Part O of the Building Regulations came into force on 15 June 2022, with the aim of ensuring that new-build dwellings can meet the future challenges of higher summer temperatures without reliance on mechanical cooling systems. The historic focus on increased levels of insulation and air tightness has created concerns on how dwellings will adapt to future climate conditions, and this aspect of design has been overlooked previously.

To ensure compliance, design amendments might include changes to:

  • The size or quantity of windows
  • The extent of the openable area of windows
  • Layout, to ensure cross-ventilation
  • Specification of glass (solar control)

Or the addition of:

  • External shading devices
  • Mechanical ventilation
  • Acoustic panels (where external noise may be an issue)
  • Window grilles or railings

Where Part O gets complicated

The simplified calculation methodology sets maximum glazing areas for both the largest glazed elevation and the largest glazed individual room, as well as minimum ventilation provisions for the whole dwelling and each individual bedroom. These are likely to be achievable for most standard developments, but there can be a number of specific categories where further detailed dynamic modelling could be required to determine compliance:

  • Where the site is in a higher risk area, such as some postcodes within Greater London
  • Where the largest glazed elevation is facing due West
  • Where secure night-time ventilation cannot be provided, such as for ground floor bedrooms
  • Where effective cross ventilation cannot be provided, such as single aspect dwellings
  • Where there is insufficient ventilation provision due to design and constraints, such as using sash windows or requiring fixed-shut fire-rated windows

Dynamic modelling can take into account external shading from other buildings, or building mounted shading devices, and can also model solar control glazing and mechanical ventilation systems for compliance.

Building Regulations Part O in practice

It is clear from working on multiple projects that Part O is requiring a greater level of design input that previously thought, with dynamic modelling becoming commonplace on even standard developments. However, dynamic modelling is also helping to provide design flexibility on projects, and highlighting those projects where summer overheating has not been adequately reviewed.

We have also noticed that there have been some differences in interpretation between Building Control Officers, particularly when assessing secure ventilation of ground floor or accessible windows. Greater consistency is still needed within the industry when it comes to the application of Building Regulations Part O.

Of course, even the most effective design can only combat overheating if it is properly implemented, and once a property is occupied there is no guarantee that this will happen. Therefore, another key element of Part O is the issue of plot-specific guidance to occupants to teach them how to manage overheating risks within their dwelling, and we hope that this forms a key part of the handover pack – this understanding is critical, for Part O to have the positive impact on wellbeing that is intended.

To find out more about how we can advise on compliance for Part O of the Building Regulations, call us on 01206 266755 or email