Flood Risk Assessment in Windsor and Maidenhead: What Officers Check

A flood risk assessment in Windsor and Maidenhead is judged first on its numbers: a design flood level and ground floor levels, all in metres above Ordnance Datum. The Royal Borough of Windsor and Maidenhead (RBWM) lies along the non-tidal River Thames. Its riverside settlements, among them Maidenhead, Bray, Windsor, Datchet and Wraysbury, all contain land in Flood Zones 2 and 3. The council is both the local planning authority and the Lead Local Flood Authority, and the Environment Agency advises on river flooding.

The notes below combine the published national and local requirements with the points that RBWM officers and the Environment Agency raised on a small number of Air & Flood Consultants applications in the borough during 2026, mostly householder extensions. They set out what is checked and how an assessment can meet it.

The standing advice test: flood levels and floor levels in mAOD

Case officers in RBWM assess householder flood risk assessments against the Environment Agency's flood risk standing advice, last updated on 28 May 2026. The advice covers minor extensions, meaning householder or non-domestic extensions of no more than 250 m² of floor space. In Flood Zone 2 or 3 it recommends floor levels at least 600 mm above the estimated flood level. The margin may be reduced to 300 mm where there is a high level of certainty about the estimated flood level. Where floor levels cannot be raised, flood resistance and resilience measures should protect the property to at least 600 mm above the estimated flood level.

The standing advice asks for every level in the assessment to be stated in relation to Ordnance Datum, and for the estimated flood level to allow for climate change over the lifetime of the development. In practice the test needs three numbers in metres above Ordnance Datum (mAOD). The first is the design flood level for the 1% annual exceedance probability (AEP) event including climate change. The second and third are the existing and proposed ground floor levels.

The comparison is made between the lowest finished floor level and the design flood level, with the freeboard stated in millimetres. Where the new floor is to match an existing floor below the required level, the assessment should give the reason and specify resistance and resilience measures to 600 mm above the design flood level. The policies applied are Policy NR1 of the Borough Local Plan 2013-2033 and policy F4 of the National Planning Policy Framework, published on 17 August 2026, which sets out when a site-specific flood risk assessment is required.

The same test applies to small non-domestic extensions, such as a shop infill, because the standing advice covers them too. Where only the garden lies within Flood Zone 2 or 3 and the house and the extension footprint lie outside the outline, the assessment should state that and attach the Flood Map for Planning extract, because the test turns on where the floor is.

Environment Agency flood levels: Product 4 and Product 6

Modelled levels come from the Environment Agency as a Product 4 (flood levels at model nodes) or a Product 6 (model output grids). Both are requested from the Environment Agency, through the Flood Map for Planning service, which offers detailed flood risk data to order for a site. An assessment in Flood Zone 2 or 3 issued before they arrive lacks the figures the officer looks for first.

For a homeowner, the practical effect is a wait at the start of the job. The Environment Agency handles a Product 4 or Product 6 request as a request for environmental information. The Environmental Information Regulations 2004 allow it up to 20 working days to respond. The floor levels cannot be checked against the flood level until the data arrives. Submitting the flood risk assessment before the Environment Agency data arrives saves a few weeks, but an assessment without flood levels risks a refusal that costs more.

Flood Zone 2: the council decides alone

For minor extensions, the council applies the Environment Agency's standing advice instead of consulting the Environment Agency on each application. For a householder extension in Flood Zone 2, the case officer therefore usually assesses the flood risk assessment without technical advice from the Environment Agency. An assessment that leaves out flood levels cannot rely on an Environment Agency "no objection" to carry it.

Under policy F4 of the NPPF a site-specific flood risk assessment is required for development in Flood Zones 2 and 3. For a householder application in Flood Zone 2, one may be asked for.

Flood Zone 3a or 3b: settled by the model, not the map

The Flood Map for Planning does not distinguish Flood Zone 3a from Flood Zone 3b, the functional floodplain. The distinction comes from the council's Strategic Flood Risk Assessment (SFRA) and the Environment Agency's modelled outlines.

Figure 1: Flood Map for Planning extract from an AFC flood risk assessment in central Windsor (2026). Flood Zone 3 is dark blue and Flood Zone 2 light blue; the map does not show whether land is in Flood Zone 3a or 3b.

The distinction can decide whether a scheme is acceptable at all. The Environment Agency's guidance on climate change allowances states that highly vulnerable, more vulnerable and less vulnerable development should not be permitted in Flood Zone 3b. Product 6 grids from the Environment Agency's Thames models give levels cell by cell across a site and show whether it is dry in the functional floodplain event. For riverside sites, the Product 6 request is worth making at instruction.

Climate change allowances

The Environment Agency's guidance on flood risk assessments and climate change allowances sets the peak river flow allowance by flood zone and vulnerability classification. In Flood Zones 2 and 3a, more vulnerable development, which includes dwellings, uses the Central allowance, and essential infrastructure uses the Higher Central allowance. In Flood Zone 3b, essential infrastructure uses the Higher Central allowance and water compatible development the Central allowance.

The percentages themselves are set for each management catchment and epoch, in the Environment Agency's table of peak river flow allowances by management catchment. The RBWM SFRA should be checked as well for any higher allowance it requires in the functional floodplain. The assessment should name the catchment, the epoch and the allowance used, and state the design flood level that results. The national position on allowances is set out in the AFC article on climate change and the NPPF 2026.

Floodplain compensation: level for level

Any new footprint or raised ground in the floodplain removes flood storage. Ground re-profiling in the landscaping counts as a loss. Where storage is lost, the Environment Agency asks for level-for-level compensation. Its usual request covers:

• the compensation set out in 100 mm or 200 mm slices up to the 1% AEP plus climate change level;

• losses and gains for each slice, with no net loss in any slice;

• a plan showing that the compensation area is hydraulically connected to the floodplain;

• the compensation excavated before construction.

The calculation should be based on a topographic survey submitted with the assessment. A statement that floodwater will pass through the building, or that floodplain storage will increase, needs a drawing of the voids, the openings and their levels to support it.

Finished floor levels should be given for every floor and for the lowest habitable room, each compared with the design flood level. The standing advice recommends floor levels at least 600 mm above the estimated flood level, or 300 mm where there is a high level of certainty about that level.

Groundwater: Thames gravels and the Chalk

On the river terrace gravels beside the Thames, groundwater risk is often high. The gravels are permeable and connected to the river. The SFRA groundwater mapping should be checked for every site, and on the gravels the groundwater section belongs in the first issue of the assessment.

West of Maidenhead the picture changes. On the Chalk, the water table can lie several metres below ground. There, the evidence for a low rating is the record from Environment Agency boreholes and wells near the site, with each reading checked against the station datum, rather than regional mapping alone.

Defences, flood warnings and safe access

The Environment Agency's Jubilee flood relief channel is part of the Maidenhead, Windsor and Eton Flood Alleviation Scheme. Flood zones are mapped without taking account of defences. A site behind the scheme is in Flood Zone 3 regardless of the defences, where the undefended outline covers it. A Product 4 for a defended site names the defences and gives their standard of protection, which may be lower than the 1% AEP event. An assessment in the defended area should record the defence, its standard of protection and the residual risk, and recommend that the occupants sign up for flood warnings.

Where a scheme in Flood Zone 3 increases the number of people at risk, for example by adding a flat, RBWM can be expected to ask for:

• a Sequential Test;

• the Exception Test;

• safe access and egress at "very low hazard" to land outside the 1% AEP plus climate change extent.

The standing advice asks for emergency access and escape plans where any part of the development is below the estimated flood level, with a safe route set above the estimated flood levels. A flood warning and evacuation plan sets out the route to land outside Flood Zone 3, the trigger for leaving, the hazard along the route and the refuge arrangements. RBWM and the Environment Agency decide whether those arrangements are acceptable. The Sequential Test is covered in the AFC article on the Sequential Test under the NPPF 2026.

The Datchet Neighbourhood Plan

The Datchet Neighbourhood Plan was adopted on 27 June 2023. It forms part of the development plan for the parish. Policy DAT10 (Flooding, Drainage and Water Efficiency) requires new development to take full account of existing flood risk from any source. It supports the retention of existing bunds. Their removal or expansion should be shown not to increase flood risk.

Paragraph 7.33 of the plan covers new buildings and extensions. In Flood Zones 2 and 3 it advises floor levels at least 300 mm above the 1 in 100-year river flood level, including climate change. For drainage, DAT10 expects SuDS that avoid infiltration, given the high groundwater table that affects parts of Datchet.

Conditions on approval

Where RBWM grants permission on a flood risk site, it can attach a condition under Policy NR1. The condition requires the mitigation and protection measures in the named assessment to be implemented and kept for the lifetime of the development. The mitigation list then becomes a planning condition binding on the owner. It should contain only measures that will be built, described specifically enough to be checked. Guidance on resistance and resilience products is in the AFC property flood resilience guide.

Before submission: a checklist for a flood risk assessment in Windsor and Maidenhead

• Flood zone confirmed from the Flood Map for Planning, with the extract attached, and 3a or 3b settled from the Environment Agency model.

• Product 4 or Product 6 levels obtained before submission, and the design flood level stated in mAOD at 1% AEP plus the climate change allowance for the flood zone, vulnerability and management catchment.

• Existing and proposed ground floor levels in mAOD, on the drawings as well as in the report.

• Freeboard of 600 mm (300 mm only with high certainty about the flood level), or resistance and resilience to 600 mm above the design flood level with the reason the floor cannot be raised.

• In Datchet, floor levels checked against paragraph 7.33 of the neighbourhood plan.

• Floodplain compensation tabulated level for level, with a plan, wherever footprint or ground levels increase.

• A groundwater section, with evidence proportionate to the geology.

• Defence standard, residual risk and flood warning registration recorded, and an evacuation route assessed where the number of people at risk increases.

The national requirements behind these points are summarised in the AFC article on flood risk assessments under the NPPF 2026.

Enquiries

Air & Flood Consultants prepares flood risk assessments, flood warning and evacuation plans, groundwater assessments and SuDS strategies for sites across Windsor and Maidenhead. Environment Agency modelled levels are requested at the start of each job. Enquiries, with the site address and the proposed drawings, can be sent through the enquiry form.