The 4 UK Flood Risk Levels Explained: What High, Medium, Low And Very Low Mean
The four flood risk levels are High, Medium, Low and Very Low. Each describes the chance an area floods in any year: High is greater than 3.3%, Medium 1% to 3.3%, Low 0.1% to 1%, and Very Low less than 0.1%.
A flood risk level describes an area rather than a single building, and each property has a separate level for rivers and the sea, surface water and reservoirs. The sections below explain each level, what a "1 in 100 year" chance means over the life of a mortgage, how the levels are calculated, and why they change.
What Are The Four Flood Risk Levels Used In The UK?
England and Wales use four flood risk levels, High, Medium, Low and Very Low, defined by the chance of flooding in any single year. Scotland uses three levels of likelihood with different thresholds, so the same word can describe different odds on either side of a border.
The flood risk levels used by each agency are compared below.
| Level | England — Environment Agency (rivers and the sea) | Wales — Natural Resources Wales (rivers) | Wales — Natural Resources Wales (sea) | Scotland — SEPA |
|---|---|---|---|---|
| High | greater than 3.3% a year (1 in 30) | greater than 3.3% (1 in 30) | greater than 3.3% (1 in 30) | at least 10% a year (1 in 10) |
| Medium | 1% to 3.3% (1 in 100 to 1 in 30) | 1% to 3.3% | 0.5% to 3.3% (1 in 200 to 1 in 30) | at least 0.5% (1 in 200) |
| Low | 0.1% to 1% (1 in 1,000 to 1 in 100) | 0.1% to 1% | 0.1% to 0.5% (1 in 1,000 to 1 in 200) | at least 0.1% (1 in 1,000) |
| Very Low | less than 0.1% (less than 1 in 1,000) | less than 0.1% | less than 0.1% | not used |
A High likelihood in Scotland starts at a 10% chance each year, three times the threshold for High in England. In Wales, sea flooding reaches Medium at a lower chance than river flooding does. Northern Ireland's flood maps are published separately by DfI Rivers; the country pages explain flood risk in Scotland and flood risk in Wales in each agency's own terms.
What Does A High Flood Risk Mean?
A High flood risk means the chance of flooding in any given year is greater than 3.3%, or more than 1 in 30. At this level, at least one flood is more likely than not over a 25-year mortgage.
| High | |
|---|---|
| Chance of flooding each year | greater than 3.3% (greater than 1 in 30) |
| Chance of at least one flood over 25 years | greater than 56.8% |
| Properties in England — rivers and the sea | 367,900 |
| Properties in England — surface water | 1,071,800 |
The Environment Agency estimates that 163,300 of the properties at High risk from rivers and the sea are likely to flood to a depth of 30cm or more. The number of properties at High risk from rivers and the sea rose by 88% in the 2024 assessment compared with the previous one.
What Does A Medium Flood Risk Mean?
A Medium flood risk means the chance of flooding in any given year is between 1% and 3.3%, or between 1 in 100 and 1 in 30. The lower edge of Medium is the 1% annual chance that planning describes as a 1 in 100 year flood.
| Medium | |
|---|---|
| Chance of flooding each year | 1% to 3.3% (1 in 100 to 1 in 30) |
| Chance of at least one flood over 25 years | 22.2% to 56.8% |
| Properties in England — rivers and the sea | 323,300 |
| Properties in England — surface water | 884,800 |
A Medium level is the point at which flood risk becomes a practical question for a buyer. Over a 25-year mortgage, the chance of at least one flood sits between about one in five and just over one in two, depending on where in the band a property falls.
What Does A Low Flood Risk Mean?
A Low flood risk means the chance of flooding in any given year is between 0.1% and 1%, or between 1 in 1,000 and 1 in 100. Flooding is unlikely in any single year, but it is not ruled out.
| Low | |
|---|---|
| Chance of flooding each year | 0.1% to 1% (1 in 1,000 to 1 in 100) |
| Chance of at least one flood over 25 years | 2.5% to 22.2% |
| Properties in England — rivers and the sea | 966,500 |
| Properties in England — surface water | 2,644,000 |
Low is the most common level among properties at risk in England, for both rivers and the sea and surface water. A Low level can reflect flood defences that exist today, which reduce the chance of flooding without removing it.
What Does A Very Low Flood Risk Mean?
A Very Low flood risk means the chance of flooding in any given year is less than 0.1%, or less than 1 in 1,000. It is the lowest of the four levels.
| Very Low | |
|---|---|
| Chance of flooding each year | less than 0.1% (less than 1 in 1,000) |
| Chance of at least one flood over 25 years | less than 2.5% |
| Properties in England — rivers and the sea | 777,300 |
| Properties in England — surface water | not published as a separate count |
Very Low for one source says nothing about the others. An address can be Very Low for rivers and the sea and still High for surface water.
What Does A "1 In 100 Year" Flood Chance Actually Mean?
A 1 in 100 year flood has a 1% chance of happening in any single year, not once a century. The chance does not reset after a flood: two such floods can happen in consecutive years, and the odds of at least one grow every year a property is exposed.
The chance of at least one flood over different periods is shown below, assuming the annual chance stays the same each year.
| Period | 1 in 30 (3.3%) | 1 in 100 (1%) | 1 in 200 (0.5%) | 1 in 1,000 (0.1%) |
|---|---|---|---|---|
| 1 year | 3.3% | 1.0% | 0.5% | 0.1% |
| 10 years | 28.5% | 9.6% | 4.9% | 1.0% |
| 25 years | 56.8% | 22.2% | 11.8% | 2.5% |
| 30 years | 63.5% | 26.0% | 14.0% | 3.0% |
| 50 years | 81.3% | 39.5% | 22.2% | 4.9% |
| 100 years | 96.5% | 63.4% | 39.4% | 9.5% |
A 1% annual chance becomes a 22.2% chance of at least one flood over a 25-year mortgage, and a 63.4% chance over a century rather than a certainty. The table assumes the annual chance stays the same, and the Environment Agency's climate projections expect the number of properties at risk to rise.
What Does A 1 In 100 Year Flood Mean Over A Mortgage?
A 1% annual chance is a 22% chance over a 25-year mortgage. The chance is not one in a hundred per lifetime — it is per year, and the years compound.
| Band | 10 years | 25 years | 40 years |
|---|---|---|---|
| High (1 in 30) | 29% | 57% | 74% |
| Medium (1 in 100) | 10% | 22% | 33% |
| Low (1 in 1,000) | 1% | 2% | 4% |
Over 25 years, a 1% annual chance is a 22% chance of flooding at least once.
This assumes the annual chance stays the same and that years are independent. In reality flood risk changes as defences, the climate and the models change, so treat these as an illustration of how the annual figure compounds, not a forecast.
How Is A Property's Flood Risk Level Calculated?
The Environment Agency calculates a flood risk level by modelling the chance of flooding across England at 2-metre resolution, taking account of the current condition of existing flood defences. Its National Flood Risk Assessment combines outputs from thousands of detailed local flood models with national-scale modelling, and it was published on 17 December 2024.
The 2024 assessment replaced one last fully updated in 2018, whose maps for rivers and the sea were limited to 50 metre resolution. It also estimates the chance of flooding to three depths: less than 30cm, between 30cm and 60cm, and greater than 60cm. Its climate projections use the 70th percentile from UKCP18, RCP 8.5 scenario.
The assessment has three limits that every flood risk level carries:
- It describes an area, not a building. The Environment Agency's service states that it does not tell you how likely it is that an individual property will flood.
- It excludes groundwater, because there are too many gaps in our understanding and evidence for groundwater flood risk.
- It does not cover flooding from blocked drains or burst pipes.
How the figures shown on each town page are derived from this data is set out in the methodology.
Why Does My Property Have A Different Flood Risk For Each Source?
A property has a separate flood risk level for each source because each source floods in a different way and is mapped separately. Rivers and the sea, surface water and reservoirs each have their own map, and groundwater has no national risk level at all.
Surface water affects more properties than rivers and the sea: about 4.6 million properties in England are in areas at risk of surface water flooding, against 2.4 million from rivers and the sea. Heavy rain can overwhelm drains on a hill far from any river, so a single address can be Very Low for rivers and the sea and High for surface water flooding. In Wales, surface water is mapped together with small watercourses.
Do Flood Defences Change A Property's Flood Risk Level?
Flood defences are counted in a flood risk level and are not counted in a planning Flood Zone. The Environment Agency's levels take account of the current condition of existing flood and coastal defences, so a home behind a well-maintained defence can have a Low level.
Defences lower the chance of flooding without removing it. Natural Resources Wales states that Flood defences reduce, but do not completely stop the chance of flooding as they can be overtopped or fail. National planning policy takes the opposite approach for Flood Zones: they Flood Zones 2 and 3, shown on the Environment Agency's Flood Map for Planning, only account for river and sea flooding, and do not account for the benefits of flood defences or the possible effects of climate change. The result is that a home can sit in Flood Zone 3a and carry a Low flood risk level at the same time, which is explained in full in why a Flood Zone 3 house can be Low risk.
Can A Property's Flood Risk Level Change Over Time?
A property's flood risk level changes for three reasons: the national model is updated, defences are built or change condition, and the climate changes. None of the three is rare, and a level recorded in a survey several years ago can differ from the level shown today.
The 2024 assessment shows how much a model update alone can move the numbers. The count of properties at risk of surface water flooding rose by 43%, and the Environment Agency states that this doesn't reflect a real-world increase in risk. It is almost entirely due to improvements in data, modelling and use of technology.
Climate change moves the numbers in one direction. The Environment Agency estimates that properties at risk from rivers and the sea could rise from 2.4 million to 3.1 million between 2036 and 2069, and those at High risk to 637,600. By mid-century, 1 in 4 properties in England could be in areas at risk of flooding.
What Does Each Flood Risk Level Mean For Insurance And Mortgages?
A flood risk level is evidence an insurer may use, not a price. The Environment Agency states that NaFRA was not developed or tested for insurance underwriting or setting insurance premiums, and that insurers set premiums with their own underwriting methods, which can combine this data with a property's characteristics, its claims history, catastrophe modelling and repair costs.
A High or Medium level therefore signals that a property needs checking with an insurer before a purchase goes ahead, rather than predicting a premium. Homes built before 1 January 2009 can be eligible for Flood Re, the scheme that helps insurers offer cover in high flood risk areas. How premiums, refused cover and previously flooded homes work is explained in flood insurance, and what a lender checks is covered in mortgage on a flood risk property.
Sources (Verified 16 Sep 2026)
| Claim | Source |
|---|---|
| England levels, property counts, depth estimates, 2024 method, climate projections, not for insurance underwriting, surface water increase explanation | Environment Agency, National assessment of flood and coastal erosion risk in England 2024 |
| Wales river and sea levels; surface water and small watercourses; defences reduce but do not stop flooding | Natural Resources Wales, Different types of flood risk |
| Scotland high/medium/low likelihood | SEPA, Flood maps FAQs |
| Area not individual property; blocked drains and burst pipes excluded | Check the long term flood risk for an area in England |
| Flood Zones do not account for defences | National Planning Policy Framework, Annex F (17 Aug 2026) |
| Flood Re build-date rule | Flood Re eligibility |
| Cumulative probabilities | Calculated: 1 − (1 − annual chance)^years, assuming a constant annual chance |