Low, and not from any river. None of Mortimer sits in a river or sea flood zone. That stays true on the map adjusted for climate change. A small number of postcodes flag on other flood measures. Here the flood question is about the drain at the end of your road, not a river.
The Environment Agency's river-and-sea flood zones cover none of Mortimer. The share of the area's postcodes in flood zone 3, the higher-probability band, is 0%. The share in flood zone 2, the medium-probability band that lies beyond zone 3, is also 0%. There is no river-flooding story for a seller to leave out of the listing, which makes Mortimer unusual in a part of Berkshire where plenty of places have one.
Surface water is different. That's rain that falls faster than the ground and drains can take it, and it floods low spots, driveways and dips in roads, whether or not a river is nearby. This page doesn't carry a separate surface-water figure for Mortimer, so the check has to be done address by address. A house at the bottom of a slope can have a flooding problem that no river map will ever show.
The Environment Agency's climate-adjusted flood zone, which projects the river-and-sea zones forward for a changing climate, also covers 0% of Mortimer's postcodes. Many places look fine on today's map and much worse on that one. Mortimer looks the same on both.
For insurance, having no postcodes in a river or sea flood zone should keep flood cover routine for most homes. Insurers price the individual address, though, and they will see any surface-water flag that applies to it. Get a quote for the specific house before you commit. Don't assume the village's clean record transfers automatically to one property.
At postcode level, about 4% of the 101 postcodes assessed rate high on at least one flood measure, including surface water. About 4% rate medium on at least one measure and about 20% low on at least one measure. These bands overlap with each other and with the zone figures above, so they can't be added together, and they aren't the same measure. With the river-and-sea zones at zero, every high flag here comes from something other than a river or the sea.
The Environment Agency's modelled flood extents put 0% of Mortimer's postcodes at high risk without defences, and 0% at high risk with defences in the model. In other words, no postcode depends on a flood defence to stay dry, because none sits in a high-risk modelled extent to start with. In the model, "defended" only means a defence exists. It never means a property is safe. In Mortimer the question doesn't arise.
The nearest tide gauge, at Portsmouth, recorded sea level rising by about 2 millimetres a year across its record from 1962 to 2023. That's a measured historic rate at a gauge a long way from an inland village, not a forecast. It has no practical bearing on a house in Mortimer.
This page's sources hold no recorded flooding for the area: the share of postcodes with historic flood records is 0%. That's reassuring. It isn't proof that no garden has ever gone under in a cloudburst, and a seller's answer on past flooding still deserves to be read.
Mortimer is one of the lower flood-risk places a Berkshire buyer will look at: river and sea risk is absent and the remaining flags are few. Check the specific address for surface water and then move on to other questions.