UK Climate Rankings
These 20 local authority areas have the highest projected number of days per year exceeding 25°C, based on UKCP18 climate projections for the 2030s.
UK housing stock was designed for a temperate climate. The 25°C threshold is widely referenced in UK overheating-risk literature, including CIBSE's overheating-risk standards, which apply to UK building types and account for the lack of mechanical cooling in most existing properties. Days exceeding 25°C don't sound extreme by Mediterranean standards, but for buildings designed without air conditioning they materially affect indoor comfort, sleep quality, and health for vulnerable residents.
UKCP18 is the Met Office's UK climate projection programme, the official reference dataset used by HM Government, the Climate Change Committee, and UK local authorities for adaptation planning. The 2021-2040 near-term period is calibrated against observed weather from the recent past, which means the projections reflect a near-term trajectory rather than a distant scenario. The RCP8.5 high-emissions pathway is the headline scenario used here; under lower-emissions pathways the numbers would be somewhat lower, but the relative ranking between councils stays broadly similar.
UKCP18 produces a probability distribution of outcomes rather than a single number. The values shown here are the 50th percentile (the central estimate), meaning there is a 50% probability the actual outcome will be lower and a 50% probability it will be higher. The 95th percentile (worst case) would show substantially more hot days; the 5th percentile (best case) substantially fewer. LocalRisk uses the central estimate because it is the standard reference figure for UK planning.
The number shown for each council is the median across all postcodes in the area. Within any council, postcodes near large parks, water bodies, or higher elevations will run cooler than those in dense urban cores. London postcodes near Hampstead Heath or Richmond Park show lower hot-day counts than equivalent postcodes near Liverpool Street or the City. For an address-specific picture, the LocalRisk postcode page shows the postcode-level projected hot days alongside other climate risks.
Heat risk is largely a built-environment problem rather than a climate problem. The most effective mitigations are external shading (awnings, brise-soleil, mature trees on the south-west side), light-coloured external surfaces, mechanical ventilation, and in some cases air conditioning. The Greater London Authority's published guidance on Overheating in Major Developments is one current UK reference. New-build properties built to the 2021 Building Regulations Part O are now required to mitigate overheating risk by design; older properties typically rely on retrofit measures.
For UK housing stock, which was largely designed for a cooler climate without mechanical cooling, days above 25°C trigger indoor overheating in many properties. CIBSE references 25°C in its overheating-risk standards. The threshold is lower than typical Mediterranean comfort thresholds because UK buildings lack the architectural features (shutters, thick masonry, ventilation patterns) that warmer climates evolved over centuries.
RCP8.5 is Representative Concentration Pathway 8.5, a high-emissions climate scenario in the UKCP18 reference set. LocalRisk uses it because it is the standard reference for UK adaptation planning by the Climate Change Committee and HM Government. Under lower-emissions pathways the hot-day counts would be somewhat lower, but the ranking order between councils stays broadly similar because the urban heat island effect dominates inter-council variation.
Projections. The Met Office UKCP18 dataset is a forward-looking climate model output for the 2021-2040 near-term period. It is calibrated against the historical climate record but extends beyond what has been observed. Observed historical hot-day counts (from ERA5 reanalysis) are different and are shown elsewhere on LocalRisk for context.
Two factors combine. First, the urban heat island effect: dense built environments absorb solar radiation during the day and release it overnight, keeping urban temperatures consistently higher than surrounding rural areas. London is the largest UK urban heat island. Second, London sits in the warmest part of England geographically. The combination pushes all 33 London boroughs into the highest projected hot-day counts in the country.
External shading is the most effective single measure (awnings, brise-soleil, mature deciduous trees on the south and west sides of the property). Other measures include light-coloured external paint and roofing, cross-ventilation strategies, and mechanical ventilation. New properties built to Building Regulations Part O (2021) must address overheating by design. The Greater London Authority publishes guidance on Overheating in Major Developments that is widely used as a UK reference.