Cosham mostly sits above the flood maps. None of the area falls inside the river or sea flood zones for medium or high probability. There is no recorded historic flooding on the addresses here. Surface water is the live risk, and it touches a small minority of postcodes rather than the bulk of them. The forward-looking climate maps are the part that earns a longer look.
For river and sea flooding, the present-day picture is effectively clean. None of the area sits inside the medium-probability flood zone, and none sits inside the high-probability zone. That reflects the geography. Most of Cosham sits on or below the southern slopes of Portsdown Hill, set back from the harbour edge, with the bulk of the residential stock on ground that the maps do not flag for tidal or river flooding under current conditions. Historic flood records for the area come back at zero too.
Surface water is where the present-day risk actually lives. About one in eighteen postcodes carries a high surface-water flood risk, and a little under one in ten carries a medium risk. Roughly three in ten carry a low risk, with the rest classed as negligible. The pattern follows the topography in detail rather than spreading evenly across the area. Streets at the bottom of a slope or near a storm-drain pinch point will sit in the affected fraction; streets a hundred metres uphill will not. The headline number is small but the per-property check matters.
The forward look is different. The climate-adjusted flood zone, which models river and sea risk against projected sea-level rise, covers around half the area. That is a 2050-and-beyond number rather than a current one, and it reflects the harbour-fringe edges getting wetter rather than the inland streets going under. The Portsmouth tide gauge, with records running since 1962, shows sea level rising at roughly two millimetres a year, which is the rate the climate map is extrapolating from. A property well inland today might still be inland in twenty years; one closer to the harbour edge will start the lender conversation earlier.
For insurance, most properties in Cosham will buy buildings cover at standard rates without a flood loading. The minority of postcodes flagged for surface water can attract a higher excess or a specific flood premium, and buyers there should check the previous owner's renewal price and the conveyancer's flood search rather than the postcode-level average. Properties inside the climate-adjusted zone will not affect cover today, but will start to feed into lender and insurer pricing as that horizon shortens.
Average for the south coast. Lower than the harbour-edge postcodes, higher than the streets at the top of Portsdown Hill. Most owners will not think about it; the ones who should will know which side of the street they are on.