Understanding Part O of the UK Building Regulations: Preventing Overheating in Homes
When people think about Building Regulations in the UK, they often focus on insulation and keeping heat in — but with hotter summers becoming more common, the regulations now also address the problem of keeping homes cool.
That’s where Part O comes in. Introduced in June 2022, Part O sets new rules for limiting overheating in new dwellings and certain major conversions. In this guide, we’ll break down what it is, when it applies, and how you can design your project to comply.
What is Part O?
Part O of the Building Regulations is officially titled “Overheating”.
Its aim is to ensure that new homes, and some types of residential conversions, are designed to avoid excessive heat build-up in summer without relying heavily on mechanical cooling (such as air conditioning).
When Does Part O Apply?
Part O applies to:
• All new residential dwellings, including houses, flats, and maisonettes.
• Certain conversions (material change of use) creating new dwellings from non-residential buildings, e.g., office-to-flat conversions.
• Student accommodation and other long-term residential units.
It does not generally apply to:
• Extensions to existing dwellings.
• Non-residential buildings.
Two Routes to Compliance
There are two official ways to show compliance with Part O:
1. The Simplified Method
Best suited to straightforward, naturally ventilated homes.
• Limits glazing areas based on orientation and location (London vs. rest of England).
• Requires minimum openable window areas for cross-ventilation.
• Has stricter limits for single-aspect dwellings (windows only on one side).
2. The Dynamic Thermal Modelling Method
For more complex designs or where glazing limits can’t be met:
• Uses detailed computer modelling following CIBSE TM59 methodology.
• Considers the building’s layout, materials, shading, and weather data.
• More flexible but requires a specialist assessor.
Key Requirements in the Simplified Method
Dwelling Type Maximum Glazing Area (South, East, West) Maximum Glazing Area (North) Minimum Openable Area (Cross-Ventilation)
Single Aspect Lower limits (approx. 15% of floor area) Higher limits 1/20th of floor area
Dual Aspect Higher limits (approx. 21% of floor area) Higher limits 1/20th of floor area
Note: Exact percentages vary based on region (London vs. rest of country) and whether there is significant shading from adjacent buildings or balconies.
Design Strategies for Part O Compliance
1. Optimise Window Orientation
• Reduce large west-facing glazing where evening heat gain is highest.
2. Incorporate External Shading
• Brise soleil, shutters, or overhangs can reduce solar gain without blocking light.
3. Increase Cross-Ventilation
• Position windows or vents on opposite walls to encourage air movement.
4. Consider Thermal Mass
• Exposed concrete or masonry inside can help absorb and slowly release heat.
5. Use Light-Coloured External Finishes
• Reflect more solar radiation and reduce external surface temperatures.
Why Part O Matters
• Comfort – Prevents homes from becoming uncomfortably hot in summer.
• Health – Overheating can be dangerous, particularly for vulnerable occupants.
• Energy Savings – Reduces the need for air conditioning.
• Future Proofing – Homes designed to handle heat waves will remain comfortable for decades.
Consequences of Ignoring Part O
• Building Control may reject your application.
• You could face costly redesigns or retrofits.
• Risk of occupant discomfort or health issues.
How Drawandplan.com Can Help
At Draw and Plan, we integrate Part O compliance checks into our design process for new homes and qualifying conversions. We:
• Assess your project early for overheating risks.
• Recommend cost-effective design changes to meet either the simplified or modelling route.
• Coordinate with accredited energy assessors for full thermal modelling where needed.
Final Thoughts
Part O isn’t just another regulatory hurdle — it’s a practical set of rules to make sure the homes we build today are fit for the climate we’ll have tomorrow. By considering overheating prevention from the very start of the design process, you’ll save time, avoid compliance headaches, and create a home that’s comfortable year-round.
Tip from drawandplan.com: Even if Part O doesn’t apply to your extension or renovation, incorporating its principles can improve comfort and reduce summer cooling bills.
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.