Part O Building Regulations: A Complete Overheating Compliance Guide

The Part O Building Regulations – sometimes referred to simply as the Part O building regs, the building regulations Part O or building regs Part O – tackle overheating in new dwellings, ensuring comfortable living spaces year-round. As UK summers get hotter, keeping a new home cool has become as important a design consideration as keeping it warm in winter.

In this blog, we’ll cover:

• Understanding overheating and its impact on new builds
• The requirements for complying with the Part O Building Regulations
• Choosing the right compliance route for your project (Simplified vs. Dynamic Thermal Modelling)
• The benefits of achieving thermal comfort in your new home
• Our insights and recommendations for navigating Part O compliance

Part O Building Regulations: The Problem of Overheating

Overheating is a serious problem in the UK, with some 1.9 million households reported to be experiencing it (English House Survey 2020-21).

While insulation and draught-proofing remain important for keeping buildings warm in winter, the focus needs to shift towards summer comfort too. As the effects of climate change become more apparent – with warmer summers and more frequent heatwaves – ensuring that homes are designed and built to prevent overheating is becoming increasingly important.

To address this growing concern, the Part O Building Regulations mandate that new homes are designed and built to prevent overheating, particularly since new dwellings are increasingly built with lightweight materials such as timber frames, which lack thermal mass.

A report by the Climate Change Committee highlights the health risks of overheating, including heatstroke, dehydration, and respiratory problems. It can also disrupt sleep and concentration – a significant concern for the growing number of people working from home.

The Part O Building Regulations

The Part O Building Regulations (introduced in 2010, with Approved Document O published 15 December 2021 and last updated 15 June 2022) focus on the risk of overheating in new dwellings. These regulations aim to ensure comfortable living spaces year-round and prevent overheating by addressing excessive heat gain during summer months. They require that buildings are designed and constructed to reduce solar gain in summer and to provide adequate means of removing heat from the indoor environment.

The Part O Regulations set out a number of requirements for new dwellings to reduce the risk of overheating. These include but are not limited to:

• Limiting the amount of solar gain in summer: Using shading like awnings or trees, or glazing with a low solar gain coefficient.
• Providing adequate ventilation: Natural ventilation through windows, or by mechanical ventilation.
• Using thermal insulation: Helping keep the heat out in summer.
• Installing cooling systems: Cooling systems should only be used as a last resort – they can be expensive to run.

How to Assess Overheating Risk

The Building Regulations provide guidance for assessing overheating risk in a particular dwelling. This overheating calculation assessment takes into account factors like the location of the dwelling, its orientation, the type of glazing, and the amount of shading.

Approved Document Part O 2021 mandates evaluating rooms individually, rather than using a previously-standard dwelling average, and it provides two methods for assessing overheating risk in dwellings:

1. The Simplified Route
2. The Dynamic Thermal Modelling Route

The Simplified Route

The Simplified Method is suitable for moderate and high overheating risk areas, with or without cross ventilation. This method is ideal for dwellings located outside of large cities and for traditional housing types. It focuses on design criteria like maximum glazed window areas and minimum ventilation openings. While no modelling is required, detailed measurements and window design are crucial.

Benefits: Quicker and easier to implement for simpler projects.

Where a design leans on large or complex glazing, though, the Simplified Method’s blanket assumptions can fail it outright – regardless of how well the shading, orientation or ventilation strategy actually performs.

Building Compliance Part O Overheating Calculations

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The Dynamic Thermal Modelling Route

This more detailed approach involves modelling the dwelling in an approved software tool, such as DesignBuilder. It uses comfort-based criteria drawn from CIBSE TM59, and offers greater design flexibility. This route is necessary when:

• Night-time noise limits or pollution prevent natural ventilation.
• Openings pose a security risk (e.g. ground floor bedrooms).
• The scheme has long corridors, long hot water pipes or is located within a large city (especially London).

Benefits: More flexibility for complex designs and challenging solutions.

We saw the security-risk scenario play out directly on a recent development in Somerset, where a bungalow failed the Simplified Method purely because security concerns limited night-time window opening – despite the design itself effectively managing overheating risk.

Building Compliance Part O Overheating Calculations

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CIBSE has since published a major revision to this methodology. Approved Document O currently still references TM59:2017 as the baseline for the Dynamic Thermal Modelling Route, but TM59:2026 introduces significant changes – including future climate data, a revised bedroom overheating criterion, and a staged, passive-first framework – and is expected to be adopted into the regulatory framework in due course. If you’re weighing up which version applies to your project, our guides on what TM59 is and whether you need it and what’s changed under TM59:2026 cover both versions in full.

Is Overheating Compliance Mandatory in the UK?

Yes. Part O of the Building Regulations mandates that new dwellings containing one or more rooms for residential purposes must:

• Limit unwanted solar gain in summer.
• Provide adequate means to remove heat from the indoor environment.

This ensures comfortable and healthy living spaces year-round. Anyone constructing a new dwelling must demonstrate compliance with these overheating requirements to building control bodies, typically by submitting an Overheating Assessment report detailing the steps taken to mitigate overheating risk. Failure to comply can result in penalties such as fines and enforcement notices.

Choosing the Right Assessment Route

Both the Simplified and Dynamic Thermal Modelling routes have specific criteria to help identify the right approach for compliance. These criteria primarily focus on:

• Noise: Night-time noise levels that might restrict opening windows for ventilation.
• Security: Concerns about security for openings in vulnerable locations like ground-floor bedrooms.
• Safety: General safety considerations related to Part K of the Building Regulations, such as protection from falls.
• Pollution: Air pollution levels that might make relying solely on natural ventilation undesirable.

What’s Next?

Whether the Simplified Method is sufficient for your scheme, or you’re dealing with the kind of noise, security or density constraints that call for Dynamic Thermal Modelling, getting the right assessment route identified early avoids costly redesigns later.

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Part O Overheating Calculations

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Our team are experienced in providing overheating assessments using both the Simplified Methodology and Dynamic Thermal Modelling – get in touch to discuss your specific project requirements.