Part O overheating rules: Glazing, vents and shading for extensions
- 1 December 2025
- News
Part O overheating rules: glazing, vents and shading for extensions
If you’re planning a new extension or a light-filled orangery, it’s easy to get caught up in stunning designs, panoramic glass, and sun-soaked spaces.
Although before the foundations can be laid, and the furniture bought, it’s important to gain an understanding of the latest legislation, particularly when it comes to Part O glazing requirements UK.
This section is all about preventing overheating in residential buildings, ensuring your new space stays comfortable during the summer months while remaining energy efficient.
Understanding these rules early can help you to make the right glazing, ventilation and shading choices, saving time, money and stress during the design phase – which is why we’ve put this helpful blog together!
Explaining Part O glazing requirements UK
Part O was introduced to address the risk of overheating in new residential buildings. Its main goal is to ensure that buildings maintain a comfortable indoor environment without relying excessively on mechanical cooling systems. According to Approved Document O, reasonable provision must be made to limit unwanted solar gain while providing adequate means of removing heat.
For designers and builders, compliance can be demonstrated in two ways:
- Simplified method – prescriptive rules based on glazing ratios, ventilation openings, shading devices, and room orientation.
- Dynamic thermal modelling method – predictive simulations that account for solar radiation, building materials, and climate to calculate potential heat gain and internal temperatures.
The benefits of following Part O include:
- Reduced overheating risk during hot summer months
- Improved energy efficiency and lower reliance on mechanical cooling
- Increased comfort for occupants in residential units, from bedrooms to living areas
Even though Part O technically applies only to new builds, understanding its principles can influence the design of extensions and orangeries for better comfort and compliance with complementary regulations like Part F (ventilation) and Part L (energy efficiency).
What do these regulations apply to?
Part O primarily covers new dwellings, including houses, flats, care homes, student accommodation, and other residential buildings. Existing homes undergoing renovations, extensions, or window replacements usually do not fall under Part O unless the work creates a fundamentally new dwelling.
However, the principles are still useful for homeowners planning:
- Conservatories and orangery-style extensions
- Glazed roofs and large floor-to-ceiling windows
- Spaces with high solar exposure, especially south-facing or west-facing rooms
By considering solar control glass, external shading devices, and cross ventilation, you can reduce overheating even if your project isn’t strictly regulated by Part O.
What does this mean if you’re planning an extension or orangery
Even if Part O doesn’t legally apply, thinking about overheating now will make your extension far more enjoyable in the summer. Key steps include:
- Choosing a reputable installer like Crown, who understands thermal performance and building regulations
- Using thermally efficient glazing, including solar control glass for south-facing windows
- Incorporating ventilation options, such as trickle vents, roof vents, or purge ventilation for long periods of warm weather
- Considering external shading, internal blinds, or lightweight materials to reduce heat gain
- Planning glazing ratios carefully so that rooms don’t become high-risk areas for overheating
Even small design adjustments can make a big difference to your comfort and energy bills. For example, a south-facing conservatory with external blinds and a small roof vent can stay cool without relying on air conditioning.
Extensions from Crown
At Crown Conservatories, Windows and Doors Ltd, we specialise in high-quality conservatories and extensions designed to suit your home and lifestyle. Our Victorian, lean-to, gable-end, Edwardian, and combination conservatories are built with thermally efficient Guardian tiled roofs and warm roof solutions to reduce heat loss and heat gain.
We offer:
- Free 3D design consultations, so you can visualise your new space
- A wide range of windows: casement, flush sash, sliding sash, tilt & turn, Georgian, and lead designs
- Thermally efficient glazing to help maintain a comfortable indoor environment
- Installation by experienced, FENSA-registered fitters, ensuring compliance with building regulations
- A 10-year warranty for peace of mind
By combining smart glazing, ventilation, and shading, we create extensions that are not just beautiful but comfortable all year round.
FAQs
Does Part O apply to my extension?
No. Part O applies only to new residential buildings, not extensions, unless a new dwelling is being created.
What is the simplified method?
A prescriptive approach using fixed rules for glazing ratios, ventilation, and shading to limit overheating.
Can I use solar control glass in an existing home?
Yes, it’s an effective way to reduce solar gain and improve comfort in any glazed extension.
Do I need mechanical ventilation?
Not always. Natural ventilation with trickle vents or roof vents can often meet comfort needs.
Are Crown conservatories energy efficient?
Absolutely. Our designs combine thermally efficient glazing, quality frames, and optional Guardian tiled roofs for maximum comfort.
How can I limit overheating in a moderate risk location?
Smart glazing design, passive means, and careful orientation help control excess heat in a moderate risk location.
Who are Crown?
Crown Conservatories, Windows and Doors Ltd has over 20 years of experience helping homeowners across Milton Keynes and the surrounding areas. We provide made-to-measure windows, doors, and conservatories, all fitted by FENSA-accredited experts. Visit our Milton Keynes showroom or contact us for a free quote.
Call us on 01908 507 778 or email info@crownwindows.co.uk to start planning your perfect, comfortable, and stylish extension today.
