Flood Risk Assessment for New Build Residential Development

The Sequential Test requires that new residential development is directed to areas of lowest flood risk. Where a site in Flood Zone 2 or 3 is being brought forward, the applicant must demonstrate that there are no reasonably available alternative sites in lower flood zones that could accommodate the development.

In some cases, where a site-specific Flood Risk Assessment demonstrates that the development would be safe for its lifetime without increasing flood risk elsewhere, and that no part of the development would be affected by flooding from any source, the Sequential Test may not need to be applied. This is typically considered on a site-specific basis.

For sites allocated in a Local Plan through the Sequential Test at plan-making stage, applicants are not required to repeat the test at application stage unless flood risk circumstances have changed or elements of the development differ from what was assessed. For unallocated sites, the developer must carry out the Sequential Test as part of the planning application.

Finished floor levels for new build residential should be set above the design flood level, with an appropriate freeboard allowance. Environment Agency standing advice typically recommends a minimum of 600mm above the design flood level, although this may be reduced (for example to 300mm) where there is a high degree of confidence in the flood level, or increased where there is greater uncertainty or residual risk.

The design flood level is the predicted flood water surface elevation for the 1 in 100 year river flood event, or 1 in 200 year tidal event, including an allowance for climate change over the lifetime of the development.

On sites with limited or uncertain model data, obtaining Environment Agency Product 4 data and undertaking site-specific analysis is often necessary to establish these levels robustly and to justify the freeboard applied.

Safe access and egress is a specific requirement for new residential development in flood risk areas. The FRA must demonstrate that a safe dry route exists for residents and emergency vehicles during the design flood event. Where a safe dry route cannot be demonstrated, the development may need to demonstrate that safe refuge above flood level is available and that residents will receive adequate flood warning, through an accompanying Flood Warning & Evacuation Plan (FWEP).

• Any new residential development in Flood Zone 2 or 3, regardless of size.
• New residential development of 1 hectare or more in Flood Zone 1.
• New residential development in Flood Zone 1 where the SFRA identifies surface water, groundwater or ordinary watercourse flood risk.
• Development requiring Sequential Test evidence for a site in Flood Zone 2 or 3.
• Development requiring Exception Test evidence for a more vulnerable residential use in Flood Zone 3a, or where residential development in Flood Zone 3b functional floodplain would not be permitted at all.
• Any residential development where the planning authority or Environment Agency has indicated flood risk is a material consideration.
• Pre-application scoping at RIBA Stage 1 or 2 to establish flood risk requirements before design is committed

New build residential is the development type where a flood risk assessment has the most potential to add value, and where a late or poorly scoped one causes the most damage. Finished floor levels set without proper flood level analysis, layouts fixed before access routes are tested, drainage strategies prepared separately from the FRA and inconsistent with it: these are the things that create problems at planning and cost money to resolve.

We obtain EA model data and establish design flood levels from that data, not from the online mapping tools. Those tools are useful for screening. They are not sufficient for setting floor levels or demonstrating safe access on a planning application, and using them as if they were is one of the most common reasons FRAs draw objections from the Environment Agency.

Early instruction is where the return is greatest. At RIBA Stage 2, when layout, levels and access are still being decided, flood risk input changes outcomes. By Stage 4, an FRA can only respond to decisions that have already been made. Finished floor levels, site layout, SuDS placement and access arrangements are all most efficiently resolved before they are fixed in the design, not after. We work with developers and architects from Stage 1 through to planning submission, so that the flood risk analysis informs the design rather than reacting to it.