Part O

What is Part O?

Part O of the Building Regulations, introduced in 2022, focuses on overheating mitigation in new residential buildings across England. Its purpose is to ensure that homes are designed and constructed to limit unwanted solar gains and provide adequate means of removing excess heat, thereby improving indoor comfort and energy efficiency. The regulation applies to dwellings and residential units, including care homes and student accommodation. Compliance with Part O requires careful consideration of factors such as window size, glazing type, shading, ventilation, and building orientation. By meeting the standards set out in Part O, developers and designers can create buildings that stay cool in summer, reduce reliance on air conditioning, and enhance overall occupant wellbeing.

With rising temperatures, overheating in homes is becoming a serious health risk — especially for the elderly, children, and those with health conditions. Part O helps ensure homes are built to be safe, healthy, and resilient to future climate conditions.

Routes to compliance

There’s currently two routes to compliance. The Simplified Method is the more straightforward and cost effective way of achieving compliance however, has a set of prescriptive design criteria which can be hard to achieve. The TM59 Dynamic Thermal Model is a more detailed assessment based on the CIBSE TM59 model which can allow for more detailed and flexible mitigation. 

Simplified Method

This method analyzes the ratios of floor area and glazed areas and equivalent areas. Other factors such as location (London or not) and whether cross-ventilation is possible are also taken into account however, the Simplified Method does have limitations and can prove to be tricky to pass.

Mitigation for the simplified method comes down to either increasing the amount of natural ventilation through more or bigger openings, and/or reducing glazed to overglazed rooms. However, these strategies may be limited by planning constraints or design preferences, which is why many projects default to a TM59 assessment.

Dynamic Thermal Model (TM59)

This method utilises the CIBSE TM59:2017 Method to provide a more in depth assessment of the properties. The requirements are based on ensuring the living rooms and bedrooms don’t exceed certain temperatures for set periods of time. These assessments look at a 3D model of the house, taking into account the fabric elements, most importantly the window specifications, M&E spec,  more precise weather conditions and occupancy profiles.

If a room doesn’t meet the requirements under the TM59 assessment, mitigation would be needed. Sometimes this may be extra ventilation, sometimes this may be changing the window specification. There are also other routes to compliance, so we work closely with design teams to explore all viable options and ensure compliance is achieved in a practical and cost-effective way. Once compliance is met, we issue a formal Part O Overheating Report, which can be submitted to Building Control as part of your regulatory documentation.

​Implications for Developers and Designers

Part O requires developers and designers to consider overheating at an early stage in the design process. This includes early design decisions and allowing time for potential design changes such as additional shading, less glazing, more openings or enhanced ventilation.