Complying with Approved Document O - Taking the heat out of building regs compliance

New Building Regulations and uplifts to Part L and F have come into force in England impacting the design and delivery of new non-domestic buildings as well as homes and existing homes.

Announced in 2021 as an interim step toward the government's Future Buildings Standard, due in 2025, these new regulations change how the design of both domestic and non-domestic buildings will be shaped in the future. The Department for Levelling Up, Housing and Communities (DHLUC) believes the new regulations will help the UK to meet its net zero target by 2050.

The interim updates to Approved Document L and F are no surprise but the recently introduced Approved Document O means developers must also comply with new requirements intended to prevent domestic dwellings and residential buildings (such as care homes and student accommodation) from overheating in warmer months.

Here, Associate Director and Head of Building Performance, Philip O’Loughlin, summarises Approved Document O and looks at what developers can do to comply with these latest requirements.  

What is Approved Document O?

Previously, regulation has focused on ensuring dwellings, or other buildings containing rooms for residential purposes, curtail avoidable heat losses in the winter months. Approved Document O addresses the flip side, looking at how we remove excess heat to prevent our dwellings from overheating in warmer months. Overheating is a growing problem in towns and cities where ambient temperatures can be significantly higher. The issue looks set to worsen in the face of global warming.

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How to demonstrate compliance

To secure Building Regulation compliance for a development that includes domestic accommodation, developers will need to meet the requirements laid out within Approved Document O. Compliance can be shown using one of two methods, the Simplified Method and the Dynamic Thermal Modelling Method.

Which one you chose will depend on the type of development and the resources available to the developer.

1. The Simplified Method

The quicker and easier, simplified method, focuses on glazing and free areas, limiting solar gains and maximising natural ventilation potential.

Before applying this method, a development's level of risk (high or moderate) must be determined. High-risk areas include parts of central London and central Manchester, a full list of postcodes can be found here in appendix C. All remaining areas in England, not considered high-risk, are considered moderate-risk locations.

1.1 Maximum Glazing Areas

Allowing large quantities of glazing increases the potential heat gain into the building as windows and doors are susceptible to much higher radiant gains than walls or other opaque surfaces. The Approved Document, therefore, provides guidance on maximum glazing areas based on orientation to limit these solar gains. 


1.2 Minimum Opening Areas

The free area provided to openings and ventilation routes allows the natural cooling effect of the external ambient air to percolate into the building and mitigate overheating by displacing warmer internal air. Approved Document O specifies prescribed free areas to be used to remove excess heat for this purpose.  

Pros and cons of the Simplified Approach


  • Cheaper to assess
  • No specialist software required
  • No experienced modeller needed


  • Can’t be used in residential buildings that contain more than one unit 
  • Restricted solutions
  • Can’t be used in residential buildings with a significant amount of horizontal heating or hot water distribution pipe work

2. Dynamic Thermal Modelling Method

The Dynamic Thermal Model must undergo and satisfy a CIBSE TM59 overheating assessment whilst also allowing for some limits set out within the Approved Document (AD).

Specialist modelling software and an experienced modeller will be required for this approach but gives developers greater flexibility by adopting a robust engineered solution.

This method involves the creation of a model of the development, using dynamic thermal modelling software and using the standard set of data inputs derived from TM59. Based on these inputs and using climate modelling from the appropriate CIBSE 2020s Design Summer Year (representative of 2010-2040), the model can be used to demonstrate compliance with the AD.

Pros and cons of the Dynamic Thermal Modelling method


  • More design flexibility
  • Engineered solution to meet the multi-faceted requirements of the project
  • Only a sample of units need to be assessed
  • Greater likelihood of a satisfactory compliant solution


  • More costly
  • Involves more parties

What factors can reduce overheating?

When designing to mitigate overheating, the following factors can be beneficial:

  • Windows and openings

Ventilation provided through windows and openings is one of key measures for mitigating overheating. But a number of factors need to be considered when designing windows and openings such as noise, security and safety.

  • Blue/green infrastructure

Local blue/green infrastructure such as parks, landscaping, water features and rivers help to reduce external air temperature which will have a knock-on effect for development in the immediate area.

  • Tall buildings/trees – shading

Is the development surrounded by tall trees and/or buildings? If so, it may benefit from the shading these provide depending on its location in relation to sun path.

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