Climate Design Data for London, UK

Direct answer

Use Heathrow's Met Office 1991–2020 normals as a transparent London-area baseline, not as HVAC design temperatures. For modelling, select the project's CIBSE 2025 zone by postcode or coordinates, use a TRY for annual assessment and the required DSY1/2/3 files for overheating, then apply the English Approved Documents and transition rules relevant to the submission date.

Public observed baseline

These are observed long-term averages for the named station. Heathrow is a defensible public London-area reference, but an airport site in west London is not automatically representative of a dense central street, an exposed riverside plot or every edge-of-city site. Record the project's distance, elevation, urban form and exposure before accepting it as context.

Translate the observations into design questions

The normals indicate a milder winter and warmer summer baseline than the Manchester and Edinburgh references in this tranche. They do not quantify a London heatwave or urban heat-island penalty. For design, split the work:

• use the public normals to communicate seasonal context and to challenge obviously mismatched inputs;
• use CIBSE's location selector and a TRY for annual operational-energy simulation;
• use the applicable DSY1, DSY2 and DSY3 files to test the relevant forms of heat event;
• model internal gains, solar exposure, window operation, security, noise and air quality rather than assuming night ventilation is available;
• obtain a named, edition-controlled design criterion from the building-services engineer for heating and cooling plant sizing.

London overheating and façade decisions

A July mean daily maximum of 23.89°C must not be treated as a worst case. CIBSE's DSY method exists because hot-event intensity and duration affect buildings differently. At concept stage, test orientation, glazing area, g-value, external shading, exposed thermal mass and purge-ventilation feasibility before relying on active cooling. Preserve a usable strategy: an opening area that is blocked by noise, pollution, security or guarding constraints is not equivalent to dependable heat rejection.

For new residential work in England, Approved Document O provides a simplified and a dynamic thermal route within its scope. The weather-file and modelling assumptions must match the selected route. Planning policy, local guidance and the project brief can add requirements, so Part O is a floor for the applicable work rather than a universal statement of good performance.

English edition checkpoint

At this page's review date, the 2021 Approved Document L incorporating 2023 amendments remains relevant for current work. The published 2026 Approved Document L has later effective dates in 2027, with a separate date for higher-risk work and transitional rules. Before fixing fabric, systems and reporting assumptions, record:

Limits

The table is useful for orientation and QA, not a compliance dataset. It contains no CIBSE hourly weather, Guide A design values, degree-days or invented percentile temperatures. A project team must license or otherwise lawfully access the required CIBSE material and document the selected site, zone, release and criterion.

London design-climate handoff sheet

Related

• UK Building Climate and Weather Data Explained
• Climate Design Data for Manchester, UK
• Climate Design Data for Edinburgh, UK

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Source & method: Observed figures are from the official Met Office Heathrow location-specific long-term averages, station Heathrow, climate period 1991–2020, retrieved 26 August 2026. The Met Office describes these pages as samples for general interest rather than formal climate monitoring. Weather-file roles and the 1994–2023 baseline observation period are from CIBSE Weather Data and the 2025 technical briefing; select the actual project zone by postcode or coordinates and obtain licensed files from CIBSE. English compliance references are the official Approved Document L collection, Approved Document L 2026 notice and Approved Document O. No CIBSE tabulated values are reproduced.