CIBSE TM59 is the industry-standard dynamic thermal modelling methodology used to assess overheating risk in new homes. If you’re delivering residential development in the UK, there’s a good chance you’ll need it – or something like it – to satisfy Building Regulations.
Updated July 2026: CIBSE has published a major revision, TM59:2026, introducing new climate data and assessment criteria. Read our full breakdown of what’s changed.
In this guide you’ll learn:
- What a CIBSE TM59 Overheating Assessment is and how it works
- The advantages compared to simpler assessment methods
- Whether you need TM59 for your project
- What’s changed with the introduction of TM59:2026
What is a CIBSE TM59 Overheating Assessment?
Think of it as a computer simulation of your home before it’s built. TM59 uses dynamic thermal modelling software to analyse how a dwelling will respond to different weather conditions across a full year, identifying where and when overheating risk is likely to occur – long before construction starts.
Unlike simpler, static assessment methods, TM59 accounts for:
- Solar gain – how much heat a home absorbs through its windows
- Building orientation – how the sun’s path affects different elevations across the day
- Internal heat sources – appliances, occupants and other heat generated inside the home
- Site-specific conditions – TM59 uses weather files matched to the building’s actual location, factoring in local shading from surrounding buildings and landscaping
This gives a far more realistic picture of thermal performance than assessments based on generic assumptions.
Do I Need a CIBSE TM59 Assessment?
All new dwellings require an overheating assessment under Approved Document O of the Building Regulations. TM59 is one recognised route to demonstrating compliance, but it isn’t automatically mandatory for every scheme – a simplified method may be sufficient for lower-risk designs.
TM59 is likely the right approach – or may be specifically required – where:
- The design is complex. Large areas of glazing, open-plan layouts, or significant internal heat sources all increase overheating risk, and TM59 assesses these far more precisely than simplified methods.
- Your local authority requires it. Some planning authorities ask for TM59 as standard on residential schemes, regardless of what the simplified method would technically permit – always check with local building control.
- The simplified method has real limitations for your scheme. In particular, it doesn’t account well for:
- Glazing g-values – the solar heat gain coefficient of your windows, which TM59 factors into the modelling directly
- Shading nuances – the real-world effectiveness of overhangs, window reveals, and shading from neighbouring buildings
We saw this play out on a recent development in Somerset, where a bungalow failed the simplified assessment purely because security concerns limited night-time window opening – despite the design itself effectively managing overheating risk. See how dynamic thermal modelling resolved it.
Not sure if you need TM59 for your project? Our experts can advise – get in touch.
Why It’s Worth Doing Properly
- Comfort. Catching overheating risk at design stage means it can actually be designed out, rather than discovered – and lived with – after completion.
- Energy efficiency. Reducing overheating risk through passive design lowers reliance on mechanical cooling and cuts running costs.
- Compliance assurance. A robust assessment avoids the delays and redesign costs that come with failing to meet Part O.
- Future-proofing. With UK summers trending hotter and more frequent heatwaves becoming the norm, an assessment that only just scrapes compliance today may not hold up over a building’s lifetime.
TM59:2017 vs TM59:2026
Since this guide was first written, CIBSE has published a major revision to the methodology. Approved Document O currently still references TM59:2017, so that remains the baseline compliance route for most projects today. However, TM59:2026 introduces significant changes – including future climate data based on 2050s weather scenarios, a revised bedroom overheating criterion, a new assessment for communal spaces, and a staged, passive-first framework – and is expected to be adopted into the regulatory framework in due course.
If you’re starting a new project now, it’s worth understanding both versions and discussing with your building control team which is appropriate. Read our full breakdown of TM59:2026 and what’s changed.
What’s Next?
A CIBSE TM59 Overheating Assessment is one of the most reliable ways to ensure a new home is comfortable, energy-efficient and compliant – both today and as the UK’s climate continues to shift. Whether TM59:2017 or TM59:2026 is the right route for your project will depend on your local authority and design stage, so it’s worth talking it through early.
We offer a full range of overheating assessments, including CIBSE TM59 under both the current and updated methodology. Get in touch to discuss your project and whether TM59 is the right fit.