Turn on the 10-, 100- and 500-year flood zones. If nothing is coloured, check whether the nearest stream was studied at all. An unmapped rambla is not a safe rambla.
- Turn on the preferential flow zone where it exists.
- Read the depth layers for the 100- and 500-year floods: raster maps with water depth in metres. Note the depth where you want to build.
- Compare with the municipal plan, which may have its own hazard zones.
- Send the screenshots to your architect before you sign the deposit contract, and ask one question: could a study change this result, or only confirm it?
If you are buying from abroad, your architect or lawyer can make the town hall and river basin authority enquiries; ask for written answers.
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What may change in 2026
On 16 July 2026 MITECO opened public consultation, until 16 September, on a draft royal decree amending the RDPH. According to the ministry and press reports it would bar new homes, garages and basements in preferential flow zones, limit below-ground space in flood zones, and add a duty to inform buyers about flood risk when a property is sold, with transitional rules. As of 2 November 2026 it has not been approved. If your plan depends on a basement or garage in a flood zone, ask for the study to cover both scenarios.
If the result is bad
The whole plot is preferential flow zone. For a new house on rural land, expect a refusal. Price it as farmland or walk away.
Part of the plot is clear. Ask the engineer to fit the building into the clear part and calculate the 500-year level there. A raised house may fit, as at point C.
The flood level is high where you can build. Price the extra structure for a raised floor and compare it with what you are paying for the land.
The map shows nothing, the study shows flooding. The study prevails: it is what the authority will assess.
The map shows flooding and you think it is wrong. A study with a detailed survey is how you challenge it. If the authority accepts it, your file is decided on that boundary.
You have already paid a deposit. Check whether the contract made the purchase conditional on the river basin permit. If not, renegotiate with the study in hand.
What the report covers
A TerrenoCheck report shows, for the plot you select on the map, the SNCZI layers (flood zones by return period, preferential flow zone where published), nearby watercourses, the cadastral plot on aerial imagery, land classification and other constraints, each with its source and date. It helps you see, before you pay for a study, whether you are likely to need one. It is not a flood risk study, does not calculate new flows or depths, does not replace the river basin authority's report, and is not legal advice or a valuation.
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