Flood risk assessment (major development)
A full flood risk assessment for larger developments, including review of modelling and detailed mitigation.
About this service
Larger developments in or near a flood zone need a fuller flood risk assessment than a domestic extension. The council, and often the Environment Agency, will want to see detailed flood levels, how the buildings will stay safe over their lifetime, how people will escape or shelter in a flood and how the scheme avoids increasing risk elsewhere. We prepare these assessments for new residential and mixed-use schemes up to five storeys across London.
The work goes beyond reading a flood map. We obtain and review the available flood modelling, including data for the tidal Thames and any river breach scenarios, and use it to set finished floor levels and access arrangements. We also assess surface water, groundwater, sewer and reservoir flooding, which can be the dominant risks on London sites well away from a river.
Flood risk and drainage are closely linked, so the assessment interfaces with the SuDS strategy and site levels. If the site is in Flood Zone 2 or 3, planning policy may also require the sequential and exception tests, which we can prepare alongside. Every assessment goes through our three-step quality control process, with final sign-off by our Head of Engineering.
When you need it
- You're developing a larger site in or near a flood zone
- The Environment Agency or council needs a detailed assessment
What's included
- Review of flood data and modelling
- Assessment of all flood sources
- Finished floor levels and mitigation
- Surface water and drainage interface
How it works
- Obtain flood dataWe request modelled flood levels and defence information from the Environment Agency where available, and review the council's strategic flood risk assessment and surface water mapping.
- Assess all sourcesWe assess tidal, river, surface water, groundwater, sewer and reservoir flooding, considering climate change allowances, the vulnerability of the proposed uses and how risk may change over the lifetime of the development.
- Set levels and safety measuresWe recommend finished floor levels, flood resistance and resilience measures, safe access and egress routes, refuge where needed and a flood warning and evacuation approach.
- Check effects elsewhereWe consider whether the development displaces floodwater or affects flow routes, and set out compensation or design changes where the scheme would increase risk to others.
- Coordinate drainage and reportWe link the assessment to the surface water strategy and issue a full report for planning, then respond to Environment Agency or council comments during the application.
What you receive
Finished floor levels and residual risk in London
Floor levels are usually the central issue. The Environment Agency normally expects sleeping accommodation to be set above the design flood level with an allowance for uncertainty, often called freeboard. On tight urban sites, raising floors can conflict with neighbouring buildings, street frontages and accessibility. Solutions such as placing non-residential uses at ground floor with homes above are common, but need agreeing early with the planning officer and the Agency.
Along the tidal Thames, much of London is protected by defences, yet planning still considers what would happen if a defence were breached. Breach modelling can show deep, fast-flowing water close to the river. Away from the river, surface water flow paths and groundwater in gravels can be more important. Your borough council, acting as Lead Local Flood Authority, will comment on those local sources.
Frequently asked questions
When is a full flood risk assessment required?
A site-specific flood risk assessment is generally needed for development in Flood Zones 2 and 3, for sites of one hectare or more in Flood Zone 1, and for sites the council or Environment Agency identifies as having critical drainage or other flood problems. Larger schemes, new homes and more vulnerable uses need a fuller assessment than minor extensions. The council's validation list confirms what it expects.
What is the difference between flood resistance and flood resilience?
Flood resistance aims to keep water out of a building, using raised thresholds, flood barriers, sealed openings and non-return valves on drains. Flood resilience accepts that water may enter and limits the damage, using materials that dry quickly, raised electrics and solid floors. Most schemes use a combination, with the balance depending on the expected flood depth and duration.
Will the Environment Agency object to my planning application?
The Agency is consulted on certain applications in Flood Zones 2 and 3 and may object if the flood risk assessment is inadequate or the development would be unsafe. Common reasons include floor levels set too low, no safe access, or loss of floodplain storage. A thorough assessment that addresses these points from the start greatly reduces the chance of an objection.
How long does a flood risk assessment take?
Our typical turnaround is 10 working days, confirmed in writing with the fee. Where we need to request modelled data from the Environment Agency, the timetable depends partly on how quickly they respond, so we request it at the start. Our full flood risk assessments start from £1,500 + VAT, with a fixed fee agreed before work begins.