For developers, architects and design teams, understanding the changes to the overheating methodology assessment now will help ensure projects are aligned with current best practice and reduce the likelihood of design changes later in the process.
As our climate continues to warm and homes become more energy efficient, designing buildings that remain comfortable throughout the summer has become an increasingly important consideration. Overheating is no longer viewed as a residual risk – it's a challenge that designers need to address today.
The publication of CIBSE TM59:2026 marks the first major update to the industry's recognised methodology for assessing overheating in homes since the original guidance was released in 2017. Drawing on almost a decade of industry experience, academic research and evolving climate data, the updated guidance refines how overheating is assessed and places even greater emphasis on designing resilient homes from the outset. That emphasis aligns closely with Approved Document O, introduced in June 2022, which brought overheating into the Building Regulations for the first time and made it a requirement for new homes to limit unwanted solar gains and provide an adequate means of removing excess heat.
For developers, architects and design teams, understanding these changes now will help ensure projects are aligned with current best practice and reduce the likelihood of design changes later in the process.
Why has TM59 been updated?
Since TM59 was first published, both the regulatory landscape and our understanding of overheating have evolved considerably.
The introduction of Part O of the Building Regulations, alongside growing evidence of overheating in homes and increasing recognition of future climate risks, has highlighted the need for a more robust and consistent assessment methodology. TM59:2026 responds by incorporating updated research, revised climate data and a clearer framework for assessing overheating risk.
Key changes in TM59:2026
While many of the core principles remain familiar, the updated guidance introduces several important changes.
A new three-stage assessment strategy
Perhaps the most significant change is the introduction of a structured three-stage approach. This encourages project teams to demonstrate that passive design measures have been fully explored before mechanical cooling is considered, reinforcing the principle that good building design should be the first line of defence against overheating.
Updated weather data
Assessments now use the latest future climate weather files, providing greater geographical accuracy and reflecting the conditions buildings are expected to experience over their lifetime. This helps ensure developments are designed for tomorrow's climate, rather than yesterday's.
Revised bedroom assessment criteria
The methodology for assessing bedrooms has been updated following research into sleep quality and thermal comfort. Rather than focusing solely on hours above a temperature threshold, the revised approach considers overheating during summer nights, recognising the importance of comfortable sleeping environments.
Recognition of ceiling fans
TM59:2026 formally recognises ceiling fans as an effective passive comfort measure in appropriate circumstances. This supports the wider emphasis on reducing reliance on energy-intensive cooling systems wherever possible.
Updated occupancy assumptions and communal spaces
The guidance also reflects changing patterns of home occupancy, including home working, and introduces additional guidance for assessing communal spaces where relevant.
What does this mean for residential projects?
For many developments, the overall design philosophy will remain unchanged. However, the updated methodology reinforces the importance of making informed design decisions early in the project.
The revised bedroom criteria, updated occupancy assumptions, allowance for ceiling fans and new weather data will all influence how a design performs in the assessment, so they need to be considered from the outset rather than treated as late-stage checks. Doing so during the concept and developed design stages gives teams greater flexibility to refine designs before they become fixed.
The updated guidance also places greater emphasis on clearly justified modelling assumptions, helping to improve consistency and transparency across overheating assessments.
Designing for a warmer future
One of the strongest messages within TM59:2026 is that overheating should not be viewed as a standalone compliance exercise.
By considering overheating alongside daylight, ventilation, energy performance and other environmental factors from the earliest stages of design, project teams are better placed to develop homes that are comfortable, resilient and energy efficient throughout their lifetime.
Rather than relying on mechanical cooling to address problems later, the updated guidance encourages a design-led approach that prioritises passive measures wherever practical.
Looking ahead
TM59:2026 reflects the industry's growing understanding of how residential buildings perform in a changing climate. While it introduces several important updates to the assessment methodology, its broader message is one of resilience: homes designed today should remain comfortable for decades to come.
For developers and design teams, the updated guidance is an opportunity to embed overheating considerations earlier in the design process, improve coordination across disciplines and create homes that are better prepared for the future.