How Groundwater Impacts Planning Applications and Site Development

FPS Environmental | May 2026

We recently attended a CIWEM and British Hydrological Society webinar on development planning and groundwater flood risk and came away feeling a mix of encouragement and frustration that anyone working in this sector will recognise.

Encouragement, because some genuinely excellent thinking is being done. Frustration, because the gap between what we know and what the planning system actually does about it remains stubbornly wide.

Groundwater flooding is something we deal with regularly at FPS Environmental. We see it underestimated, misread, and in many cases simply missed at the planning stage, only to become a serious problem once development is underway or complete. Here’s our take on where things stand, and why it matters to anyone involved in planning, development, or flood risk in the UK.

Key Takeaways

• Groundwater flooding is the only major source of flood risk with no national, freely available mapping in England, it wasn’t included in the
• Environment Agency’s NAFRA 2 assessment.
• No statutory consultee exists for groundwater flood risk in development planning.
• National groundwater flood risk maps are screening tools, not predictions and they are frequently misread in planning applications as being more definitive than they are.
• The September 2025 update to the Planning Practice Guidance now explicitly requires groundwater to be considered in the Sequential Test.

The Forgotten Flood Risk at the Heart of UK Planning

Ask most developers or planners about flood risk and they’ll think about rivers, coastlines, and surface water runoff. Groundwater rarely enters the conversation, until it starts entering the development. It’s the source the flood industry knows least about, invests least in, and assesses least consistently. And in our experience, that gap has a habit of becoming very expensive, very quickly.

Unlike surface water flooding, which responds quickly and visibly to a rainfall event, groundwater builds slowly and silently. In areas underlain by permeable geology, chalk downlands, sandstone, river gravels, sustained winter rainfall gradually recharges aquifers until water begins to emerge at the surface. That emergence can happen weeks or even months after the rainfall that caused it, in places that may have seen little flooding.

When it does come, it tends to stay, events can last for weeks or months. Conventional flood defences are largely ineffective because the water isn’t coming over a wall or down a channel; it’s rising from underneath. And the damage it causes to below-ground structures, foundations, and buried infrastructure can be significant. Groundwater is a genuinely widespread risk that the planning system has historically struggled to get to grips with.

Why the Planning System Keeps Getting This Wrong

There’s No One Responsible

One of the issues in the current planning framework is that no statutory consultee exists for groundwater flood risk. The Environment Agency must be consulted on developments in river and tidal flood zones. Lead Local Flood Authorities have statutory consultee status for surface water drainage in major developments.

In practice, that means groundwater can and does fall through the gaps. A planning application can progress through the system without anyone with specific groundwater expertise ever formally reviewing it. Given that groundwater flooding can persist for months and has in the past taken out critical infrastructure, it’s an issue that needs addressing.

The Mapping Is Misunderstood

National groundwater flood risk datasets exist, produced by flood risk organisations. These are valuable tools. But they are broad screening tools, built on regional-scale modelling that cannot reliably predict what will or won’t happen at any individual site.

We see this misunderstood in both directions in planning applications. On one side, a site within a groundwater susceptibility zone is sometimes treated as automatically high-risk, when in reality the mapping may be significantly overestimating the likelihood of an actual flooding event. On the other, the absence of a clear red flag on a map is sometimes read as confirmation that groundwater isn’t an issue, when it might well be.

The Language of Risk Doesn’t Translate

There’s also a more subtle problem that doesn’t get talked about enough: the risk classifications used in groundwater mapping don’t mean the same thing as those used for surface water or fluvial flooding. A “high risk” designation on a groundwater map is not equivalent to a “high risk” designation on a surface water flood map. The underlying models, the return period assumptions, and the physical mechanisms involved are simply different, and the comparison isn’t a straightforward one.

This matters because planners, developers, and insurers often need to make decisions that involve comparing flood risks from different sources. If we’re treating groundwater risk classifications as directly comparable to fluvial risk classifications, we’re potentially drawing the wrong conclusions.

What the Policy Landscape Looks Like Now

The good news is that policy is moving in the right direction. The December 2024 revision to the National Planning Policy Framework strengthened the overall approach to flood risk in planning, and the September 2025 update to the Planning Practice Guidance (PPG ) took a meaningful step by explicitly naming groundwater, alongside surface water, sewer flooding, and reservoir risk, as a source that must be considered as part of the Sequential Test.
This is significant. For years, the Sequential Test in practice was largely focused on fluvial and tidal flood zones, with other sources treated inconsistently. Groundwater now has to be part of that conversation at site selection stage, not just flagged later in a flood risk assessment.

The underlying data gap is real and significant. The Environment Agency’s National Flood Risk Assessment 2, the most comprehensive picture of England’s flood risk published to date, does not include groundwater flooding. That means national risk statistics, strategic investment decisions, and policy benchmarks are all built on a picture that omits one of the four main flood sources entirely. Until that changes, groundwater will continue to be under prioritised.

What Good Practice Actually Looks Like

In our view, the right approach to groundwater flood risk in development planning follows a clear hierarchy and it starts much earlier in the process than most developers currently engage with it.

Start With the Right Data and Know Its Limits

National groundwater risk datasets should be used as a first-pass screening tool, not a final answer. They are useful for flagging areas where further investigation is warranted. They are not useful for concluding that groundwater flooding won’t happen at a specific site.

If national data indicates potential susceptibility, the next step is to bring in local knowledge: Strategic Flood Risk Assessment data from the relevant Local Planning Authority, historical records, local geological understanding, and where uncertainty remains significant, site-specific groundwater monitoring.

Follow the Hierarchy

Once the risk is understood, the response should follow a structured order of priority:
Avoid – can the site layout or location be adjusted to steer development away from the highest-risk areas? This should always be the first question.
Adapt – if some development must occur in a groundwater susceptible area, can the design be adapted to reduce its exposure? This might mean raising floor levels, reconsidering basement designs, or relocating the most vulnerable uses.
Accommodate – design the development so that groundwater can move as it naturally does without causing damage. This means building in resilience rather than trying to hold the water back.
Resist – flood resistance measures (barriers, waterproofing, pumping systems) should be the last resort, not the first response. They require ongoing maintenance and they often simply displace the problem elsewhere.

Think About SuDS and Groundwater Together

One area where we regularly see problems is the interaction between sustainable drainage design and groundwater. SuDS are now the default approach for surface water management in new development, but not all SuDS are appropriate in all locations. Infiltration-based systems such as soakaways and permeable paving work by directing water into the ground. Where the water table is already seasonally high, that can actively worsen groundwater conditions. Infiltration suitability must be assessed before drainage design is finalised, not as an afterthought.

The Bigger Picture

What struck us most at the webinar was not any single technical point, but the broader picture it painted: a hazard that is genuinely significant, increasingly well understood within the specialist community, and yet still operating largely below the radar of the planning system as a whole.
Project Groundwater – a six-year, Environment Agency-funded programme covering the chalk areas of Buckinghamshire, Hertfordshire, Oxfordshire, and surrounding authorities, is doing important work to develop practical guidance, improve monitoring, and influence national policy. The recent input into RTPI and TCPA planning guidance on flood risk is a meaningful step. But the project concludes in 2027, and without sustained institutional commitment to the issue, there’s a real risk that momentum is lost.

The Environment Agency’s R&D programme currently contains no project specifically focused on groundwater flooding. NAFRA 2 doesn’t include it. No one has statutory responsibility for it in planning. These are structural weaknesses in how England manages one of its least understood flood risks.