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Building Regulations Part O


Reading time: 8 minutes


What is Part O of the Building Regulations?

Building Regulations Part O (also known as Approved Document O) relates to overheating, and how to design new properties to protect users from becoming overheated. As well as limiting ‘solar gain’ (heat from the sun’s rays), developers have to consider ventilation and ways to remove excess heat from indoor areas. 

The Part O Building Regs (Overheating) took effect from 15th June 2022, and only covers new residential properties in England. 


The Part O Building Regs is divided into two main sections:

  • Requirement O1: Overheating Mitigation

  • Regulations: 40B (providing information on the above)

When looking at ways to keep buildings cool and well ventilated, Building Regs Part O says that the following must also be considered:


  • Noise at night

  • Pollution

  • Security

  • Protection from falling

  • Protection from entrapment


Building Regs Part O overlaps with several of the other Approved Documents, particularly Part F (ventilation) and Part L (glazing).


What types of buildings are covered under Part O?

Part O only relates to residential dwellings, including:

  • Houses

  • Flats

  • Other places where people sleep on the premises, such as boarding schools, halls of residence, care homes or other living accommodation for adults or children


The Part O Building Regs does not cover extensions or renovations. It applies only to new residential properties built after 15/6/2022. Building Regs Part O does not normally apply to change of use for buildings. However, it may apply if you are changing the building's energy status. Your local Building Control Team can help. Please note that conservatories added to existing buildings are not covered under Part O, but that conservatories on newly built homes do come under the regulations.


What is Requirement 01 Overheating Mitigation?

Requirement O1 Overheating Mitigation of the Building Regs Part O says that homes, or other buildings used for residential purposes, must make reasonable provision to: 

  • Limit unwanted solar gains in summer

  • Provide ways to remove heat from indoors


Building Regulation Part O gives detailed advice on how to meet Requirement 01, as well as stating:

  • Account must be taken of occupant safety and their enjoyment of the home

  • Mechanical cooling may only be used where insufficient heat is capable of being removed from the indoor environment without it


Regulation 40B states that information must be provided to show that Part O has been followed:

  • ‘The person carrying out the work must, not later than five days after the work has been completed, give sufficient information to the owner about the provision made in accordance with Part O so that the systems in place can be operated in such a manner as to protect against overheating’


How is Part O calculated?

There are two ways to calculate the building’s overheating risk. In Building Regulation Part O Section 1 there is a Simplified Version, with a chart to help limit solar gain (looking at the area of the UK and whether this makes it moderate or high risk, the % of glazing to floor area, whether there’s cross ventilation and whether it’s north or south facing). 


Here, the regulations state that the building’s glazing must have a maximum G-Value of 0.4 and a minimum light transmittance of 0.7. The G-value (also known as the G-factor or ‘total solar energy transmittance’) is a measurement of how much heat from the sun’s rays is transmitted through a window. This information should be available from your glazing supplier.


Section 2 covers ‘dynamic thermal modelling’, which is an alternative and more in-depth way to calculate the risk of a building overheating. The following guidance should be followed:

  • CIBSE’s TM59 methodology for predicting overheating risk

  • The limits on the use of CIBSE’s TM59 methodology (in Part O paragraphs 2.5 and 2.6)

  • The acceptable strategies for reducing overheating risk (in Part O paragraphs 2.7 to 2.11)


Should I use simplified method or dynamic thermal modelling?

Developers can choose to use the more straightforward Simplified Method, or dynamic thermal modelling which gives more flexibility when it comes to design. The Simplified Method to calculate a building’s risk of overheating is usually a more popular approach for standard dwellings, as dynamic thermal modelling can be more costly. 

Dynamic thermal modelling is ideal for properties with large areas of glazing, or if the design already includes mitigation measures such as sun shades. The Simplified Method is allowed to be used for high or moderate risk areas, but it’s not suitable for multi-residential properties that share a communal heating or hot water system with a certain amount of pipework - so the dynamic thermal modelling system will be needed in some cases.


Disclaimer: Information displayed in this article is correct at the time of publication, but note that legislation changes periodically. Please refer to the latest publication of each approved article. The information contained on this page is intended as an overall introduction and is not intended as advice from a professional building control officer. The definition of ‘building work’ and when Building Regs approval is required is set out here. Travis Perkins aims to avoid, but accepts no liability, in the case that any information stated is out of date. Always consult the approved local authority building control team when considering any exemptions, and before undertaking any work.