Part O - Overheating: What UK Developers Need to Know

Why Overheating Matters

Overheating in homes is no longer a future concern. With rising summer temperatures and increasingly airtight, well-insulated dwellings, the risk of overheating has become a major health and comfort issue. To address this, Approved Document Part O of the Building Regulations was introduced in June 2022, setting out requirements for new residential buildings to limit overheating.

But what does Part O actually mean in practice, and how can developers, architects, and housebuilders ensure compliance without compromising design?

What is Approved Document Part O?

Part O focuses specifically on reducing overheating risk in new dwellings. It requires all new homes in England to be designed so that internal temperatures remain within safe limits.

Unlike Part L (energy) or Part F (ventilation), Part O is not about fuel use or indoor air quality, it’s about preventing health risks from excessively high internal temperatures.

Two Routes to Compliance

1. Simplified Method

• Based on prescriptive rules: maximum glazed area-to-floor ratios, orientation limits, and minimum window opening sizes.
• Works well for conventional housing with moderate glazing.
• Cannot take account of design innovations such as overhangs, balconies, or external shading.
• Quick and cost-effective, but limited flexibility.

2. Dynamic Thermal Modelling (CIBSE TM59)

• Uses advanced computer modelling to simulate internal temperatures with real weather data.
• Suitable for complex, highly glazed, or urban developments.
• Allows more design flexibility: shading devices, glazing g-values, and passive strategies can be tested and validated.
• Requires specialist software such as IES VE or DesignBuilder.

Rule of thumb: If glazing ratios exceed the simplified thresholds or if there are constraints on window openings (due to noise, pollution, or security), then TM59 dynamic modelling will be required.

Design Considerations Under Part O

When designing for compliance, a number of practical issues must be factored in:

• Noise – Bedrooms must remain habitable with windows open at night. High background noise may force closed-window strategies.
• Pollution – In urban areas, external air quality may restrict reliance on natural ventilation.
• Security – Ground floor and easily accessible bedrooms must ensure safe window operation (restrictors, lockable louvres).
• Safety – Openings must comply with fall protection requirements.
• Shading – External shading (louvers, overhangs, shutters) is more effective than internal blinds, which cannot be used for compliance.

Part O vs Part L: Friends or Foes?

Part L prioritises energy efficiency, airtightness, and heat retention. Part O focuses on removing heat in summer. At first glance, they seem to conflict.

• Glazing with low g-values helps reduce overheating (Part O), but can negatively affect winter heating demand (Part L).
• Airtight envelopes improve efficiency (Part L), but increase overheating risk (Part O).

In reality, they must be considered together. Integrated design – balancing orientation, shading, glazing specifications, and ventilation – is the only way to achieve compliance without costly retrofits.

Frequently Asked Questions (FAQ)

What is Part O of the Building Regulations?
It’s the section that sets out requirements to prevent overheating in new residential buildings.

When should an overheating assessment be carried out?
As early as possible in the design RIBA Stage 3/4. Waiting until post-planning limits options, as elevations and layouts are usually fixed.

Does Part O apply to extensions or change of use projects?
No. Part O applies only to new residential dwellings, not extensions, conservatories, or conversions.

What’s the difference between the Simplified Method and Dynamic Thermal Modelling?

• Simplified = prescriptive checklist (window sizes, orientation, glazing ratios).
• Dynamic = advanced modelling (CIBSE TM59), more flexible, suitable for complex or high-risk buildings.

When must dynamic modelling (TM59) be used?

• If glazing ratios exceed simplified thresholds.
• If noise or pollution prevent windows from being relied on.
• If multiple dwellings share heating/hot water distribution.
• If passive measures need validating (e.g. shading, special glazing).

What information is needed for an assessment?

• Site and floor plans (orientation, layouts).
• Elevations and sections.
• Window and door schedules (sizes, openings, g-values).
• U-values and ventilation details.
• Any proposed shading devices.

Does MVHR help with overheating?
Not directly. MVHR is designed for winter heat recovery and indoor air quality. Boost functions can help with airflow, but if configured as active cooling, it falls outside Part O.

Why are bungalows particularly challenging?
All bedrooms are “easily accessible,” meaning open windows at night raise security concerns. This often requires additional shading or alternative ventilation solutions.

What types of homes are most at risk?

• High-rise flats (stack effect, greater solar exposure).
• Homes without cross ventilation.
• Highly glazed or west-facing dwellings.

Final Thoughts

Part O is not just another box to tick – it’s a critical regulation that directly impacts occupant comfort, health, and building usability. Approached early in design, it can be an opportunity to shape better, more resilient homes.

At Energy Report we help developers, architects, and housebuilders achieve compliance while supporting good design. Whether through simplified assessments or detailed TM59 modelling, we provide practical solutions that balance regulation, comfort, and cost.