As overheating becomes a growing concern in new UK homes, the government has introduced Building Regulation Part O – a regulation aimed at reducing excessive indoor heat during warm weather. If you are planning a new build, designing residential units, or installing windows and doors, Part O is now a critical part of the compliance process.
At Colin’s Sash Windows, we are already seeing how this regulation influences glazing choices, ventilation layouts, and overall window design. This article summarises the key points you need to be aware of.
What Is Part O?
Part O is a regulation that aims to ensure new homes remain safe and comfortable in hot weather by:
- Limiting overheating risk
- Controlling solar heat entering through windows
- Ensuring dwellings can cool down naturally or mechanically
Part O applies to:
- New-build houses
- New-build flats
- Buildings converted into residential units
- Student accommodation and care homes
Part O does not apply to:
- Extensions
- Alterations or refurbishments that do not create new dwellings
Why Part O Matters?
Homes with large glazing, poor ventilation, or exposure to urban heat can become uncomfortably warm, especially in cities like London. Part O introduces rules to prevent these issues through improved design, better glazing, and sufficient ventilation.
Key Requirements of Part O
- Limiting Unwanted Solar Gains
Designers must prevent excess heat entering a home using measures such as:
- Reducing window sizes on south- and west-facing elevations
- Using low-g or solar-control glass
- Adding external shading (overhangs, shutters, brise-soleil)
- Designing internal layouts to minimise overheating in key rooms
This is already reshaping how architects specify windows and doors for modern homes.
- Providing Adequate Ventilation to Remove Heat
Homes must be able to purge heat effectively. This can be achieved by:
- Installing windows with sufficient free area (usable opening area)
- Providing cross-ventilation where required
- Using mechanical ventilation if natural ventilation is insufficient
- Using mechanical cooling only when all other methods fail
Important: Trickle vents do not contribute to the required ventilation free-areas.
High-Risk Overheating Areas: What’s Different?
Large parts of the South East, especially London, fall within the high-risk classification. In these areas, Part O often requires:
- Larger opening windows
- Cross-ventilated layouts
- Smaller fixed panes on hot elevations
- More dynamic overheating assessments
- Wider use of solar-control glass
This typically results in more design constraints and more detailed modelling.
How Part O Interacts with Noise, Security & Safety?
Part O must be balanced with:
- Part Q (security)
- Part K (fall protection)
- Acoustic requirements for noisy sites
For example:
- Windows cannot be left open at night if external noise levels are too high
- Safety restrictors may be needed, provided they still meet free-area rules
- PAS24 security remains mandatory for new dwellings
This creates more complex design decisions, particularly in urban settings.
Impact on Window & Door Choices
Part O is influencing product selection across the industry:
- Side-hung casements are increasingly specified for maximum free-area
- Tilt-and-turn windows are being used more frequently on high-risk plots
- Fixed glazing is being reduced on certain elevations
- Solar-control glass is becoming standard in many new homes
- More projects require detailed free-area calculations for Building Control
Colin’s Sash Windows now works closely with designers to help calculate and specify compliant opening sizes and glazing types.
Download the Full Technical Guide
We have produced a detailed PDF guide covering:
- Full technical explanations
- Free-area calculations
- Glazing specifications
- Shading strategies
- Cross-ventilation examples
- Mechanical vs natural ventilation considerations
- Documentation required for Building Control
Download the full detailed Part O guide
