Flood Risk Assessment in Richmond upon Thames: What Officers Ask For

A flood risk assessment in Richmond upon Thames is read by three bodies, and each looks for something different. The London Borough of Richmond upon Thames is both the local planning authority and the Lead Local Flood Authority (LLFA), so the same council judges flood risk and surface water drainage. The Environment Agency advises on river and tidal flooding from the Thames, the River Crane and the borough's smaller rivers. Below Teddington Lock the Thames is tidal. Above it, it is not.

The borough's Local Plan was adopted on 7 October 2025, with flood risk and drainage set out in Policy 8 (Flood Risk and Sustainable Drainage). The notes below combine the published requirements with the points that Richmond officers, the LLFA and the Environment Agency raised on a dozen Air & Flood Consultants jobs in the borough during 2025 and 2026, most of them householder extensions, and on one condition decision taken from the planning register.

Drainage goes in with the FRA, in every flood zone

On AFC's Richmond jobs, the most frequent request is for drainage, not flood levels. Any proposal that adds impermeable area, front or rear, is expected to come with a sustainable drainage strategy. The council looks at the change across the whole site, not only at the new roof. A rainwater harvesting tank on its own still needs runoff calculations behind it.

The strategy is tested against London Plan Policy SI 13, which asks for greenfield runoff rates and runoff managed as close to its source as possible, following the drainage hierarchy. Richmond agents and clients expect the London Sustainable Drainage Proforma with it, and some asked for it before instructing.

On one householder extension in Flood Zone 1, the officer asked at decision stage for:

• peak runoff limited to the greenfield rate, or 2 l/s at most;

• calculations showing no flooding on site in the 1 in 30-year event and no flooding of buildings in the 1 in 100-year event with a 40% allowance for climate change;

• a drainage layout showing where the water goes;

• a completed London Sustainable Drainage Proforma.

On another, where the impermeable area did not change, the officer still asked for a 50% betterment calculation on existing runoff, or a written reason why it could not be achieved. An FRA that offers only qualitative SuDS suggestions tends to draw a second request, first from the agent before submission and then from the officer during determination. Pricing and scoping the FRA and SuDS strategy together avoids both.

Figure 1. Runoff table from AFC's FRA and SuDS strategy for an office-to-residential conversion in East Sheen, permitted in September 2026. Peak runoff in the 1 in 100-year event falls from 16.63 l/s to 2.39 l/s.

Infiltration: test it or rule it out

Much of the borough sits on London Clay, Langley Silt or Thames gravels with high groundwater susceptibility. Infiltration is the second destination in the hierarchy, after re-use, and the LLFA treats it accordingly. Where a strategy relies on a soakaway, the LLFA asks for BRE Digest 365 infiltration test results, and for a measured rate of at least 1 × 10⁻⁶ m/s. An assumed rate is not accepted.

On a householder SuDS condition in the borough, the details were refused about three weeks after validation, a month before the target date, with no request for further information. The submission included a proforma, a hierarchy justification, a soakaway, a maintenance regime and a drawing. The LLFA's objection asked for:

• BRE 365 test results;

• greenfield, existing and proposed runoff for the whole site area, not only the new roofs;

• exceedance routes;

• confirmation of who maintains the SuDS.

Two practical points follow. Where a soakaway is proposed, the test belongs before submission, and the client should be told it is needed. Where the ground is clay or silt, or groundwater susceptibility is high, the strategy should state plainly that no infiltration is proposed and why. The alternative is sealed storage, such as lined permeable paving, planters or a tank, with a flow control to the surface water sewer. A dry pond on untested clay drew a request for infiltration testing on one scheme, and the client replaced it with a tank rather than commission the test. On an office-to-residential conversion in East Sheen, London Clay and groundwater flood susceptibility of 75% or more ruled infiltration out. Storage there is 8.9 m³ in the drainage layer under the permeable paving, with a sedum roof and water butts, and the outfall is to the surface water sewer. No tank was needed.

Figure 2. The discharge hierarchy as recorded in the same East Sheen strategy, with infiltration ruled out and the outfall to the surface water sewer.

Critical Drainage Areas: the full package first time

Parts of the borough lie in Critical Drainage Areas. Here even a small householder extension is examined as a drainage scheme. One householder application was refused on flooding grounds with six items listed as missing:

• a justification for not using infiltration;

• rainwater harvesting;

• a drainage drawing showing the discharge point and the flow control;

• calculations based on the total site area, showing 24-hour drain-down and the 1 in 100-year event with climate change;

• an exceedance route drawing;

• a maintenance schedule naming who is responsible.

The appeal was dismissed on flood risk. On that job, a fresh application with a revised drainage strategy covering each item was approved in under four months. Where a householder scheme is refused only for missing drainage detail, a resubmission that answers each item is worth weighing against an appeal. An AFC case study sets out a SuDS condition in a Critical Drainage Area at Teddington. Infiltration was ruled out there, and rainwater planters provide the attenuation.

Flood levels: what the Environment Agency supplies

The Environment Agency's flood risk standing advice asks for floor levels at least 600 mm above the estimated flood level for minor extensions in Flood Zones 2 and 3, or 300 mm where there is high certainty about that level, with all levels stated against Ordnance Datum. Getting that flood level in Richmond now takes longer than it used to.

For the River Crane at Twickenham, the Environment Agency no longer issues a Product 4 (a table of flood levels) on request. It supplies shapefiles and the River Crane model data (Product 5 and 6) under a conditional licence, by temporary download link, and asks for the FRA to include labelled 1D and 2D node maps with level tables for the relevant events. On one site it stated that, given the model's condition, floor levels should be the 1% annual exceedance probability (AEP) plus climate change level plus 300 mm. One request took about three and a half weeks to answer.

For the tidal Thames, a Product 4 request is answered with raw model grids (Products 5, 6 and 7) rather than a sheet of levels. Design flood levels, breach levels, depths and hazard then have to be extracted from the grids in GIS. Both cases need the request made at instruction and GIS time allowed in the programme.

The flood zone can change between two issues of the same report

The Environment Agency updates the Flood Map for Planning as new models are adopted. After the River Crane 2024 model came in, a site near the river in Twickenham was in Flood Zone 2 in February 2026 and Flood Zone 3 in October 2026. The revised scheme then needed a Flood Zone 3a or 3b check, floodplain compensation and a flood warning and evacuation plan that the first report had not needed. The Flood Map extract should be regenerated for every revision of an FRA, even a few months on.

Datum is the other trap. On one site the architect's topographical survey used an arbitrary local datum about 3.3 m away from Ordnance Datum. Across two successive schemes, a floor level that looked 3 m above the flood level on the drawings was, once converted, below it in the first and only 0.2 m above it in the second. Survey levels should be checked against Environment Agency LiDAR before floor levels are set, and the drawings should show both datums.

Tidal Thames: resilience below, habitable floors above

Along the tidal Thames at Barnes, householder schemes that add upper floors have gone through with a short FRA. Each gave resilience measures for the non-habitable ground floor and showed that the new habitable floors sit above the extreme water level in the Thames Estuary 2100 plan. The floor level of the new habitable floor should be stated in mAOD against that level, and taken from the architect's drawings rather than assumed.

On sites fronting the river, such as Thames-side plots at Twickenham, the tidal section needs real levels. Owners and agents should check that any flood report describes the tidal Thames and the site's actual flood zone. It should compare Environment Agency tidal and fluvial levels with the topographical survey, and it should not call tidal risk negligible on a riverside site. An existing cellar on a Flood Zone 3 site should be covered even when the proposal adds no basement: how water could enter, the pumps or non-return valves fitted, and confirmation that the cellar is not habitable.

Small structures and basements

Garden rooms, domes, saunas and pods in a riverside garden in Flood Zone 3 still need an FRA. On one application the council first advised that none was needed and then asked for one during determination. An FRA submitted with the application avoids that delay. For a garden building on a suspended floor in Flood Zone 3a, the full footprint should be counted as lost floodplain storage, with compensation planned by lowering garden ground or removing existing structures.

Basements are covered by Policy 54 of the Local Plan. It requires a Screening Assessment by a chartered professional in the throughflow and groundwater policy zones, or where mapped groundwater flood susceptibility is 25% or more. Where the screening finds a possible impact, a full Basement Impact Assessment follows. It points back to Policy 8 on flood risk. Clients compare basement proposals against the council's basement assessment guidance, so a proposal should say which stages a desktop screening covers and which need site investigation.

Before submission: a checklist for a flood risk assessment in Richmond

• Flood Map for Planning extract generated for this issue of the report, not reused from an earlier one.

• Environment Agency model data requested at instruction, with GIS time allowed to extract the levels.

• Floor levels in mAOD, checked against the survey datum and Environment Agency LiDAR.

• A SuDS strategy with the FRA wherever impermeable area increases, with greenfield, existing and proposed runoff for the whole site and the London Sustainable Drainage Proforma.

• Infiltration either tested to BRE 365 before submission or ruled out in writing, with the reason.

• In a Critical Drainage Area: drainage drawing, flow control, exceedance routes, 24-hour drain-down and a maintenance schedule naming who is responsible.

• Habitable floors on tidal sites stated against the TE2100 extreme water level.

The national requirements behind these points are set out in the NPPF chapter on flood risk and summarised in the AFC articles on flood risk assessments under the NPPF 2026 and SuDS under the NPPF 2026.

Enquiries

Air & Flood Consultants prepares flood risk assessments and SuDS strategies for sites across Richmond upon Thames and the rest of London. Environment Agency model data is requested at the start of each job. Enquiries, with the site address and the proposed drawings, can be sent through the enquiry form.