Part O Regs explained!

If you’re involved in building or renovating homes in England, you might’ve heard people talking about “Part O” and wondered what all the fuss is about.

Don’t worry it’s not some complicated legal stuff. It’s actually pretty straightforward but super important, especially with our summers getting hotter. So let’s break it down in plain English.

 

What Is Part O?

“Part O” is all about overheating. It was introduced in June 2022 to make sure new homes are designed in a way that doesn’t turn them into ovens during summer. With UK heatwaves becoming more common, some modern homes, especially small flats with lots of glazing , were getting dangerously hot. Part O helps prevent that by setting design rules to keep indoor temperatures down naturally.

 

What buildings does it apply to?

• New residential buildings (houses, flats, care homes, student digs, etc.)
• Some change-of-use projects, like turning an office into flats
• However it doesn’t apply to non-residential buildings, like shops or offices.

 

Part O wants homes to:

• Avoid overheating
• Provide proper ventilation
• Use shading or smart design to stop excess heat coming in

 

You can meet the requirements in two ways

1. The Simplified Method: This is the easy checklist approach — like following a recipe. If your design follows the rules for window sizes, glazing orientation, shading, and ventilation then you are compliant.
2. Dynamic Modelling (For complex designs): Computer simulation to check overheating risk can be undertaken.

 

There’s one extra detail that catches a lot of people out

The Window Height Rule (Yes, This Is a Big One). To cool a home naturally, windows need to open wide enough to let fresh air in and hot air out but there’s a safety rule in Part O too: if a window opens more than 0.35m², and the bottom of that opening is less than 1.1 metres from the floor, then it’s considered a fall risk — especially in high-rise buildings or upper-floor flats.

 

Window Heights: Safety vs. Ventilation

To cool rooms, windows must open wide enough but at he same time if a window opens more than 0.35m², and the bottom is under 1.1m from the floor...It's a Fall Risk!

Solutions:

•  Raise window sill to 1.1m
• Add safety restrictors or guards
• Use smaller openings (but they may not ventilate enough!)
   

Clash with Part B of the Building Regulations (Means of Escape)

Part B says:

Bedrooms on upper floors of a two storey home must have escape windows (often starting at low sill heights)

Part O says:

Those same low windows may now be unsafe if opened wide. Conflict!

 

You need:

• Safe ventilation (Part O)
• Low escape windows (Part B)

 
 Design Tip

We at Level Architecture must balance both rules sometimes using the following techniques: 

• Alternative escape routes
• Protected stairwells
• Fire-resistant glazing + sprinklers
• Mechanical ventilation (MVHR) in tricky urban areas

What About Homes in Cities?

In “high-risk areas” (usually noisy or polluted city spots), windows might not be practical to open. In these cases, we will need to:

• Use mechanical ventilation systems
• Add external shading, like shutters or brise soleil
• Think carefully about the layout and orientation of windows

 

Why Part O Matters

Part O isn’t about making life harder (although the conflict between part O and Part B would say otherwise) it’s about making homes safer and more comfortable in the long run. With rising temperatures, it's essential to build homes that:

• Stay cool in summer
• Don’t rely on energy-hungry air conditioning
• Are safe for everyone, including children and vulnerable people

 

Our Final Thoughts

Part O might sound like a dry bit of building regs, but really, it’s about creating better homes for a warmer world. Whether you’re designing a new house or converting an old office into flats, Part O helps make sure your building doesn’t cook its future residents.