British homeowners may be adding to the country's surface water flooding problem by paving over gardens, according to new research commissioned by Flood Re, the reinsurance scheme that backs affordable flood cover, with even modest rainfall capable of sending bathtub-loads of water into already strained drainage systems.
The findings were released ahead of this year's Chelsea Flower Show, where Flood Re is exhibiting its "Flood Re: Contain the Rain" garden. Surface water flooding is among the fastest-growing flood risks in the UK.
Surface water flooding occurs when rainfall overwhelms local drainage systems and can happen far from rivers or coastlines, even at the top of a hill. More intense downpours linked to climate change are driving the trend.
The warning lands against a wet start to 2026, with Cornwall recording its wettest January on record and parts of Devon, Somerset, and South Wales seeing rainfall well above long-term averages. Successive extra-tropical cyclones left soils saturated and river levels elevated heading into spring.
A 5mm rainfall event on a 20m² paved garden can generate about 100 liters of runoff, the rough equivalent of a full bathtub directed into drains rather than absorbed into the ground.
That runoff is additional to water already entering drains from roofs, gutters, and household use. Across a residential street, the combined load can overwhelm systems and push water back toward homes, in some cases backing up through pipes.
The Environment Agency estimates around 4.6 million properties in England are at risk of surface-water flooding, including roughly 1.1 million at high risk, underscoring why the reinsurance industry is paying closer attention to runoff at the household level.
Awareness of the risk remains limited. Flood Re's research found 82% of British garden owners believe their outdoor space would cope with heavy rainfall, while 31% do not realize that paving and other hard surfaces can raise flood risk. Just 3% say they consider flood risk when designing their garden.
Drainage design treats impermeable paving as near-total runoff, while permeable surfaces and planted gardens slow, store, and absorb rainfall at source. The average flood repair bill now exceeds £30,000.
Kelly Ostler-Coyle (pictured above), director of corporate affairs at Flood Re, said households risk "sleepwalking into a flooding problem without realizing it." She added that hard surfaces send water racing into drains while greener, more permeable designs help slow, store, and absorb it.
Dr. Peter Melville-Shreeve, associate professor at the University of Exeter, said intense rainfall alone does not cause flooding. "It's all about what happens to rain when it hits the ground," he said, describing the loss of green space to paving as "urban creep."
Flood Re is encouraging homeowners to check local flood maps, reduce unnecessary hard surfacing, and add features that absorb and store rainwater, framing household-level action as a complement to wider reinsurance and resilience efforts as flooding pressures build.
