Flood Risk Assessments in London

The Thames Tidal Defences protect central London to a high standard at present, but the EA Flood Map for Planning shows flood zones without defences. Virtually every riverside site in London appears in Flood Zone 3 on the standard map. A London FRA must go significantly further: breach and residual risk analysis, tidal defence failure scenarios, and direct engagement with the individual borough SFRA. The West London SFRA covers the boroughs of Brent, Ealing, Hammersmith and Fulham, Hounslow and Richmond. The East London SFRA covers Barking and Dagenham, Havering, Newham, Redbridge, Tower Hamlets and Waltham Forest. Every other borough has its own SFRA. There is no shortcut.

Surface water and sewer flooding are the fastest-growing planning concerns across inner London. The GLA Mandatory Drainage Plan requirement under London Plan Policy SI13 means surface water strategy is now a first-stage planning obligation on most new development. Many boroughs have designated Critical Drainage Areas where additional assessment is mandatory regardless of flood zone. Hillingdon, where we have delivered flood survey work, has specific ordinary watercourse flooding issues where becks and field drains overflow into residential areas, the kind of flooding that does not appear on the standard flood zone map at all.

Groundwater flooding from the Thames gravels affects parts of south and west London including Twickenham, Kingston, Merton and parts of Wandsworth. Basement development in any London flood zone requires a specific basement flood risk assessment addressing groundwater, surface water and tidal sources together. Requirements vary significantly by borough and a single London-wide approach does not exist.

• Development in Flood Zone 2 or 3 in any of the 33 boroughs, covering most riverside and low-lying development
• Development in a Critical Drainage Area in boroughs including Southwark, Lewisham, Brent, Croydon and others
• Basement development in any flood zone, requiring a specific basement FRA
• Development of 1 hectare or more in Flood Zone 1
• Any development requiring a GLA Mandatory Drainage Plan under London Plan Policy SI13
• Sites where breach and residual tidal risk must be assessed beyond the standard Flood Map

Flood Risk Assessments in London are not just about identifying flood zones. They require alignment with borough-level SFRAs, the London Plan, and detailed technical evidence that stands up to scrutiny. Our work is advisory-led from the outset. We start by understanding the planning context, flood mechanisms affecting the site, and the proposed use, then define a proportionate scope of work. This may include Exception Test evidence, consideration of the Surface Water Drainage Strategy, or site-specific measures such as flood warning and evacuation strategies where required.

We obtain and interrogate Environment Agency model data, review borough SFRA datasets, and create detailed GIS mapping to understand surface water flow paths and flood depths. The site is then considered in the context of the wider catchment, not in isolation.

We regularly work within London borough requirements, including areas influenced by the tidal Thames, West London SFRA datasets, and the GLA’s drainage policies. Flood Risk Assessments and drainage strategies can be prepared together so that submissions are technically consistent and ready for planning.

This is not a box-ticking exercise. Our role is to challenge assumptions, work with the design team, and develop a strategy that makes the development safe for its lifetime without increasing flood risk elsewhere.

We have delivered Flood Risk Assessments across London, including schemes in Southwark where Exception Test evidence supported approval in Flood Zone 3, and project work for the London Borough of Hillingdon.