What the daylight standard actually measures
BS EN 17037 is the standard that sets out how daylight in buildings is assessed, and it looks at the room as a whole rather than the size of one window.
The standard covers four things: how much daylight reaches the room, the view out, access to sunlight, and protection from glare. For daylight provision it sets targets for the light falling on a working surface across a share of the floor, and grades a room as minimum, medium or high depending on how far it goes beyond them. Designers use software or measured surveys to check a room against those targets. The specific figures sit in the standard itself and are calculated for the room in question, so a rule of thumb copied from the internet is no substitute for looking at the real space.
For a homeowner the useful point is simpler. The standard judges daylight by how much of the room is well lit and for how long, not by how bright the one spot under the window gets. A room with a single bright pool and dim corners scores worse than one with gentler light spread evenly. That is exactly the difference between a well placed rooflight and a badly placed one.
Kitchens, living rooms and bedrooms compared
Each room needs a different kind of daylight, because each is used at different times and for different tasks.
| Room | Daylight need | When it matters | Watch for |
|---|---|---|---|
| Kitchen | High, even light on worktops | Morning to early evening | Shadow from wall units, glare on the hob |
| Living room | Moderate, soft and spread | Afternoon and evening | Glare on the TV wall, summer heat |
| Bedroom | Lower, calm light | Morning, dark at night | Early summer sunrise |
| Home office | High, steady, little direct sun | Working hours | Reflections on screens |
A kitchen is where task light matters. You chop, read labels and cook on surfaces that sit under wall cupboards, so light needs to land on the worktop, not just the floor in the middle of the room. A rooflight over the working run, or a lantern set towards the back of a side-return extension, does more than the same glass placed centrally.
A living room is used later in the day and needs comfort more than brightness. The risk is too much direct sun across a sofa or a screen. A bedroom needs the least daylight of the three and the most control, because a skylight that lights the room beautifully at noon will also let in the dawn at five in June unless it has a blackout blind.
Why a skylight sees more sky than a wall window
A roof opening faces the brightest part of the sky, so the same area of glass brings in noticeably more daylight overhead than it does in a wall.
A vertical window can see at most half the sky dome, and in a Cambridge street much of that half is blocked by the house opposite, a boundary wall or the neighbouring rear extension. Its light also enters low and falls off quickly towards the back of the room. A rooflight or flat skylight looks straight up towards the zenith, which on an overcast day is the brightest part of the sky, and nothing on a flat Fenland horizon gets in its way. The light arrives from above and spreads down across the whole floor.
This is why a deep terraced house in Romsey or off Mill Road, lit only from the front and a narrow rear window, can be transformed by one well placed roof window over the middle of the plan. It is also why a smaller skylight often does a job a bigger wall window cannot. For the full story of fixing a gloomy room, see our guide to skylights for dark rooms.
How Cambridge sun angles change the answer
How much daylight a room gets from the roof also depends on the season, because the sun over Cambridge climbs to very different heights through the year.
At noon on the winter solstice the sun is only about 14 degrees above the horizon. At the spring and autumn equinoxes it reaches about 38 degrees, and at the summer solstice about 61 degrees. A rooflight that catches strong direct sun in June may see very little of it in December, and relies instead on light from the sky, which is why north-facing roof glazing gives such steady results all year.
So the right amount of glass is not a fixed number. It is the amount that gives the room good light on a grey February morning without overheating it on a July afternoon. That balance changes with the slope the rooflight faces, which is why orientation matters as much as size.
Sizing the glass for the room
More skylight area does not automatically make a better room, and the right size comes from the room's use, its orientation and the glass chosen.
A large south-facing pane can leave one blinding pool of sun on the floor while the corners stay dim, and add heat that a blind then has to fight. Two smaller rooflights, spaced so their light overlaps, tend to give a more even room than one oversized opening. Solar control glass can let in daylight while cutting heat, and a light-coloured reveal bounces more of it into the room. The trade-offs are covered in our answer on whether a skylight can be too big for a room, and in how to stop glare from a skylight.
At a light survey we look at the room itself: its depth, which way the roof faces, what blocks the sky, where you work and sit, and when the room is used. The written light plan that comes with every quote sets out the position, size, glass and shading we recommend, and why. You can read how we do this on how we plan the light, or start from our rooflight and skylight installation overview. When you are ready, book a light survey or message us on WhatsApp.
Questions people also ask
Does BS EN 17037 apply to my extension?
It is a published standard rather than a Building Regulations requirement for a home extension, though architects and planners sometimes refer to it. It is still a useful way to think about whether a room is well lit, even when nobody is asking you to prove compliance.
Is a north-facing skylight bright enough?
For many rooms, yes. A north-facing rooflight sees open sky all day, and that light is steady and free of glare. It brings in less direct sun, so it can be sized a little larger than a south-facing one to give a similar feel.
Is one big rooflight better than two small ones?
Not always. Two smaller rooflights spread light more evenly across a long room and avoid the single hot spot a large pane can create. One larger unit can suit a square room, or a view of the sky you want to keep whole.
Can a sun tunnel light a room on its own?
A sun tunnel suits small spaces such as landings, bathrooms and hallways. It will not light a kitchen or living room by itself, but it can bring daylight into a space with no room for a full skylight above.
