Flood Risk Assessment in Old Windsor

Unda prepare flood risk assessments and drainage strategies for sites in Old Windsor and the Thames-side ground around it. A flood risk assessment in Old Windsor 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).

Old Windsor sits on one of the largest stretches of low, undefended Thames floodplain in England, where the river splits around Ham Island and the Battle Bourne drains the land behind the village. We read every source that matters here — the Thames itself, the New Cut and the ditches that struggle to discharge while the river is high, the surface water that backs up off hard ground, and the groundwater sitting close beneath the gravels — then deal directly with the borough's flood team and the Environment Agency 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.

Old Windsor has been managing its relationship with the Thames for more than a thousand years — from a Saxon mill leat cut across a loop of the river to the steel embankment that guards the village today — and on this undefended floodplain the question is rarely whether a site can flood, but how deep, how fast and how often.

Flood risk assessment in Old Windsor at a glance

• The Thames defines the village. Old Windsor lies on a broad, low Thames floodplain about two miles downstream of Windsor, where the river divides around Ham Island and rejoins below Old Windsor Lock.
• Around 2,000 homes sit largely in Flood Zones 2 and 3. The Old Windsor Neighbourhood Plan records a community of roughly 5,000 people living alongside the river in medium and high flood-probability land.
• The Old Windsor Lock gauge reached a record 5.324 metres on 11 February 2014, when the Thames burst its banks, homes and businesses flooded, and parts of Ham Island were abandoned.
• 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 Old Windsor

The Old Windsor parish 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 Old Windsor: the Thames, the Battle Bourne and Ham Island

River flooding from the Thames is the defining risk in Old Windsor. The river runs along the village on a low, broad floodplain, divides around Ham Island through the New Cut, and rejoins below Old Windsor Lock. 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.

Old Windsor's geography is the product of two centuries of engineering. Ham Island was created in 1822 when the Thames Navigation Commissioners dug the New Cut across a meander to build Old Windsor Lock, leaving a roughly 50-hectare island now home to about 37 dwellings reached only by Ham Bridge. The Battle Bourne, a small tributary, drains the low ground behind the village to the river. Knowing which channel drives the risk on a given site, and how it behaves in a flood, is the heart of a credible fluvial flood risk assessment.

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. Our notes on the sources of flood risk set out how these sources interact.

• Fluvial (river) flooding. The Thames is the controlling source. When it rises, water spreads across the floodplain into the heart of Old Windsor and onto Ham Island well before the higher ground inland is affected.
• Surface water (pluvial). Now the most widespread flood risk in England. Intense rain overwhelms drains across the built-up village, 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. See our note on surface water and planning.
• Groundwater. The Thames gravels hold water close to the surface, so levels rise quickly in a wet winter — a point the Old Windsor Neighbourhood Plan makes about the village's seasonally high water table. We treat groundwater as a source in its own right, not an afterthought.

A history of flooding in Old Windsor

Old Windsor has 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 the village watches the river so 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, inundating thousands of properties along the river through Windsor and the villages downstream, Old Windsor among them.
• 2003 and the recurring pattern. The Thames flooded notably in 1947, 2003 and the winter of 2013–14 — a reminder that significant floods recur every few years here, not once a lifetime.
• February 2014 — the recent benchmark. Prolonged winter rain put the Thames over its banks for weeks. The Old Windsor Lock gauge reached a record 5.324 metres on 11 February; homes and businesses flooded, parts of Ham Island were abandoned and a riverside horse-rescue centre was evacuated.

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

The Battle Bourne embankment, residual risk and why a protected site still needs an assessment

Old Windsor is not wholly undefended. The Battle Bourne embankment runs from Datchet Road to the Old Windsor lock cut along the southern bank of the Battle Bourne — about 1,300 metres of steel sheet piling that better protects roughly 700 properties from the Thames and its tributary. It is a genuine asset, but it manages the risk; it does not remove it.

This matters for development. The embankment is built to a standard and can be overtopped by a larger event, and because the Environment Agency maps flood risk as though defences were not present, protected parts of Old Windsor stay firmly within the flood zones on the planning map. That remaining risk — the chance of a flood beyond the design standard, or of a defence behaving 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 defence changes the odds, not the rules: because the flood maps ignore embankments, much of Old Windsor stays in Flood Zone 2 or 3, and an assessment has to show a site is safe even when the defence is overwhelmed.

Datchet to Hythe End: future defences and the planning warning

Old Windsor sits within one of the largest areas of undefended developed floodplain in England, and the scheme meant to address it is still at an early stage. The Datchet to Hythe End flood improvement measures — a partnership between the Environment Agency and the Royal Borough — are exploring how to reduce flood risk to around 2,500 properties across Datchet, Horton, Wraysbury and Old Windsor.

This reach was originally Channel 1 of the River Thames Scheme but was removed in 2020, and the borough's funding was redirected to a separate local project. Two broad options are under investigation — flood walls and embankments, or a flood relief channel — alongside property-level resilience. None of it 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 in Old Windsor must stand on the protection that exists now, not the protection that might arrive later.

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

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; Policy QP3 backs this up with sustainable design and blue and green infrastructure.

Old Windsor also has its own neighbourhood-level policy. The Old Windsor Neighbourhood Plan (2019–2033) recognises that the village is highly susceptible to flooding and expects new development to manage surface water on site and respect the seasonally high groundwater table. 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.

Local development context in Old Windsor

Development here is shaped by the river, by Green Belt that wraps the village, and by ground that holds water close to the surface. Most schemes are 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.
• Ham Island plots. Development on the island sits within the flood extent and depends on safe access across Ham Bridge in a flood, which makes access and egress a central part of any assessment.
• Householder extensions. Even modest works in the floodplain 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.

How Unda helps with flood risk assessment in Old Windsor

We are not based in Old 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 Ham Island and the Battle Bourne 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.

Flood risk assessments for every situation in Old Windsor

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 hub, read how flood risk works across the wider Windsor and Eton area and Berkshire, and try our free live river levels map, ground conditions checker, drainage calculators and BRE 365 infiltration calculator.

Local flood risk management and emergency contacts in Old Windsor

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 an Old Windsor address for free Environment Agency flood warnings; the River Thames at Old Windsor warning area covers the riverside village and Ham Island.
• Watch the river yourself. The Old Windsor Lock gauge shows how the Thames is behaving; our free live river levels map brings the local gauges together by postcode.
• Riparian responsibilities. If your boundary contains or adjoins a ditch or the Battle Bourne, you carry riparian maintenance duties that the borough and the Environment Agency will expect to see addressed.

Frequently asked questions about flood risk assessment in Old Windsor