Best Practice on Daylighting Extensions

There is a minimum amount of glazing every new living space should feature in order to achieve consistent natural daylight levels.

By Larry Bohan on 28 June 2019

velux integra roof windows installed in loft

It’s not just a lack of living space that is driving the renovation market. It’s a shortage of natural lighting too. Daylight is becoming more and more of a focus when it comes to extension and loft conversion planning.

This has been partly fuelled by the increasing amount of time families are now spending indoors, along with a greater awareness of the health benefits of regular daylight exposure.

Aesthetically speaking, getting the level of daylight right right in a room can enrich any room and transform it into a lighter and brighter space. And then there is the added bonus of potentially significant savings that comes with less reliance of artificial lighting.

But just how much natural light does a converted loft bedroom, en-suite bathroom or open plan living-dining area need?

The 15% Rule

velux white roof windows installed in extension

We recommend that a room’s overall glazing should cover a minimum of 15-20% of the overall floorspace. Any less than this leave projects susceptible to dark spots that could spoil the natural flow of daylight running through a room.

This 15% rule is also followed by VELUX, market leader for roof windows. The glazing area can be made up of pitched roof windows, flat glass rooflights, casement windows and exterior doors, in any combination.

When applying this rule, an 10m² en-suite bathroom would require 1.5m² of glazed area. This it the equivalent of 2x VELUX PK08 (94x140cm) roof windows or 3x VELUX MK06 (78x118cm) windows.

Likewise, a larger loft space 25m² in size would need 3x UK08 (134x140cm) windows or 4 x PK10 (94x160cm) units.

If planning to daylight a project this way, remember it’s the visible glazing area spec that is the important spec to look at rather than the overall window size which, of course, also includes frame. 

This grid by VELUX highlights the glazed area measurements that each VELUX standard sized roof window will provide.