How Much Daylight Can a Flat Roof Window Add?

April 2026

Flat roof windows and lanterns can dramatically transform how light moves through your home, especially in extensions where natural light is often compromised. If you’re planning a kitchen extension, loft conversion, or rear addition, understanding how daylight works is key to creating a bright, usable space.

How Flat Roof Windows Increase Natural Light

Flat roof windows (also called skylights or rooflights) bring in up to 2 - 3 times more daylight than vertical windows of the same size. This is because they capture light from above, where sunlight is strongest and least obstructed. 

Key daylight benefits: 

• Light enters deeper into the room 

• Reduces reliance on artificial lighting 

• Creates a brighter, more open feel 

• Enhances wellbeing and mood 

As a general rule: 

Glazing equivalent to around 20% of the floor area will provide sufficient natural light for most living spaces. 

Why Extensions Often Feel Dark Without Roof Windows 

When you build an extension, you push the original external wall further back, which means: 

• The existing windows are now deeper inside the home 

• Natural light has to travel further 

• The original room can become dark and gloomy 

Daylight typically only travels around 2 metres effectively before it starts to significantly diminish. Beyond this point, spaces rely heavily on artificial lighting unless additional daylight sources are introduced. 

This is why flat roof windows are so important, they reintroduce light into the centre and rear of the extension.

Extensions With vs Without Flat Roof Windows 

Without Roof Windows: 

• Dark zones in the middle of the extension 

• Gloomy original rooms 

• Increased need for artificial lighting 

• Less inviting living space 

With Flat Roof Windows: 

• Even light distribution throughout 

• Bright kitchen/diner and existing rooms 

• Reduced energy usage (less artificial lighting) 

• Visually larger and more appealing space 

One Large Rooflight vs Two Smaller Rooflights 

While one large rooflight might seem like the best option, using two smaller flat roof windows often delivers better results and may be the optimal window count for interior spaces. 

Why? 

• More balanced light distribution across the room 

• Reduces dark corners and shadowing 

• Better alignment with room layout (e.g., over kitchen zones) 

• More design flexibility 

This approach is especially effective in long or open-plan kitchen diners.

Managing Heat & Overheating 

Glass brings light, but also heat.

Considerations: 

• Too much glazing can lead to overheating, especially in south-facing extensions 

• Use solar control glazing where needed 

• Install internal sun screening blinds to regulate temperature 

Modern flat roof windows often come with: 

• Integrated blinds 

• Solar reflective coatings 

• Ventilation options 

Ventilation & the Chimney Effect 

Adding an opening flat roof window is a game-changer, especially in kitchen diners. 

Benefits: 

• Allows hot air to escape naturally 

• Improves air quality 

• Reduces condensation 

• Creates a chimney stack effect:
  Warm air rises and exits through the rooflight
  Cooler air is drawn in from below
  This creates natural cooling without relying on mechanical ventilation. 

Final Thoughts: Transforming Your Space with Daylight 

Flat roof windows aren’t just a design feature, they’re a functional necessity in modern extensions. 

Key takeaways: 

• Daylight fades after 2m, rooflights solve this problem 

• Aim for approx. 20% glazing relative to floor area 

• Use multiple smaller windows for better light balance 

• Consider ventilation and solar control to manage comfort 

By introducing natural light from above, you can turn a potentially dark extension into a bright, vibrant, and valuable living space.