TM59 Overheating Assessment Requirements if you’re designing or building homes today, you can’t afford to ignore overheating.

Rising summer temperatures and tighter building rules mean one thing: homes must be cooler, smarter, and more resilient. where the TM59 Overheating Assessment comes in. It’s more than a box-ticking exercise. It’s your blueprint for keeping buildings habitable and compliant especially in the age of climate change.

Let’s explore what TM59 overheating assessment requirements are, how it works, and why developers, architects and contractors across the UK are taking TM59 seriously.

What is TM59?

Developed by CIBSE (Chartered Institution of Building Services Engineers), TM59 is a standardised method for measuring how likely a new or refurbished residential building is to overheat in hot conditions.

Will it get too hot to live in?
If the answer’s yes, planners and regulators will want solutions before you build.

It uses dynamic simulation modelling to predict internal temperatures across various spaces in a home. And it’s becoming increasingly important in meeting Building Regulations Part O and local planning conditions.

What Are the TM59 Requirements?

Let’s break it down.

1. You Need a Dynamic Simulation Model

A TM59 assessment must be done using specialist software that simulates:

• Hourly internal temperatures
  
  
• Summer weather patterns (e.g. DSY1 2020 London)
  
  
• Occupancy and usage patterns
  
  
• Internal heat gains from appliances, people and lighting
  
  

2. You Must Pass These Temperature Limits

Two main thresholds must be met:

• Living rooms: Must not exceed 26°C for more than 3% of occupied hours
  
  
• Bedrooms: Must not exceed 26°C for more than 1% of night hours (10pm–7am)
  
  

These thresholds are based on adaptive comfort, not fixed targets.

If your model exceeds these? You’ll need to revise the design.

3. You Must Account for Ventilation and Shading

A key part of TM59 is showing how your building deals with heat. That includes:

• Window sizes and how they open
  
  
• Natural ventilation or mechanical systems
  
  
• Night-time purge ventilation
  
  
• Use of blinds or solar shading
  
  
• Building orientation and external obstructions
  
  

It’s not enough to say “the windows open.” You have to prove they make a difference.

4. Applies to These Residential Types

You’ll need a TM59 assessment for:

• Apartments or flats
  
  
• Care homes
  
  
• Student accommodation
  
  
• High-density housing
  
  
• Urban developments with limited airflow
  
  
• Refurbishments where natural ventilation is limited
  
  

If it’s a home and it’s likely to trap heat you’ll need this.

5. Your Final Report Must Be Detailed

The report should include:

• Room-by-room comfort results
  
  
• Floor plans and zones
  
  
• Weather file references
  
  
• Internal gains assumptions
  
  
• Explanation of mitigation measures used (e.g. solar shading)
  
  
• Summary of whether thresholds are met or exceeded
  
  

Your local authority, planning officer or funder might request this as part of your submission.

Why Is It So Important?

A 2023 report by the UK Climate Risk Assessment warned that heat-related deaths could triple by 2050 if buildings don’t adapt.

Older adults, babies, and people with pre-existing conditions are most at risk.

But even beyond the human impact, non-compliance can cause:

• Planning delays or refusals
  
  
• Expensive redesigns
  
  
• Unhappy occupants or poor sales
  
  
• Long-term complaints and liability
  
  

By getting TM59 right, you’re not just following rules you’re protecting your people, your project, and your reputation.

How Gradwell Group Helps?

At Gradwell Group, we work with developers, architects and contractors to deliver accurate, planning-ready TM59 reports.

What you get:

• Expert thermal modellers
  
  
• Guidance on design tweaks for compliance
  
  
• Fast turnaround and clear reporting
  
  
• Help with planning submissions and sustainability goals
  
  

Explore our TM59 Overheating Assessment Services

FAQs

1: Is TM59 a legal requirement?

A: It’s not in law, but it’s often required to demonstrate compliance with Part O of Building Regulations.

2: How long does a TM59 report take?

A: Depending on the complexity, usually between 5–10 working days.

3: What if my building fails the assessment?

A: Most failures can be resolved by improving ventilation, reducing glazing, or adding shading strategies.

4: Can mechanical cooling help me pass TM59?

A: It’s allowed, but passive measures must always be prioritised.

Final Thoughts

Overheating isn’t just about comfort it’s about safety, sustainability, and standards.
TM59, you have a clear, science-based method to predict problems before they happen. And with the right support, passing doesn’t have to be painful.

Need help with TM59? Contact Gradwell Group today we’ll help you keep your building cool, compliant and future-proof.

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