As UK summers become hotter and heatwaves more frequent, designing homes that stay comfortable without relying on air conditioning has become increasingly important.
The Government introduced Part O of the Building Regulations in June 2022. Rather than focusing on keeping heat in—as many Building Regulations do—Part O aims to prevent homes from overheating during warmer weather.
If you’re building a new home or converting a commercial building into residential accommodation, understanding Part O is essential. This guide explains what Part O is, when it applies, how compliance is achieved and the practical design strategies that can help keep your home cool for years to come.
What Is Part O?
Part O (Overheating) is part of the Building Regulations for England. Its purpose is to reduce the risk of excessive indoor temperatures during the summer while avoiding unnecessary reliance on mechanical cooling such as air conditioning. Instead, Part O encourages good building design that naturally limits heat gain and promotes effective ventilation. The result is homes that are:
- More comfortable.
- More energy efficient.
- Better prepared for future climate change.
For full technical guidance, see the Government’s Approved Document O (Overheating).
When Does Part O Apply?
Part O applies primarily to:
- New‑build houses.
- New‑build flats and maisonettes.
- Residential developments.
- Student accommodation.
- Certain material change of use projects that create new dwellings, such as office‑to‑flat conversions.
Part O does not generally apply to:
- Extensions to existing houses.
- Most home renovations.
- Commercial buildings that remain non‑residential.
Although many home extensions are exempt, applying Part O design principles can still improve comfort significantly.
Why Was Part O Introduced?
Modern homes are becoming increasingly airtight and well insulated. While this reduces heating costs during winter, it can also trap unwanted heat during summer. Additional factors such as larger areas of glazing, south‑facing windows, urban heat islands and climate change have increased the risk of overheating. Part O seeks to balance excellent thermal insulation with effective summer comfort.
Two Routes to Compliance
The Simplified Method
Most straightforward housing developments can use the simplified method. This approach limits glazing areas, solar gain, window orientation and ventilation provision. If the proposed design stays within these limits, additional thermal modelling is generally unnecessary.
Dynamic Thermal Modelling
More complex projects may require dynamic simulation. This involves specialist computer modelling following CIBSE TM59: Design Methodology for the Assessment of Overheating Risk in Homes. The assessment considers building orientation, window sizes, internal layouts, thermal mass, external shading, local weather data and ventilation strategy. Although more expensive, this route offers greater design flexibility.
The Simplified Method Explained
Window Size
Excessive glazing increases solar heat gain. Part O therefore limits glazing, particularly on south‑, west‑ and east‑facing elevations. North‑facing glazing is generally less restricted because it receives less direct sunlight.
Ventilation
Adequate natural ventilation is essential. Most dwellings require sufficient openable window area to promote cross‑ventilation. Single‑aspect homes—those with windows on only one elevation—are often subject to stricter requirements because natural airflow is more limited.
Design Strategies That Improve Compliance
Optimise Window Orientation
- Reduce west‑facing glazing.
- Balance glazing across different elevations.
- Avoid excessive uninterrupted glass areas.
External Shading
External shading is generally more effective than internal blinds because it stops solar heat before it enters the building. Examples include roof overhangs, brise‑soleil, external shutters, adjustable louvres and deep window reveals.
Encourage Cross‑Ventilation
Providing windows on opposing elevations allows air to flow naturally through the building, one of the simplest and most effective methods of reducing internal temperatures.
Use Thermal Mass
Materials such as concrete, brick and masonry can absorb heat during the day and release it gradually overnight, helping to moderate indoor temperatures.
Choose Lighter External Finishes
Light‑coloured roofing and wall finishes reflect more sunlight than darker materials, reducing external surface temperatures.
Benefits of Part O
- Improved Comfort: Homes remain cooler during hot weather.
- Better Health: Reducing overheating particularly benefits older occupants, young children and people with medical conditions.
- Lower Energy Bills: Naturally cool homes require less mechanical cooling, reducing electricity consumption.
- Future‑Proof Design: Climate projections suggest UK summers will continue becoming warmer; homes designed today should remain comfortable for decades.
The Government’s Future Homes Standard programme also aims to improve the energy efficiency and resilience of new homes.
What Happens If You Ignore Part O?
Failure to demonstrate compliance may result in delays during Building Control approval, requests for redesign, additional modelling costs and expensive remedial works during construction. Considering Part O early in the design process is almost always more economical than trying to resolve overheating later.
For a broader explanation of the approval process, see the Government’s guide to Building Regulations approval and the Planning Portal’s Building Regulations guide.
Does Part O Affect Extensions?
In most cases, Part O does not apply to domestic extensions, but many of its principles remain good design practice. Limiting excessive west‑facing glazing, providing opening rooflights, designing effective cross‑ventilation and incorporating external shading can greatly improve comfort, especially in highly glazed kitchen extensions and garden rooms.
For related advice, see our guide Can a Home Extension Be Designed to Be Net Zero Carbon?.
How Draw & Plan Can Help
At Draw & Plan, we consider overheating risks from the earliest design stages, helping clients avoid costly redesigns later in the approval process. Our Building Regulations Drawings service incorporates the technical information needed to demonstrate compliance with the Building Regulations, including Parts L and O where applicable.
Where thermal modelling is required, we work closely with accredited energy assessors to coordinate the necessary assessments alongside the architectural design. If your project also requires planning permission, our Planning Drawings service develops designs that balance planning policy, daylight, energy efficiency and long‑term comfort.
Ready to discuss your project? Contact us today for a free, no‑obligation quotation.
Further Reading
Disclaimer: This article is for general informational purposes only and does not constitute legal or professional advice. Always consult relevant professionals and local authorities before undertaking any development or change of use.
