About one in ten Steyning postcodes carries a high surface-water flood indicator. Nothing sits in the highest Environment Agency river-flood zone. About one and a half percent sits in the next zone down. The Adur was a working port until it silted up in the fourteenth century, which is a way of saying the water went somewhere and mostly stayed there. Real risk at the address level, average risk at the town level, and the two are not the same number.
The Environment Agency's river-flood zones are the first place to look, and Steyning does well on them. Nothing in the area sits inside Flood Zone Three, the highest fluvial category. About one and a half percent sits inside Flood Zone Two, the next band down, which is the Adur's flood plain doing what a flood plain does along a valley floor. Historic recorded flooding covers under one percent of the area. Tidal flooding is not the primary question because the town sits roughly six kilometres inland from the coast at its nearest point. The nearest long-record tide gauge at Portsmouth has been logging sea-level rise at around two millimetres a year across six decades of record, which is a number that matters over the life of a mortgage and does not matter this month.
Surface water is the actual risk register in Steyning. About one in ten postcodes carries a high indicator for surface-water flooding, roughly one in seven carries a medium one, and around a third carry at least a low one. This is what happens when a small town sits at the foot of the South Downs. Rain runs off the chalk, hits the settled valley floor, and finds whatever gutter, drain, or garden path it can. The distribution matters more than the average: two properties on the same street can carry completely different profiles depending on their gradient, their driveway pitch, and their relationship to the storm drains that were dug when the town was smaller and the rainfall patterns were kinder.
The climate-adjusted flood zone, which factors in projected changes to rainfall and river behaviour over the coming decades, covers just over two percent of the area. That's an increase on the current picture but not a transformation of it. The forward look is telling Steyning that the river will do slightly more of what it already does, not that the geography is about to reinvent itself. The projections are not reassurance. They are context: the risk grows, from a low base, on a slow timeline.
For insurance, what matters is whether your specific address sits inside a flood-risk polygon, not whether the town does. Postcodes with high surface-water risk face harder conversations at renewal, larger excesses on flood claims, and sometimes flood-specific loadings on the premium. Postcodes with clean flood histories usually renew on standard terms. The town-average number is not the number your insurer prices from. Your postcode is. Ask the vendor for the last five years of the property's flood history before the survey, and then ask the surveyor to look at drainage, ground levels, and any local reports of ponding after heavy rain. A one-hour walk around the plot after a storm tells you more than a decade of dashboard numbers.
Three out of five on flood risk. Steyning has fluvial history but no fluvial headline, a real surface-water register, and geography that has been settled since the Adur silted up. The town-level number is average. The property-level number is the one that decides whether the house is a good buy.