Flood Risk Assessment and SuDS Strategy — New Residential Dwelling, Twickenham

Flood Risk Assessment and SuDS Strategy — New Residential Dwelling, Twickenham
Location: 73 Pope's Grove, Twickenham, London Borough of Richmond upon Thames | Client: McLaren.Excell | Services: Flood Risk Assessment, SuDS Strategy

Two Problems to Solve Before Planning Could Progress
This project at Pope's Grove, Twickenham, arrived with two distinct technical challenges already in play. The London Borough of Richmond upon Thames had requested that the finished floor level be raised by 300mm above the maximum flood level — a condition the technical evidence did not support for a Flood Zone 1 site. And the proposed new dwelling increased the impermeable area of the plot by 67m², requiring a proportionate and policy-compliant drainage strategy before the application could proceed. Both issues needed resolving, and resolving them properly required a combined Flood Risk Assessment and SuDS Strategy built on sound technical evidence rather than conservative assumptions.

We were appointed by McLaren.Excell to prepare both documents, demonstrating compliance with NPPF flood risk requirements, Richmond Local Plan Policy LP 21, and London Plan Policy SI 13 — and, where the technical evidence warranted it, to push back on planning conditions that the data did not justify.

Flood Risk Assessment — and Challenging an Unsupported Requirement
The site at Pope's Grove sits within Flood Zone 1, confirmed on the Environment Agency Flood Map for Planning. Flood Zone 1 carries the lowest probability of fluvial or tidal flooding — less than 0.1% annual probability — and the site lies well outside the modelled extents for the 1-in-100-year plus climate change flood event. Fluvial and tidal risk are genuinely low and were addressed accordingly.

Against that backdrop, the council's request to raise the finished floor level by 300mm above the maximum flood level was difficult to reconcile with the evidence. A 300mm freeboard above design flood level is a standard and well-established requirement for sites in Flood Zones 2 and 3, where the probability of fluvial inundation is material and the freeboard provides a meaningful safety margin above the modelled water surface. On a Flood Zone 1 site outside the design flood extent, it serves no equivalent purpose — the design flood simply does not reach the site, so specifying a level 300mm above it adds cost and design constraints without delivering any demonstrable flood risk benefit.

The FRA addressed this directly. The assessment demonstrated that the site falls outside Flood Zones 2 and 3, is not subject to any fluvial or tidal flood mechanism that the 300mm requirement is designed to protect against, and that the surface water risk — identified at medium susceptibility with localised modelled depths of up to 0.2m — was being effectively managed through the proposed SuDS measures. Imposing a finished floor level uplift on the basis of a surface water risk of this magnitude and the proposed drainage response in place would have been disproportionate to the risk and unsupported by the technical evidence.

The council accepted the technical justification. The finished floor level requirement was dropped, avoiding unnecessary construction cost and design constraint for the client. It is worth noting that this kind of outcome — a planning condition overturned on evidential grounds rather than negotiated away — depends entirely on the quality and rigour of the technical case. A weaker or less precisely argued FRA would not have achieved the same result.

All remaining flood sources were assessed in full. Groundwater risk was classified as medium and informed elements of the drainage design. Sewer flooding risk was negligible, with no significant historical incidents recorded in the area. Risk from artificial sources, including reservoir breach scenarios, was low and did not influence the design or resilience specification.

SuDS Strategy — Making the Most of Atypical Ground Conditions
The proposed development increased the site's impermeable area from 153m² to 220m² — a net addition of 67m² that required a drainage response demonstrating that post-development runoff rates and volumes would not exceed pre-development conditions. On a London site, that requirement often runs straight into a familiar constraint: London Clay subsoils with negligible permeability make infiltration-based drainage effectively non-viable, and the options narrow quickly towards attenuation and source control.

Pope's Grove was different. Whilst the underlying geology is London Clay, the surface soils here are freely draining and loamy — a condition that, whilst not transforming the site into an ideal infiltration candidate, does open up drainage options that would not be available on a typical London Clay site. Infiltration is achievable to a meaningful degree, and that widened the palette of SuDS components available for the scheme considerably.

The drainage design drew on that advantage through a multi-component scheme. A 6m² green roof provides rainfall interception at source, reducing the volume and rate of runoff leaving the roof surface before it enters the drainage system at all. Two rainwater planters with a combined storage volume of 600 litres capture roof runoff for garden irrigation — delivering source control, reducing demand on the combined sewer network, and providing a practical amenity function that ensures the feature will be maintained by future occupants. A shallow dry pond at 0.4m depth in the rear garden acts as a detention basin, attenuating peak flows during intense rainfall events and allowing gradual infiltration into the underlying soils over time — making productive use of the site's atypical drainage characteristics. Permeable paving in the rear garden manages surface runoff across the hardstanding areas, whilst the existing front paving was retained without modification, avoiding any unnecessary disruption to street-facing surfaces.

Taken together, the scheme ensures that post-development runoff rates and volumes match or improve pre-development conditions across all modelled storm events. A two-stage water quality treatment train was incorporated for the roof and hardstanding drainage, consistent with CIRIA SuDS Manual C753 recommendations for residential surface types. Long-term maintenance responsibilities were assigned clearly to the homeowner, with guidance on inspection frequencies and routine tasks for each drainage component — an important element of SuDS compliance that is sometimes insufficiently addressed in residential drainage strategies but which planning authorities and the Lead Local Flood Authority are increasingly looking for.

Outcome
Planning permission was granted by the London Borough of Richmond upon Thames. The FRA successfully overturned the council's finished floor level requirement on technical grounds, demonstrating the value of a well-evidenced and assertively argued assessment over one that simply accepts pre-application conditions at face value. The SuDS strategy delivered policy-compliant surface water management for the increased impermeable area, making effective use of the site's atypical drainage characteristics to provide a genuinely multi-functional scheme — contributing to improved water quality, biodiversity net gain through the green roof and planting, and amenity value through the integrated drainage features. For a single new dwelling on a brownfield plot, that is a strong set of outcomes.