Flood Risk Assessment in Windsor and Eton

Unda prepare flood risk assessments and drainage strategies for sites across Windsor, Eton and the Thames-side villages around them. A flood risk assessment in Windsor and Eton shows the Royal Borough of Windsor and Maidenhead, and the Environment Agency, that a development will stay safe for its whole lifetime without pushing flood risk onto a neighbour. We prepare these reports for planning, for a property purchase, for a mortgage or insurance, and for BREEAM, alongside sequential and exception tests and surface-water drainage strategies (SuDS).

Few places in England live closer to their river than Windsor and Eton. The Thames runs between the two towns, the floodplain reaches into Datchet, Old Windsor and Wraysbury, and the Jubilee River relief channel skirts the northern edge. We assess every source that matters here — the Thames itself, the brooks and ditches that drain Eton Wick and Dorney, the surface water that backs up off hard ground, and the groundwater that sits close beneath the gravels — then deal directly with the Environment Agency and the borough's flood team on your behalf. We guide developers, planning consultants, architects, landowners, home buyers and businesses through to a decision they can rely on.

If you need a quote or want to talk a site through, contact one of our experienced flood risk consultants on 01293 214444.

Flood risk assessment in Windsor and Eton at a glance

• The Thames flows straight between Windsor and Eton. Both towns, and the villages of Datchet, Old Windsor and Wraysbury, sit on a low Thames floodplain that has flooded repeatedly across the last century.
• The Jubilee River protects the towns but does not remove the risk. The 11.6 kilometre relief channel diverts up to 180 cubic metres of water a second around Maidenhead, Windsor and Eton — reducing the chance of flooding, not ending it.
• Around 1,700 homes were affected when the Thames flooded in February 2014, with Datchet, Wraysbury and Old Windsor among the worst hit and soldiers brought in to help.
• The Royal Borough of Windsor and Maidenhead is the planning authority and the Lead Local Flood Authority. It sets the local flood and drainage policy your application has to satisfy, and we work with it on every report.

Check flood risk across Windsor and Eton

The Royal Borough of Windsor and Maidenhead is outlined below. Screen any postcode against Environment Agency data for rivers and sea, surface water and the planning flood zones, with a climate outlook to 2125. It is an indicative first pass, not a substitute for a site-specific assessment.

Flood risk in Windsor and Eton: the Thames, the Jubilee River and local watercourses

River flooding from the Thames is the defining risk in Windsor and Eton. The river runs straight between the two towns, then widens onto a low, broad floodplain through Datchet, Old Windsor and Wraysbury — one of the largest stretches of developed, undefended floodplain anywhere in England. Surface water and groundwater add to the picture, because the ground here is river gravel that sits close to the water table and drains poorly once the Thames is high. Most sites face more than one source at once, which is why a single map is never enough.

Alongside the Thames, several smaller channels shape local risk. The Jubilee River carries high Thames flows around the towns; the Cut drains land to the north towards the river near Bray; and the Roundmoor Ditch winds through Dorney Common and Eton Wick, where the Environment Agency and partners have worked to restore it. Knowing which watercourse drives the risk on a given site, and how it behaves in a flood, is the heart of a credible assessment. Our notes on the sources of flood risk set out how these interact.

On this floodplain the question is rarely whether a site can flood, but how deep, how fast and how often — and that is exactly what a site-specific assessment answers.

• Fluvial (river) flooding. The Thames is the controlling source. When it rises, water spreads across the floodplain into Datchet, Old Windsor and Wraysbury well before it reaches the historic cores of Windsor and Eton.
• Surface water (pluvial). Now the most widespread flood risk in England. Intense rain overwhelms drains across the built-up parts of the towns, and since the 2025 update to national guidance, surface water on the Flood Map for Planning can trigger an assessment even on a Flood Zone 1 site.
• Groundwater. The Thames gravels hold water close to the surface, so levels rise quickly in a wet winter. We treat groundwater as a source in its own right, not an afterthought.

A history of flooding in Windsor and Eton

Windsor and Eton have flooded from the Thames for as long as records exist, and the dated events are the clearest proof of why an assessment here has to be taken seriously. The flood history also explains why local communities watch the river closely — and why a report that ignores the local story rarely convinces a planning officer or an objector.

• March 1947 — the benchmark flood. The most severe Thames flood of the twentieth century followed a hard winter and rapid thaw. Across Maidenhead, Reading and Windsor, more than 4,000 properties were flooded.
• 2003 and the years between. The Thames flooded notably in 1947, 1968, 1993, 2000, 2003, 2007 and 2014 — a reminder that significant floods recur every few years, not once a lifetime.
• February 2014 — the recent benchmark. Prolonged winter rain put the Thames over its banks for weeks. Around 1,700 homes were affected, with Datchet, Wraysbury, Old Windsor and Horton worst hit; the borough ran its emergency centre around the clock and the Army was deployed to Wraysbury.

Each of these events sits inside living memory for the villages on the floodplain, and a flood risk assessment in Windsor and Eton is far stronger when it speaks to that record rather than around it.

The lesson runs deeper than the figures. As we argued in our piece on the gap between how institutions and communities understand flooding, the most effective reports separate modelled risk from lived experience and address the local narrative head-on — something the villages downstream of the Jubilee River have demanded for two decades.

The Jubilee River, residual risk and why a protected site still needs an assessment

Opened in 2002, the Jubilee River is an 11.6 kilometre flood relief channel that takes water off the Thames above Maidenhead at Taplow weir and returns it below Windsor near Datchet. It can carry up to 180 cubic metres a second and was built to lower flood risk to roughly 3,000 properties in Maidenhead, Windsor and Eton. It is a genuine asset — but it manages the risk, it does not remove it.

This matters for development. The channel is designed to a standard and can be overtopped by a larger event, and because the Environment Agency maps flood risk as though defences and channels were not present, riverside Windsor and Eton stay firmly within the flood zones on the planning map. That remaining risk — the chance of a flood beyond the design standard, or of the channel being managed differently in an extreme event — is exactly what an assessment has to characterise. Our explainer on residual flood risk sets out how this is done, usually through finished floor levels, safe access and egress, and resilient construction.

A relief channel changes the odds, not the rules: because the flood maps ignore defences, riverside Windsor and Eton stay in Flood Zone 2 or 3, and an assessment has to show the site is safe even when the channel is overwhelmed.

There is also a local sensitivity to handle. After the floods of 2003 and 2013–14, communities downstream around Datchet questioned whether the Jubilee River had made their flooding worse. Independent assessment attributed only a small share of the effect to the channel, but the debate has shaped how flood risk work is received here. A report that acknowledges it, rather than glossing over it, lands far better with both the regulator and the neighbours.

Future flood defences: the River Thames Scheme and Datchet to Hythe End

Two major projects sit in the background of any flood risk work downstream of Windsor, and both come with a planning warning. The River Thames Scheme would build new flood relief channels through Runnymede and Spelthorne to better protect around 11,000 homes and 1,600 businesses, lowering levels by up to 0.9 metres in places. It has consumed more than £104 million in development without construction starting, and still needs a Development Consent Order before funding is confirmed.

The stretch of the Thames from Datchet to Hythe End — taking in Old Windsor, Wraysbury and Horton — was removed from that scheme and is now covered by a separate Environment Agency and borough project, the Datchet to Hythe End flood improvement measures, looking at how to protect roughly 2,500 properties on one of the largest areas of undefended developed floodplain in England. Neither project changes the position on the ground today.

Planning decisions increasingly lean on promised future defences — but a scheme that is delayed or altered transfers the risk straight back to the developer and the occupier, so an assessment must stand on the protection that exists now.

Planning and drainage policy for a flood risk assessment in Windsor and Eton

The Royal Borough of Windsor and Maidenhead is both the local planning authority and the Lead Local Flood Authority, so it decides your application and reviews the surface-water drainage behind it. Its adopted Borough Local Plan (February 2022) carries Policy NR1, Managing Flood Risk and Waterways, which requires development to account for climate change over its lifetime and to design in sustainable drainage and flood-prevention measures; Policy QP3 backs this up with sustainable design and the use of blue and green infrastructure. The borough's Strategic Flood Risk Assessment maps where these requirements bite hardest.

National rules set the baseline. Under the NPPF and its Planning Practice Guidance, a flood risk assessment is expected for development in Flood Zones 2 and 3, for any site of a hectare or more even in Flood Zone 1, and for smaller sites where surface water or the borough's records flag a risk. You can confirm a site's zone on the Environment Agency's check for flooding service and against our own flood risk map, but the published zone is only a starting point.

The borough both decides your application and, as lead local flood authority, signs off the drainage behind it — so a sound flood and surface-water case is what keeps a decision moving.

Local development context in Windsor and Eton

Development here is shaped by a tight, historic townscape and a river that is rarely far away. Windsor's centre is dominated by the Castle and a dense conservation area; Eton is a small, low-lying town wrapped around its college; and the surrounding villages are constrained Green Belt and floodplain. Most schemes are therefore conversions, infill, replacement dwellings, extensions and basements rather than large greenfield releases — and each raises its own flood and drainage questions.

• Riverside and basement schemes. Below-ground space on the Thames gravels brings groundwater and flood-depth issues to the front; finished floor levels and resilience usually decide whether a scheme is acceptable.
• Heritage and conservation constraints. In Windsor and Eton's conservation areas, flood resilience has to be designed sympathetically, which is where early advice saves redesign later.
• Householder extensions. Even modest works in the floodplain villages can need an assessment; our note on whether you can prepare your own report explains where the line falls.
• Property purchase and lending. With this much mapped flood risk, buyers and their lenders increasingly want certainty, and flood history is reflected in local house prices.

Flood risk assessments for every situation in Windsor and Eton

People need flood and drainage reports here for very different reasons, and the right report depends on the question being asked. A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct reports. They often travel together in a planning submission, but they answer different questions: is the site safe, should the development be here, and where does the surface water go. We prepare each of them.

• Flood risk assessment for planning. A site-specific planning flood risk assessment that satisfies the NPPF and the Environment Agency, demonstrating the development is safe for its lifetime.
• Flood risk for a property purchase. An independent homebuyer flood risk survey that tells a buyer what the risk means for the building itself, beyond the broad conveyancing search.
• Flood risk for mortgage and insurance. A property-level flood risk report for insurance for when cover or a mortgage is in question.
• Flood risk for BREEAM. An assessment structured around BREEAM Pol 03 so a scheme keeps the credits it is targeting.
• Sequential and exception tests. Where a site sits in a higher-risk zone, a sequential or exception test addresses whether the development should be located there at all.
• Drainage strategy and SuDS. A surface-water drainage strategy that follows the SuDS hierarchy, with discharge of drainage conditions and preliminary drainage design where a scheme needs them.

Where national maps are not enough, we bring in flood modelling to derive accurate levels and depths, and assess groundwater within the wider assessment rather than as a separate exercise. You can see the full range on our flood risk assessment and Berkshire pages, and try our free live river levels map, ground conditions checker, drainage calculators and BRE 365 infiltration calculator.

How Unda helps with flood risk assessment in Windsor and Eton

We are not based in Windsor, and that is rather the point: our value is being an accurate translator between your scheme and the people who decide it. Our chartered flood risk consultants know this stretch of the Thames, the way the Jubilee River and the floodplain villages behave, and what the borough and the Environment Agency expect to see in a report. We have prepared thousands of assessments and drainage strategies, and a senior consultant signs off every one. Where a difficult site needs it, we stay involved past the report to answer objections and keep the application moving.

Local flood risk management and emergency contacts in Windsor and Eton

Several bodies share responsibility for flooding here, and knowing who does what saves time both in a planning application and in an emergency. The table below sets out the main contacts; in a flooding emergency that threatens life, always call 999.

• Sign up for flood warnings. Register a Windsor, Eton, Datchet, Old Windsor or Wraysbury address for free Environment Agency flood warnings covering the River Thames from Maidenhead to Windsor and Eton.
• Watch the river yourself. Local Thames gauges at Boveney Lock and Romney Lock show how the river is behaving; our free live river levels map brings them together by postcode.
• Local action. Parish flood groups in Datchet and the riverside villages, set up after the 2014 floods, remain a useful local resource alongside the statutory bodies.

Frequently asked questions about flood risk assessment in Windsor and Eton