LONDON CLIMATE INSIGHTS
21 March 2026 - LocalRisk
Analysis of Environment Agency, BGS and Defra data across all 33 London boroughs and 180,920 postcodes reveals a city with four distinct and material climate risks. While 18.1% of postcodes fall in high flood zones, the more striking finding is that 71.2% sit on probable shrink-swell clay - the highest subsidence exposure of any major UK city in our analysis.
London flood risk, subsidence and air quality data across 180,000 postcodes. 71% probable subsidence exposure, the worst urban air quality in England, and
The Thames Barrier protects roughly 18% of London postcodes from tidal flooding. The risk affecting 71% of the capital - clay subsidence beneath foundations - rarely makes headlines.
Analysis of Environment Agency, British Geological Survey and Defra data across all 33 London boroughs and 180,920 postcodes reveals a city with four distinct and material climate risks: widespread clay subsidence, highly variable flood exposure, urban heat that exceeds comparable cities, and air quality that leads England by a considerable margin. No other major UK city in our analysis carries significant exposure across all four categories simultaneously.
Seven London boroughs record 100% probable shrink-swell clay risk across every one of their postcodes: Westminster, Southwark, Lambeth, Brent, Kensington and Chelsea, Hammersmith and Fulham, and the City of London. Across Greater London as a whole, 71.2% of postcodes are assessed as probable risk by the British Geological Survey, with a further 22.9% assessed as possible - meaning 94% of London postcodes carry some assessed level of clay subsidence exposure.
The cause is geology. London sits on London Clay - a thick, reactive Eocene marine formation that underlies most of inner and central London. London Clay shrinks during dry summers and expands during wet winters. This cycle of contraction and expansion causes ground movement beneath buildings, which in turn causes structural settlement. Shrink-swell clay movement is the most common cause of ground movement insurance claims in England, and London Clay is the most extensive such formation in the country.
Clay subsidence risk can be a material factor in property valuations and insurance terms, and is increasingly considered a material factor in property transactions. See our subsidence guide for more on what shrink-swell risk means for property owners.
Brighton records zero clay subsidence risk. Bristol records zero. Cambridge records 35.3% probable. London records 71.2% - the highest of any major city in our analysis. The boroughs with lower probable risk - Bromley (17.5%), Hackney (23.0%), Bexley (25.1%), Hounslow (28.4%) - sit partially on chalk, greensand or river alluvium, but most still carry significant possible exposure in many postcodes.
London's flood picture is defined by contrast. Barking and Dagenham records 61.4% of postcodes in high flood risk zones - one of the highest rates among all English councils, and the most exposed borough in Greater London. The City of London, sitting on higher ground at the heart of the Square Mile, records just 2.4%.
| Borough | High flood risk % | |---|---| | Barking and Dagenham | 61.4% | | Hackney | 33.2% | | Harrow | 29.2% | | Brent | 26.9% | | Hillingdon | 26.7% | | Haringey | 25.4% | | Tower Hamlets | 8.5% | | Newham | 8.5% | | City of London | 2.4% |
_Source: Environment Agency flood zone data, analysed by LocalRisk._
The highest-risk boroughs fall into two distinct groups. Eastern boroughs facing tidal and fluvial risk from the Thames system - Barking and Dagenham, Havering, parts of Greenwich - account for the most extreme exposure. Northern and western boroughs carrying river flood risk from the Lea, Colne and their tributaries - Hackney, Haringey, Brent, Hillingdon, Harrow - form the second cluster.
The Thames Barrier has cut central London's tidal flood exposure since it opened in 1982, but it does not protect against all flooding scenarios and is being closed more frequently as sea levels rise and storm surge risk increases. Our flood risk guide explains how Environment Agency flood zones are defined and what they mean for mortgage lenders and insurers. Across Greater London as a whole, 18.1% of postcodes fall within high flood risk zones and a further 9.7% within medium zones.
London averages around 11.5 days above 25°C per year on the ERA5 1991-2020 baseline - more than Bristol (5.5) and Manchester (4.0) on the same baseline, and broadly comparable to Cambridge (10.9). For a city at 51.5° north, this is notable and has a specific cause: the urban heat island effect.
London's density of heat-absorbing surfaces - roads, rooftops, concrete, glass - means the city retains warmth overnight and during heat events in a way that rural and coastal areas do not. During the July 2022 heatwave, Heathrow recorded 40.2°C - among the highest temperatures measured in England that day, and the first occasion 40°C was exceeded anywhere in the country.
The effect is unevenly distributed within London. Outer boroughs with more green space and tree canopy - Richmond, Kingston, Bromley - typically sit several degrees cooler than dense inner boroughs during heat events. The hottest boroughs in our recent data (2023-2025 observed average) are Redbridge, Bexley and Camden, each averaging 27.7 hot days per year over that period - well above the city-wide long-term ERA5 baseline of 11.5 days, reflecting both the anomalously warm 2023-2025 period and the urban heat island intensifying at a borough level.
London's annual rainfall - based on Met Office station data - is approximately 563mm at central London (Greenwich Observatory, 1991-2020), with variation across the wider borough spread. This places London drier than Manchester (~850mm), Bristol (782mm) or Brighton (799mm), and broadly comparable to Cambridge (548mm). Hot, dry summers in a dense built environment also accelerate London Clay shrinkage - the same conditions that drive the heat figures worsen the subsidence risk. See our heat risk guide for more on urban heat exposure and what it means for properties.
Every London borough exceeds the WHO PM2.5 annual guideline of 5 µg/m³. The City of London records 11.0 µg/m³ - the only council in our entire dataset to carry a High air quality band. Inner London boroughs typically record between 9.0 and 9.8 µg/m³. Outer London boroughs range from 8.1 to 8.7 µg/m³.
| Area | PM2.5 (µg/m³) | Band | |---|---|---| | City of London | 11.0 | High | | Tower Hamlets | 9.8 | Medium | | Westminster | 9.7 | Medium | | Hackney | 9.7 | Medium | | Manchester | 7.7 | Medium | | Cambridge | 7.4 | Medium | | Brighton | 6.7 | Medium | | Bristol | 6.6 | Medium |
_Source: Defra UK-AIR PM2.5 monitoring data, analysed by LocalRisk._
Road traffic is the primary contributor, alongside building heating systems and the reduced wind dispersal that comes with dense urban building geometry. The City of London's figure reflects the combination of high traffic density and the tightly packed layout of the Square Mile. Our air quality guide covers what PM2.5 levels mean for respiratory health and how to check your local reading.
London's four climate risks compound each other. Hot, dry summers shrink the clay beneath foundations while raising heat health risk and straining air quality. River flooding in the east tends to coincide with periods of intense rainfall that overwhelm ageing drainage systems. The density that creates the heat island also amplifies air pollution.
No single risk dominates across the whole city. But subsidence is the one that affects the most London properties, is most frequently encountered in conveyancer searches, and - based on the data - the one that receives comparatively little attention in property searches relative to flood risk, which is more widely reported in the media.
> London's most widespread climate risk is geological - the clay beneath 71% of the capital's postcodes, which responds directly to the hotter, drier summers that projections show becoming more frequent.
Met Office UKCP18 climate projections put London at around 40 days above 25°C as a central estimate for the 2030s (2021-2040 period) - more than three times the ERA5 1991-2020 baseline of around 11.5 days, and the highest projected figure of any city in this analysis. These are probabilistic projections - the 50th percentile is the central estimate within RCP8.5; the full range of modelled outcomes is wide and lower emissions scenarios would produce lower figures. London's combination of the urban heat island effect, extensive dark surfaces, and limited tree canopy in many inner boroughs means heat risk compounds faster here than in any other UK city.
London's climate profile varies more dramatically by borough than any other city in this analysis. Choosing the right area matters more here than anywhere else.
Flood risk follows a clear east-west pattern. East London boroughs carry the highest flood exposure: Barking and Dagenham at 61.4%, Hackney at 33.2%, Haringey at 25.4%. The City of London (2.4%) and outer south boroughs like Bromley and Kingston carry far lower exposure. If flood risk is a priority concern, east and north-east London require careful postcode-level checking before buying.
Subsidence is harder to avoid in London than in any other UK city. Inner boroughs - Westminster, Southwark, Lambeth, Kensington and Chelsea - sit on 100% probable London Clay. The boroughs with meaningfully lower probable exposure are Bromley (17.5%), Bexley (25.1%) and Hounslow (28.4%), which sit partially on chalk or gravel. Everywhere else, a structural survey and understanding of any underpinning history is a worthwhile step before exchange.
Heat is most intense in dense inner boroughs and lowest in outer areas with tree canopy and green space. Richmond, Kingston, and Bromley consistently record lower urban heat island temperatures than Camden, Hackney or Tower Hamlets. For households with young children or older relatives, this difference is practically meaningful during summer heat events.
Air quality follows population density. Worst in the City of London (11.0 µg/m³), Tower Hamlets (9.8) and Westminster (9.7). Outer suburbs are typically 8.1-8.7 µg/m³ - still above the WHO guideline, but materially better than central zones.
Check any London postcode at localrisk.co.uk for a full five-risk breakdown across flood, heat, subsidence, air quality and coastal erosion.
Risk varies between London boroughs - and within them. You can check flood, heat, air quality, subsidence and coastal erosion risk for any London postcode free at localrisk.co.uk.
For borough-level statistics and postcode comparisons, explore: Westminster · Hackney · Barking and Dagenham
Data sources: Environment Agency flood zone data (NaFRA2, January 2026); British Geological Survey shrink-swell clay risk assessment; Defra UK-AIR PM2.5 monitoring (2024); Met Office UKCP18 climate projections; Met Office 1991-2020 station-based climate averages (Greenwich Observatory: 563mm annual rainfall); Open-Meteo Historical Weather Archive (ERA5 reanalysis), 1991-2020 long-term baseline; 2023-2025 recent observed. Analysis by LocalRisk. _Methodology: Annual rainfall uses Met Office 1991-2020 station normals (Greenwich Observatory, 563mm). Hot-day and cold spell counts use Open-Meteo ERA5 reanalysis (1991-2020 baseline). Climate projections use Met Office UKCP18 (RCP8.5, 50th percentile, 2021-2040 period) - the same source as LocalRisk postcode risk scores. The 50th percentile is the central estimate; the full range is wide, and outcomes under lower emissions scenarios would be materially less. Days above 25°C are used as the hot-day threshold for consistent cross-city comparison._
You can check the flood, heat, air quality, and subsidence risk for any UK postcode - free - at localrisk.co.uk. The full Westminster breakdown is at localrisk.co.uk/council/westminster - or explore any London borough via the local areas page.
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Climate risk varies between postcodes within Westminster. Central addresses by district:
Use the postcode search to check any UK address.