Groundwater Flood Risk and Planning Applications: What Developers Need to Submit

Groundwater is the flood source planners forget about, right up until it stalls a scheme. It rarely makes the headlines the way a burst riverbank does, yet on the wrong site it is the single issue that holds up a decision, forces a redesign or rules out a basement. This guide sets out how groundwater flood risk is handled in the English planning system: what the Environment Agency and Lead Local Flood Authorities (LLFAs) expect to see, and what you actually need to submit. If you want the underlying mechanism first (why water tables rise, and where), start with our explainer on what groundwater flooding is.

When does groundwater flood risk need to be assessed?

If your site needs a Flood Risk Assessment at all, that FRA has to consider every source of flooding, groundwater included. This is not optional: the National Planning Policy Framework and the Planning Practice Guidance on flood risk and coastal change both require an assessment of all sources. In practice, groundwater moves from a box-ticking line to the leading issue when one or more of the following applies:

• the site sits over a principal aquifer (chalk, limestone or sandstone) or significant superficial sand-and-gravel deposits;
• there is a local history of groundwater emergence, springs or seasonal ponding;
• the proposal includes a basement or other below-ground structure;
• the LLFA or Environment Agency raises groundwater in pre-application advice; or
• the site falls within an area mapped as susceptible to groundwater flooding (see below).

Get this wrong at the screening stage and the cost lands later, usually as an LLFA holding objection that prevents determination.

The data: what the maps show, and what they don’t

This is where many submissions start on the wrong foot. The familiar national tools, the Environment Agency’s Flood Map for Planning and the public Check long-term flood risk service, map flooding from rivers, the sea and (since the 2025–26 updates) surface water. None of them maps groundwater. The same is true of NaFRA2: the second National Flood Risk Assessment covers rivers, sea and surface water with climate change, but it is not a groundwater dataset. So “we checked the Flood Map and groundwater wasn’t flagged” is not an assessment. The map was never going to flag it.

For a screening view of groundwater, the standard reference is the British Geological Survey’s Susceptibility to Groundwater Flooding dataset (often called Areas Susceptible to Groundwater Flooding). It is a hazard dataset, derived from geology, permeability and historic groundwater levels at roughly 50-metre resolution, and it tells you whether groundwater is a credible issue worth investigating, not whether a given plot will flood. Your local authority’s Strategic Flood Risk Assessment (SFRA) usually carries the same screening at borough scale.

Screening only takes you so far. Where groundwater matters, the EA and LLFA will expect site-specific evidence: borehole and well records from the BGS GeoIndex and any historic ground investigation, a season of groundwater monitoring, and the published aquifer and superficial-geology mapping for the site. Monitoring is a subject in its own right, and its seasonal timing trips up more applications than anything else. We cover it in detail in our guide to groundwater monitoring for planning.

What to submit: the groundwater section of your FRA

For a site with a genuine groundwater constraint, an FRA that satisfies the EA and LLFA carries a dedicated groundwater section. In many report templates this sits at Section 5, alongside the other flood sources. A robust one does five things:

A reviewer reading that section should be able to follow the logic from “is groundwater a risk here?” through to “here is the level we have designed against, and why.” If they can’t follow it, expect questions, and the delay that comes with them.

Two sites, two groundwater problems

A chalk site in the south. On chalk, water tables can swing several metres between summer and late winter, and a level taken from a single dry-season borehole can sit well below the true winter peak. We have seen a basement designed against an autumn reading that would have sat below the following February’s groundwater level. We only caught it because monitoring ran across the wet season. The fix is rarely “no basement”; it is a defensible design level and the right waterproofing approach.

A sand-and-gravel site in a river valley. Superficial sand-and-gravel deposits over a floodplain behave like a shallow, responsive aquifer that rises with the adjacent river. Here the trap is drainage: a soakaway-based SuDS scheme that looks fine on a dry day fails when the water table is within a metre of the surface, and the LLFA objects. The answer is early infiltration testing and a drainage strategy that does not assume the ground will take the water.

Dewatering, discharge and abstraction near groundwater bodies

On sites close to a groundwater body the issues run beyond flood risk, and they can produce a separate objection if they surface late. Two come up most often:

• Dewatering during construction. Pumping groundwater to keep an excavation dry is a form of abstraction. Short, small-scale dewatering may be covered by an exemption or a regulatory position statement, but larger or longer operations need a water abstraction licence from the Environment Agency, supported by a Hydrogeological Impact Appraisal that shows the effect on nearby abstractors, wetlands and watercourses.
• Discharge to ground. Putting water back into the ground, or disposing of the water you have pumped, can require an environmental permit. Where the site sits in a groundwater Source Protection Zone the constraints tighten, because the aquifer is used for supply.

Flagging these alongside the flood-risk work, rather than after the FRA is signed off, keeps them from becoming a late surprise. Where supply abstraction or a Source Protection Zone is involved, the EA’s position can be decisive.

Common mistakes that trigger objections

• Treating groundwater as part of surface-water runoff rather than a separate source with its own evidence.
• Designing soakaway SuDS on a high-water-table site without infiltration testing.
• Setting basement or finished floor levels against an average rather than a credible maximum groundwater level.
• Relying on screening data alone where site-specific monitoring was clearly needed.
• Assuming the national flood maps would have flagged groundwater. They don’t.

None of these are exotic. They are the everyday reasons a groundwater objection lands, and every one is avoidable with the right evidence in the right order. Our analysis of a listed-building groundwater case shows how costly it gets when the baseline data simply isn’t there.

How Unda can help

Unda prepares Flood Risk Assessments that treat groundwater as a distinct source: interpreting EA and BGS data, specifying and reviewing monitoring, and designing finished floor levels, basements and drainage against a defensible groundwater level. We engage the LLFA and Environment Agency early, so concerns are resolved before they become objections. A well-evidenced FRA from an experienced consultant is often the difference between a clean approval and a costly redesign.