The way we design homes is changing. As the UK's climate continues to warm, overheating is no longer viewed as a future challenge, it is a current design risk that must be considered alongside energy efficiency, occupant wellbeing and operational performance. The publication of CIBSE TM59: Overheating Risk in Dwellings – A Design Stage Methodology (2026) reinforces this shift, introducing updated guidance that reflects both the latest climate projections and almost a decade of industry experience.
The revised methodology arrives at a significant moment. In 2026, the UK Climate Change Committee elevated the risk of overheating in buildings to its highest priority category, recognising that rising temperatures present an increasing threat to health, wellbeing and resilience. For the construction industry, designing homes that remain comfortable during hotter summers is no longer simply good practice, it is becoming an essential part of delivering sustainable, future-ready developments.
Moving Beyond Compliance
TM59 has become the recognised methodology for assessing overheating risk through dynamic thermal modelling, providing designers with a consistent approach for evaluating residential buildings before construction begins. While it supports compliance with building regulations, its wider value lies in helping project teams understand how a building will actually perform during future summers.
One of the most significant changes introduced in the 2026 update is the requirement to assess every dwelling using 2050s Design Summer Year (DSY1) weather data under a high-emissions climate scenario. Rather than demonstrating that a home performs adequately in today's climate, designs must now prove they remain resilient under the hotter conditions expected in the decades ahead. The updated weather guidance also introduces new UK climate zones and improved weather datasets based on the latest UKCP18 climate projections, providing a more representative basis for overheating analysis.
This represents an important change in mindset. Instead of relying on mechanical cooling to solve overheating once it has been identified, TM59 encourages design teams to optimise passive measures from the earliest stages of a project. Building orientation, glazing ratios, solar shading, ventilation strategy, thermal mass and internal layouts all play a critical role in reducing overheating before active systems are ever considered.
A Stronger Focus on Passive Design
A key principle running throughout the revised guidance is that overheating should be prevented through good design wherever possible. TM59 now introduces a structured three-stage modelling strategy that requires buildings to first demonstrate compliance using passive measures alone before mechanical ventilation or cooling can be considered where genuine site constraints exist.
The updated methodology also reflects advances in research and industry practice. A revised bedroom overheating criterion, developed using research from Loughborough University, places greater emphasis on protecting sleep quality during warmer nights, while updated guidance on modelling ceiling fans recognises their effectiveness as a low-energy means of improving thermal comfort. Together, these changes encourage solutions that enhance occupant comfort without increasing operational carbon or energy demand.
Alongside the revised methodology, CIBSE has also published supplementary guidance including weather file requirements and a TM59 compliance checklist. These documents help improve consistency across the industry by reducing common modelling errors, ensuring the correct weather data is used, and strengthening quality assurance throughout the assessment process.
Designing Homes That Are Ready for the Future
As homes become more airtight and energy efficient, the risk of overheating will only continue to grow if thermal comfort is not considered alongside operational performance. The latest TM59 guidance reinforces that climate resilience should be embedded into design decisions from day one, rather than addressed through costly mitigation later in the project.
At IN2, dynamic thermal modelling is integrated into our building performance approach from the earliest design stages. By assessing overheating risk alongside energy performance, daylighting and ventilation strategies, we help clients create buildings that are comfortable, resilient and prepared for the future climate.
The 2026 edition of TM59 is more than an update to a modelling methodology, it signals a broader shift in how residential buildings should be designed. As climate change accelerates, successful projects will be those that place passive design, occupant wellbeing and long-term resilience at the heart of every decision.