Retrospective New Dwelling Justified in a River Ems Floodplain

A private applicant had built a new house at the end of a quiet cul-de-sac on the Hampshire and West Sussex border, a short distance inland from Chichester Harbour. The dwelling was already standing, tucked behind a row of two-storey homes, when a flood risk assessment was needed to support a retrospective planning application.

The Project

The complication was the site’s position on the map. It falls within Flood Zone 2, and partly within Flood Zone 3, on the Environment Agency’s Flood Map for Planning, with the risk coming from the River Ems around 105m to the east. Because a new home counts as “more vulnerable” development, national flood risk policy required clear evidence that the building was safe and would not make flooding worse elsewhere. Unda was instructed to provide that evidence for a house that had already been constructed.

The Challenge

A retrospective application changes the usual order of things. On a normal scheme, flood risk advice shapes the design before anything is built, so levels, layout and drainage can be adjusted to suit the site. Here the house was finished. There was no opportunity to raise floor levels or move the footprint. The assessment had to work with the building as it stood and show, after the fact, that it already met the tests a planner and the Environment Agency would apply.

That mattered because the mapped position looked unpromising at first glance. A more vulnerable dwelling sitting in Flood Zones 2 and 3 is exactly the sort of case that can attract an objection, a request for redesign that is no longer possible, or a refusal that would leave a completed home without consent. The site also sat within the Flood Zones plus climate change extent, so the future picture had to be addressed as well as the present day.

The real question was narrower than the Flood Zone label suggested: where did the actual flood risk fall on the plot, and was the house itself within it? Answering that needed detailed modelling rather than the broad-brush zones alone.

What We Did

Unda requested detailed modelled flood levels and extents from the Environment Agency, drawn from the River Ems Hydraulic Modelling. This gave node-by-node flood levels across the site instead of the coarse Flood Zone outline, which is the difference between knowing a general area is at risk and knowing whether a particular building is.

The modelling reframed the case. The dwelling sits entirely within Flood Zone 2 and entirely outside Flood Zone 3. More importantly, it lies outside the modelled 1 in 100 year flood extent at the present day, and stays outside it under every climate change allowance provided, including the 25%, 36% and 64% increases in river flow. The only modelled event that reaches the building at all is the much rarer 1 in 1000 year flood.

Even then, the numbers were comfortable. The modelled 1 in 1000 year level is 6.27mAOD. Ground levels around the house range from 6.77 to 7.28mAOD, so the dwelling sits at least half a metre above that extreme event, and the threshold levels at the doors, between 7.30 and 7.54mAOD, are more than a metre clear of it. Surface water risk to the building was assessed as “very low”, both now and into the 2040 to 2060 period, and the risk from sewers and reservoirs was similarly low, with no record of the site having flooded before.

We then set out how the completed dwelling still satisfied the wider policy tests. Safe escape was demonstrated on foot to higher ground that stays dry in the 1 in 100 year event with climate change. Because there had been no land raising within that flood extent, there was no unacceptable loss of floodplain storage, and any new fencing was specified as hit-and-miss or palisade so that floodwater could still pass. The applicant also agreed to register for the Environment Agency’s free flood warning service and to add flood-resistant and resilient measures where they were practical on a building already in place. The house includes a green roof, which supports a sustainable approach to surface water on the site.

The Outcome

The assessment gave the retrospective application a clear, evidence-based footing. Rather than resting on the Flood Zone label, it used the Environment Agency’s own modelling to show that the completed dwelling sits outside the 1 in 100 year flood extent, above the 1 in 1000 year level, and at very low risk from surface water, with safe escape available and no loss of floodplain storage.

On that basis Unda concluded that the development was suitable in flood risk terms, subject to the agreed management measures. For a home that was already built, that evidence was the route to resolving flood risk as a planning issue rather than leaving it unanswered.

This project shows how detailed modelling can separate the risk to a specific building from the broad Flood Zone it sits within, even when the assessment has to follow construction rather than guide it.