Flood Zone 1 with Surface Water Risk: Why It Still Matters

There is a persistent assumption in development that Flood Zone 1 equals no flood risk. It is understandable — Flood Zone 1 is defined as land with less than a 1 in 1,000 annual probability of river or sea flooding. For fluvial and tidal purposes, it is the lowest risk category.

But flood risk does not begin and end with rivers and the sea. Surface water flooding — caused by rainfall that overwhelms the drainage network or runs off the land before it can infiltrate or be conveyed away — is now the most prevalent source of flood risk in England. The Environment Agency estimates that 3.2 million properties are at risk from surface water flooding, compared to 2.7 million from rivers and the sea.

A site can be in Flood Zone 1 and still face significant, material flood risk from surface water. Ignoring this risk leads to planning objections, costly redesign, and in the worst cases, flooding of completed developments.

Why Flood Zones Do Not Tell the Whole Story

The Environment Agency’s Flood Map for Planning shows flood zones derived from fluvial and tidal modelling only. It does not include surface water, groundwater, or sewer flooding. A site that is pristine Flood Zone 1 on the Flood Map for Planning might show deep, fast-flowing surface water flooding on the Risk of Flooding from Surface Water (RoFSW) mapping.

This distinction is important because:

• Planning policy now considers all flood sources. The September 2025 update to Planning Practice Guidance made clear that the Sequential Test should consider all sources of flooding, not just fluvial and tidal. LPAs and LLFAs are increasingly applying this in practice.
• The LLFA may object. Even if the EA has no concerns (because the site is in Flood Zone 1), the lead local flood authority can object to a planning application on the basis of inadequate surface water flood risk assessment or drainage design.
• Building Regulations Part H requires it. Regardless of planning, the drainage design must manage surface water safely. If the site has surface water flood risk, this must be addressed in the design.

What Does Surface Water Flood Risk Look Like?

Surface water flooding occurs when rainfall intensity exceeds the capacity of the drainage system (whether natural or built) to convey it. The resulting water flows overland, following topographic low points, accumulating in depressions, and potentially entering buildings.

On the RoFSW mapping, surface water flood risk is shown at three return periods:

• High risk (1 in 30 year): Flooding that has approximately a 3.3% chance of occurring in any given year.
• Medium risk (1 in 100 year): Approximately 1% annual probability.
• Low risk (1 in 1,000 year): Approximately 0.1% annual probability.

Even “low risk” surface water flooding can be significant. A 1 in 1,000 year flood depth of 300mm or more flowing through a proposed building footprint is a material constraint that needs to be addressed in design.

Common Scenarios

Urban Infill Sites

Urban infill sites are particularly susceptible to surface water risk. Surrounding development has already increased impermeable area, reducing natural infiltration. Existing drainage networks may be at or beyond capacity. The proposed development adds further impermeable area and may block existing overland flow paths.

We frequently assess sites in London, Birmingham, Manchester, and other major cities where the Flood Zone 1 designation gives a misleading impression of low risk. The reality is that these sites sit within complex urban drainage catchments where surface water flooding is the dominant concern.

Sites at the Base of Slopes

Topography is the primary driver of surface water flood risk. Sites at the base of slopes, in valleys, or at topographic low points collect surface water runoff from a wide catchment area. Even a modest slope above the site can generate significant overland flow during intense rainfall.

These sites often show concentrated flow paths on the RoFSW mapping — narrow bands of deep water running through the site from higher ground. These flow paths must be maintained through the development; blocking them will divert water onto neighbouring properties or cause it to pond on site.

Former Greenfield Sites

When greenfield land is developed, the natural capacity of the ground to absorb rainfall is replaced with impermeable surfaces — roofs, roads, driveways, car parks. Without adequate SuDS provision, the additional runoff from the development increases surface water flood risk both on site and downstream.

This is why the LLFA requires surface water management for virtually all major developments in Flood Zone 1. The development itself creates the risk, even if the site had minimal surface water issues in its greenfield state.

What the LLFA Expects

For major developments (10 or more dwellings, or sites over 0.5 hectares), the LLFA is a statutory consultee on surface water drainage. Their expectations typically include:

Surface Water Drainage Strategy

A drainage strategy demonstrating how surface water will be managed during and after the development. This must follow the SuDS management train hierarchy: prevention, source control, site control, and regional control.

Discharge Rate Limitation

For greenfield sites, runoff from the development must be limited to the greenfield runoff rate (typically calculated using the IH124 or FEH methods). For brownfield sites, a reduction from the existing brownfield rate is usually required — many LLFAs expect a minimum 50% reduction, and some require discharge to be limited to greenfield rates regardless of the existing situation.

Climate Change Allowance

All drainage calculations must include an allowance for climate change. The current EA guidance specifies a range of uplift percentages depending on the river basin district and the epoch (the time period being assessed). For a residential development with a design life of 100 years, the upper end climate change allowance may be 40% or more.

Exceedance Management

The drainage strategy must demonstrate how the site will cope when the design capacity is exceeded — that is, during storms more severe than the design event. Exceedance flow paths must be identified and managed to ensure that flooding is directed away from buildings and towards areas of low consequence (open space, roads, car parks).

What You Should Do

If your site is in Flood Zone 1 but shows surface water flood risk on the RoFSW mapping, or if the site exceeds 1 hectare, the following steps will help you manage the risk and avoid planning delays:

1. Check the Surface Water Mapping

Before assuming the site is unconstrained, review the Risk of Flooding from Surface Water mapping. Identify any flow paths, ponding areas, or deep-water zones that affect the site.

2. Commission an FRA Early

Even if an FRA is not strictly required by the flood zone trigger, a surface water flood risk assessment and drainage strategy will be needed for any major development. Commission this early so the findings can inform the site layout and levels strategy.

3. Design with Surface Water in Mind

Use the surface water flood risk mapping to inform the masterplan. Keep buildings out of flow paths and ponding areas. Direct overland flow through open space, landscape corridors, and other areas where temporary flooding is acceptable. This approach converts a constraint into a design asset — exactly the approach we took on Capital Gate, Ilford, where surface water constraints were used to create biodiversity-rich open space.

4. Engage the LLFA

For major developments, early engagement with the LLFA is as valuable as early engagement with the EA. The LLFA will confirm their expectations for discharge rates, SuDS features, climate change allowances, and adoption arrangements.

5. Consider Direct Rainfall Modelling

If the RoFSW mapping shows significant surface water flood risk, the broad-scale mapping may not be detailed enough to inform the design. A bespoke direct rainfall model using TUFLOW or similar software can provide site-specific flood extents, depths, and flow paths at much higher resolution than the published mapping. This is particularly valuable for larger sites or sites with complex topography.

The Bottom Line

Flood Zone 1 is not a guarantee of no flood risk. Surface water flooding is widespread, damaging, and increasingly scrutinised by planning authorities. If your site shows surface water risk, address it proactively through early assessment, informed design, and a robust drainage strategy.

The cost of addressing surface water risk at the design stage is modest compared to the cost of redesign, planning delays, or post-completion flooding. If you need to understand the surface water risk on your Flood Zone 1 site, contact us for a rapid assessment.