Portsmouth is a city on an island, so parts of it flood and parts of it never will. Most of the built-up area sits outside the river and sea zones today. Roughly one postcode in eight rates high on at least one flood measure. Once the climate allowance goes in, close to half the city falls inside a zone. The useful question here was never whether Portsmouth floods. It is whether your bit does.
On river and sea flooding, the Environment Agency's mapping puts about 8% of the area's postcodes in flood zone 3 and about 3% in zone 2. Zone 2 being the smaller number is not a mistake. Zone 3 is the higher-probability core, and zone 2 is the additional medium-probability fringe sitting outside it, so the two are separate bands rather than one inside the other.
Surface water is not published here as a share of its own. It is folded into the combined postcode indicators further down, which means the river and sea figures above describe rivers and sea only, and the combined figures describe something broader. Reading one as the other is how people end up thinking a page contradicts itself.
The forward look is the number worth sitting with. Once the climate allowance is applied, roughly 47% of postcodes fall inside a flood zone. That is not a prediction about next winter. It is the mapped extent when the modelling accounts for the sea the coast is expected to be dealing with later this century, and it covers a great deal of ground that is currently dry and unremarkable.
For insurance, a high rating or a zone 3 address changes the conversation rather than ending it. Expect the premium and the excess to reflect it, expect the insurer to want the specific address rather than the postcode, and expect a lender to take an interest. Getting an indicative quote before instructing a solicitor is cheaper than finding out afterwards.
Across the combined postcode indicators, which fold surface water in alongside rivers and sea, about 12% of postcodes rate high on at least one measure, around 15% medium and roughly 37% low. Those bands overlap with the zone figures above rather than adding to them, because a postcode can appear in both counts.
Counted as homes rather than postcodes, the Environment Agency's rivers-and-sea mapping lists roughly 19,300 properties across the 792 postcodes that carry any risk at all. Of those, about 2,400 are rated high, just over 1,000 medium and roughly 6,100 low, with around 1,750 of the high-rated ones residential. Postcodes with no risk do not appear in that list, so it describes the exposed part of the city rather than the whole of it.
Whether anything stands between the water and the house is the part most flood summaries skip. About 11% of postcodes sit inside a high-probability flood extent with no defence present in the model, against about 4% where a defence is present. That second figure is not reassurance. It records that the model knows a defence exists, not that it will hold, not that it protects a particular building, and not that the standard it was built to still matches the sea it faces.
The coast is close by design. Around 28% of postcodes sit within 500 metres of it, a threshold the mapping defines rather than a judgement about any address, with the median postcode about 800 metres away and the nearest about 7 metres. The whole area is covered by a shoreline management plan, and the dominant policy is to hold the line across the twenty, fifty and hundred year horizons. A stated policy is an intention to maintain defences over that period. It is not a defence that already exists, and it is not a promise.
At the Portsmouth tide gauge, records running from 1962 to 2023 show sea level rising at about 2 millimetres a year. That is a measured historic rate over six decades, not a forecast, and it is the baseline the coastal policy above is written against.
No postcode in the area falls inside a recorded historic flood outline. One flood event is on file, from the sea, in March 2008. Absence from the recorded outlines is genuinely worth something, and it is not the same as proof that a street has never taken water.
Flood risk is the middling entry on Portsmouth's card. Not the disaster an island city invites you to assume, not something to wave away with a line about the seafront being lovely. Put the exact address into the Environment Agency's long-term flood risk tool before you fall for the house, because the city-wide answer and your answer are different documents.