Flood Zone 3 Flood Risk Assessment, Riverside Sauna, Whichford Mill, Stratford-upon-Avon
We prepared a site-specific Flood Risk Assessment for the retrospective change of use of agricultural land to leisure use (Class E) at Whichford Mill, Warwickshire, a rural mill site within Flood Zone 3, bounded by the River Stour. The works comprise a sauna in a converted horse box (no foundations) and a compost toilet; the existing dwelling on site is unchanged. The FRA compared the sauna's position with the Environment Agency's flood zones plus climate change extent and found it outside the 1 in 100-year plus climate change design flood extent. As a change of use, the scheme did not need the Sequential Test, and so not the Exception Test either (national policy now sets this out in NPPF Policy F5, paragraph 2(c), and Policy F6, paragraph 2(a)). Flood resilience measures were recommended for the high fluvial and surface water risk.
Flood Zone 3 Flood Risk Assessment for a Retrospective Change of Use to a Riverside Sauna at Whichford Mill, Warwickshire
Location: Whichford Mill, Warwickshire | Services: Flood Risk Assessment
A Small Leisure Use in Flood Zone 3
Whichford Mill is a rural mill site on a valley floor, bounded by the River Stour and a sluice. The Environment Agency's Flood Map for Planning places the site in Flood Zone 3. The application sought retrospective permission for the change of use of agricultural land to leisure use (Class E), with a sauna in a converted horse box (no foundations) and a compost toilet; the existing dwelling on site is unchanged.
The question for the FRA was where the sauna sits in relation to the modelled flood extent. The Flood Zone 3 designation covers the site, but the extent of the 1 in 100-year plus climate change event does not necessarily reach every structure on it.
Establishing the Planning Framework
We prepared the FRA in accordance with the NPPF, Planning Practice Guidance, the Stratford-on-Avon District Core Strategy (2011 to 2031), and the Level 1 (2020) and Level 2 (2021) Strategic Flood Risk Assessments for the district.
As a change of use, the scheme did not need the Sequential Test, and so not the Exception Test either (national policy now sets this out in NPPF Policy F5, paragraph 2(c), and Policy F6, paragraph 2(a)).
NPPF chapter 18, Managing flood risk and coastal change: https://www.gov.uk/guidance/national-planning-policy-framework-chapter-18-managing-flood-risk-and-coastal-change-f1-9
Flood Risk from All Sources
The FRA assessed tidal, fluvial, surface water, groundwater, sewer and artificial sources of flooding.
Fluvial flood risk was rated high, because the site lies entirely within Flood Zone 3. Environment Agency flood mapping shows that the site does not benefit from formal flood defences providing protection up to a 1 in 100-year (1% annual probability) event.
Surface water risk was assessed as high, and sewer and artificial-source risk as low. The Environment Agency surface water mapping shows high risk areas on parts of the site. The Stratford-on-Avon SFRA reservoir breach mapping shows the site outside the flood extent from artificial sources. Tidal risk is negligible.
The site is underlain by the Dyrham Formation, interbedded siltstone and mudstone, with slightly acid loamy and clayey soils with impeded drainage, so infiltration may not be possible. The SFRA groundwater mapping places the site in an area of no risk of groundwater flooding in a 1% annual exceedance probability event, and groundwater risk was rated low.
The Sauna and the Design Flood Extent
The FRA compared the position of the sauna with the Environment Agency's flood zones plus climate change extent, read with LiDAR ground levels, which fall towards the north of the site. The site lies within the design flood extent for the 1 in 100-year plus climate change event, but the sauna structure is outside it. The development therefore does not encroach on or reduce the floodplain storage associated with the modelled design event.
Flood Resilience Measures
The FRA recommended precautionary measures for river and surface water flooding: external ground levels profiled to direct overland flow away from building entrances; resilient materials for skirtings, doorframes and window sills; service entry points sealed against water ingress; ground profiles that remove local depressions and direct runoff toward open waterways; sealed thresholds to doors and windows; moisture-resistant plaster or plasterboard, insulation with low water absorption and well-sealed pipework; and electrical sockets at least 1.0 m above finished floor level.
Outcome
Stratford-on-Avon District Council granted permission with conditions on 3 February 2026 (25/03040/FUL). The FRA concluded that, subject to the mitigation measures it sets out, the development can proceed without being subject to significant flood risk.