Part O became a legal requirement in England on 15 June 2022. There was a transitional period for applications submitted before this date.

Part O applies to new residential buildings, including houses, flats, and certain types of student accommodation. It does not generally apply to extensions to existing dwellings unless they significantly alter the thermal performance of the entire dwelling.

Any building with no security, noise or pollution issues can be assessed with either method. Simplified assessments can prove to be harder to pass so choosing to opt for a TM59 may be preferable.

If bedroom windows are impacted by security, noise or pollution issues, there may be a requirement for these windows to be assessed as closed during the Part O assessment meaning they require a TM59 assessment. Ground floor bedroom windows, bedroom windows above a flat roof, windows which are affected by noise sources such as roads or bedroom windows affected by pollution sources such as factories are all examples of where the windows would have to be modelled as closed during the assessment.

The implementation of Part O may involve some additional costs for certain design elements (e.g., external shading, higher-performance glazing). However, these costs can often be offset by the long-term benefits of more comfortable and healthier homes, and potentially reduced energy consumption for cooling.

The official guidance for Part O can be found in Approved Document O (Overheating), which is available on the UK government's website. You may also find guidance from industry bodies and professional organisations.