CIBSE has publishedTM59: Overheating risk in dwellings – a design stage methodology (2026), or TM59:2026 for short – the most significant update to the residential overheating assessment methodology since TM59 was first introduced in 2017. If you’ve used TM59 before, it’s worth treating this less as a minor revision and more as a new assessment framework: new climate data, new criteria, and a fundamentally different approach to how compliance is demonstrated.
For architects, developers, housebuilders, building services engineers and anyone specifying residential schemes, understanding what’s changed now – ahead of wider regulatory adoption – will make it much easier to design homes that pass first time.
In this blog you’ll learn:
- Why CIBSE updated TM59, and what’s driving the change
- The headline changes: climate data, bedroom criteria, communal spaces and the new three-stage framework
- What’s new for mitigation – constrained ventilation, ceiling fans and home offices
- What this means for compliance under Part O
Why the TM59:2026 Update?
The UK has now seen three consecutive years of temperatures exceeding 35°C. What used to be treated as an exceptional heatwave is increasingly the norm, and overheating risk in homes is rising with it. CIBSE’s TM59:2026 responds to this by assessing homes against the climate they’ll actually experience over their lifetime, not the climate of today.
The Headline Change: Future Climate Data
Previously, TM59 assessments were run against current-day weather data. Under TM59:2026, assessments must now use weather files based on a 2050s climate scenario as standard, reflecting the fact that homes built today will still be in use decades from now.
Alongside this shift, CIBSE has also refreshed its weather dataset:
- UK climate zones increase from 16 to 28, for far greater geographical accuracy
- New 2025 weather files are used as the baseline
- Projections are now available for the 2030s, 2050s and 2080s, allowing schemes to be tested against a range of future scenarios
The practical effect: building types that already struggle with overheating – highly glazed schemes, single-aspect apartments, homes with limited cross-ventilation – will find compliance harder. Façade design, glazing specification, shading and ventilation strategy all need to work harder, and earlier in the design process.
Revised Part O Overheating Criteria
TM59:2026 changes how bedrooms are assessed and introduces an entirely new criterion for shared spaces.
Bedrooms. The old criterion – hours spent above 26°C – is replaced by an assessment of the mean overnight bedroom temperature. A dwelling now fails if this average exceeds:
- 26°C for Category 1 (Temporary Accommodation), or
- 27°C for Category 2 (Residential Accommodation)
on more than four nights a year. This change is informed by Loughborough University research linking sustained elevated night-time temperatures to poorer sleep quality and health outcomes – a more meaningful measure of comfort than short overheating spikes.
Communal spaces (new Criterion D). For the first time, TM59 sets dedicated requirements for corridors and other shared circulation spaces, particularly relevant to higher-density schemes. Corridor ventilation, smoke ventilation integration, and heat build-up in communal areas now need explicit consideration, not just individual dwellings.
A New Three-Stage Assessment Framework
TM59:2026 introduces a staged approach designed to keep passive design front and centre, with mechanical cooling only permitted as a last resort:
- Stage 1 – Passive measures only. No site-specific constraints (noise, air quality, security) are applied to window opening. This tests the building’s inherent resilience through orientation, glazing, shading, ventilation and fabric alone.
- Stage 2 – Realistic constraints. Site-specific restrictions on natural ventilation are introduced, and low-energy measures such as ceiling fans or mechanical ventilation can be considered.
- Stage 3 – Mechanical cooling. Only permitted if the dwelling still fails after Stages 1 and 2 – reinforcing that active cooling is a last resort, not a default.
This structure rewards good design early and makes it much harder to “specify your way out” of poor fundamentals with cooling systems bolted on later.
A More Realistic Ventilation Approach
Rather than the previous all-or-nothing treatment of restricted windows, TM59:2026 introduces a partially constrained ventilation approach: where noise, security or air quality issues are present, windows can be assumed open for 50% of the time, rather than fully open or fully closed. This better reflects how people actually behave in hot weather – often tolerating some noise or poor air quality in exchange for comfort – and avoids both over- and under-estimating real-world overheating risk.
Ceiling Fans: Now a Recognised Mitigation Measure
Ceiling fans are formally recognised in TM59:2026 as a legitimate, low-energy mitigation strategy. They improve comfort through increased air movement without the energy penalty of full mechanical cooling, and are likely to become a common feature in schemes where full air conditioning isn’t desirable, practical, or aligned with energy targets.
Home Offices Now Assessed
With hybrid working now firmly embedded, TM59:2026 introduces specific occupancy profiles and criteria for home offices, ensuring these spaces are tested against realistic working-hours use rather than being ignored or folded into general living space assumptions.
What Does This Mean for Compliance?
At present, Approved Document O still references TM59:2017, and formal adoption of TM59:2026 into the regulatory framework is expected in due course. In the interim, some local authorities and building control bodies may be willing to accept TM59:2026 assessments voluntarily, recognising the value of testing against more representative future climate conditions.
Given the scale of the changes – updated climate data, revised criteria, a new staged framework – schemes that comfortably passed under TM59:2017 may need re-testing, and design decisions made early (glazing ratios, orientation, ventilation strategy) will carry more weight than ever.
What’s Next?
TM59:2026 represents a genuine step change in how residential overheating risk is assessed, not an incremental tweak. Getting ahead of it now – testing schemes against future climate scenarios and building in passive design measures from the outset – will save time, cost and redesign later, whether or not your local authority is asking for it yet.
If you’d like to discuss what TM59:2026 means for your project, or need a TM59 Overheating Assessment carried out under the new methodology, get in touch with our team.