TM59 is the industry-standard dynamic thermal modelling methodology used to assess overheating risk in new homes, published by CIBSE. If you’re delivering residential development in the UK, you might need 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 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 TM59 Overheating Assessment?
Think of it as a computer simulation of your home before it’s built. An TM59 overheating assessment 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.
What TM59 Accounts For That the Simplified Method Doesn’t
Unlike the simplified method, 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 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. Here’s how dynamic thermal modelling resolved it.
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 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 risk assessments, including TM59 under both the current and updated methodology. Get in touch to discuss your project and whether TM59 is the right fit.
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