Flood Risk Assessment and SuDS Strategy — Five-Flat Conversion, Peckham
We prepared a combined Flood Risk Assessment and SuDS Strategy for the conversion of a detached dwelling into five flats at 39 Consort Road, Peckham — achieving 50% surface water betterment through a green roof, rainwater harvesting and permeable paving despite high pluvial and reservoir breach risk.
Flood Risk Assessment and SuDS Strategy — Five-Flat Conversion, Peckham
Location: 39 Consort Road, London, SE15 2PH, London Borough of Southwark | Client: Matra Group Ltd | Services: Flood Risk Assessment, SuDS Strategy
High Surface Water Risk and Reservoir Breach in Flood Zone 1
Flood Zone 1 doesn't always mean low risk. At 39 Consort Road in Peckham, the site carries high risk from two sources — surface water flooding immediately in front of the property and reservoir breach on the wet-day scenario — making this a more demanding FRA than the flood zone classification might suggest. The proposed conversion of a detached Victorian dwelling into five self-contained flats, with rear extensions across basement, ground and first floor and a new loft storey, also created a net increase in impermeable area that needed a robust SuDS strategy.
We were appointed by Matra Group Ltd to prepare the combined FRA and SuDS Strategy for the London Borough of Southwark.
Site Context and Ground Conditions
The site is a detached residential property at grid reference E: 534582; N: 176415, bounded by dwellings on all sides with railway infrastructure to the south. The underlying geology comprises Lambeth Group bedrock with Interglacial Lacustrine Deposits (clay and silt) as superficial deposits. A BGS borehole approximately 500m south (reference TQ37NW3020) confirmed silty clay, brown sand, silty sand and grey clay, with groundwater at 2.1m depth. Soilscapes mapping indicated slowly permeable, seasonally wet soils — limiting infiltration potential. The Southwark SFRA (2017) and the Southwark Plan 2019-2036 informed the assessment.
Flood Risk Assessment — All Sources
Surface water (pluvial) risk was high — EA mapping confirmed significant ponding and overland flow routes immediately in front of the site during extreme rainfall events. Reservoir breach (artificial source) risk was also high — the site falls within the modelled wet-day breach flood extent. Fluvial, groundwater, and sewer flooding risks were all assessed as low. No historic flooding was recorded at the site itself.
SuDS Strategy — 50% Betterment Through a Multi-Component Management Train
The proposed extensions created a net increase in impermeable area. To offset this, we developed a comprehensive source-control SuDS strategy incorporating three complementary components:
Approximately 47m² of green roofing — providing interception storage, water quality treatment, and biodiversity benefit. 280 litres of rainwater harvesting via a water butt system — providing grey water recycling to the basement WC, reducing both potable water demand and surface water discharge. Approximately 130m² of permeable paving — providing attenuation and water quality treatment through the sub-base.
Collectively, these measures achieved a 50% reduction in peak surface water runoff relative to existing site conditions. The system was designed to the 1-in-100-year storm event with a 40% allowance for climate change, delivering a significant betterment in both on-site and downstream flood risk.
Outcome
The combined FRA and SuDS Strategy demonstrated full NPPF compliance, providing the London Borough of Southwark with the technical evidence to support the planning application. The 50% betterment in surface water runoff — achieved through a diversified SuDS management train rather than a single attenuation solution — delivered genuine environmental benefit alongside the planning requirement.