Approved Document O (Part O) of the Building Regulations was introduced in June 2022 and applies to new residential buildings in England. It aims to limit unwanted solar gains and ensure adequate means of removing excess heat, thereby reducing the risk of overheating in dwellings, especially in the context of climate change.

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Scope and Applicability

Part O applies to:

• New residential buildings, including:

  • Houses

  • Flats

  • Student accommodation

  • Care homes

  • HMOs (Houses in Multiple Occupation)

• It does not apply to:

  • Extensions to existing dwellings

  • Non-residential buildings

It is applicable at the design and construction stage and must be addressed to obtain Building Control approval.

 

Objectives of Part O

1. Reduce Solar Gains:

   • Limit internal temperature rise through control of window size, orientation, and shading.

2. Promote Effective Ventilation:

   • Ensure that dwellings can remove excess heat through natural or mechanical means.

 

Simplified Method

This is a prescriptive approach, suitable for most standard residential designs.

Key Requirements:

• Maximum glazed area: Limits based on orientation and room type.

• Minimum openable area: Proportion of openings required for ventilation.

• Cross ventilation: Required in certain layouts (e.g., single-aspect flats).

• Solar shading: External shading or recessed windows in certain orientations.

 

Dynamic Thermal Modelling (TM59) Method

This is a more flexible, performance-based route using CIBSE TM59 methodology.

Key Requirements:

• Modelled using dynamic simulation software (e.g., IES VE, DesignBuilder).

• Dwellings must pass both TM59 criteria:

  • Limit on hours above 26°C (living spaces)

  • Maximum allowable peak temperature (bedrooms)

Required When:

• The simplified method cannot be met.

• The design involves complex geometry, extensive glazing, or mechanical cooling systems.

• Located in high-risk zones with challenging layouts (e.g., single-aspect, high-rise flats).