Marylebone carries some flood risk, and nearly all of it comes from rain. The rivers and the sea don't reach it. About one postcode in ten still rates high on at least one flood measure. Heavy downpours on hard pavement are the problem. It is a middling risk, concentrated in particular streets and particular basements.
The Environment Agency's river-and-sea mapping puts 0% of Marylebone's postcodes in Flood Zone 3, the highest-probability band, and 0% in Flood Zone 2, the medium-probability fringe that lies beyond it. A parish that took its name from a stream, St Mary-burne, has ended up with no homes in a river flood zone, and the stream's old course is now mostly street names.
Surface water is the risk that is left. When a storm drops more rain than the drains can take, the water collects on the lowest ground, and in a neighbourhood built almost entirely of pavement, roof and road, there is little open ground to absorb it. The old Tyburn ran north to south through the parish, and low ground keeps its memory longer than maps do.
The Environment Agency's climate-adjusted flood zone, which adds an allowance for future river and sea levels, also covers 0% of the area's postcodes. On that forward measure, the rivers and the sea still don't reach Marylebone.
For insurance, the question will mostly be surface water and the exact position of the flat, and a basement or lower-ground flat is a different conversation from a third-floor one. Run the exact address through the Environment Agency's long-term flood risk service and get an indicative insurance quote, because a postcode average covers a whole block and the lowest flat in it has the most at stake.
Counting every measure together, rivers, sea and surface water, about 11% of Marylebone's postcodes rate high on at least one flood indicator, about 6% rate medium and about 13% rate low. A postcode lands in those shares if any single one of the measures flags it. That is the route by which rain shows up in a neighbourhood the river maps leave blank.
The modelled flood extents put 0% of postcodes at high risk with defences in place and 0% at high risk without them, so the question of whether a flood wall protects the house never arises. A defence in the model would describe what is assumed, not what keeps a kitchen dry, and here the model has nothing to assume. Walls and embankments do nothing about water falling from the sky in any case.
The nearest tide gauge is at Portsmouth, where sea level rose by about 2 millimetres a year across the record from 1962 to 2023. That this is the closest gauge says a lot about how far Marylebone is from the sea.
There is no recorded historic flooding anywhere in the area, with 0% of it inside the Environment Agency's recorded flood outlines. That is reassuring, as long as nobody reads it as proof of anything. A record covers what was mapped, and short, sharp surface flooding in a city street often goes unmapped.
Marylebone's flood risk is middling. The rivers and the sea are no threat, and the real exposure depends on which floor of which building you buy.