The short answer
- Choose a roof lantern for a feature: a pitched glass roof over a kitchen or dining table that adds height and sheds rain and dirt well.
- Choose a flat rooflight for a minimal look from inside and almost nothing visible from outside or from the windows above, or where the roof height is limited.
- Choose an opening rooflight where you want night cooling or ventilation in summer.
Side by side
| Roof lantern | Flat rooflight | |
|---|---|---|
| Shape | Pitched glass on slim rafters | One flat pane on an upstand |
| From inside | A glass ceiling feature with visible rafters | A clean opening in the ceiling |
| From outside and above | Clearly visible, adds height | Barely visible |
| Rain and dirt | Run off the pitched glass | The glass needs a slight fall; dirt can collect at the edge |
| Typical size | Up to c. 3 × 6 m | Single panes, or several side by side |
| Opening | Usually fixed | Fixed, opening or sliding hatch |
1. Light and the room below
Light from above reaches the middle of the room in a way that wall windows can’t, which is why rooflights suit long terrace extensions. A lantern also adds volume: the glass rises above the ceiling, so the room feels taller under it. A flat rooflight keeps the ceiling line. If the ceiling is deep, a splayed lining around the opening spreads more light into the room.
2. Rain, dirt and cleaning
On a lantern, rain runs off the pitched glass and takes most of the dirt with it. Flat glass rooflights are made with a slight fall so water drains, but leaves and dust can sit at the lower edge, and you’ll see them from below. Self-cleaning coatings help on both. Either way, the rooflight sits on an insulated upstand that keeps it above the roof finish, usually at least 150 mm.
3. Heat in summer
Glass in a roof catches the high summer sun, so a large lantern over a south-facing kitchen can overheat the room. Solar control glass cuts the heat while keeping the light, and an opening rooflight lets warm air out at night. New homes must show they won’t overheat under Approved Document O, and the glass figures feed into that assessment.
4. Building Regulations
- What counts as a rooflight. Approved Document L defines a rooflight as a glazed unit installed out of plane with the roof on a kerb or upstand, sometimes called a skylight. Lanterns and flat rooflights both count. A roof window sits in the plane of a pitched roof.
- U-value. New rooflights in existing homes and extensions need a U-value of 2.2 W/m²K or better. It is worked out on the developed surface area of the rooflight, which for a lantern is larger than the hole in the roof. The 2026 edition of Approved Document L, which takes effect on 24 March 2027, keeps the same limit.
- The 25% rule for extensions. Windows, roof windows, rooflights and doors in an extension together shouldn’t exceed 25% of the extension’s floor area, plus the area of any windows and doors the extension covers up. Above that, the extension has to be shown to perform as well as one that meets the rule, usually through better U-values elsewhere.
- Safety. A laminated inner pane in roof glazing keeps broken glass in place.
For large glazed doors under the same rules, see our design guide to large sliding doors.
5. Planning
On a new extension, the lantern is part of the extension, so its height counts in the extension’s permitted development limits or in your planning drawings. On the original pitched roof of a house, rooflights are usually permitted development if they protrude no more than 0.15 m beyond the roof slope and no higher than the highest part of the roof. Listed buildings, flats and some conservation areas need consent; see the conservation areas guide.
Our roof glazing
We supply roof lanterns and flat and opening rooflights to order.
Questions
Is a roof lantern or a flat rooflight better for a kitchen extension?
A lantern makes a feature and sheds rain and dirt well; a flat rooflight looks minimal and keeps the ceiling line. Both bring light to the middle of the room. Over a south-facing kitchen, choose solar control glass with either.
What U-value does a new rooflight need?
2.2 W/m²K or better in existing homes and extensions, under Approved Document L. The value is based on the developed surface area of the rooflight, not the roof opening.
Does a roof lantern count towards the 25% glazing limit?
Yes. Rooflights, roof windows, windows and doors in an extension count together against 25% of its floor area, plus the area of any windows and doors the extension covers up.
Do I need planning permission for a roof lantern?
On a new extension, the lantern counts in the extension’s height, so check it against the permitted development limits or include it in the planning drawings. Listed buildings and flats need consent.
Sources
The official documents this guide is based on. Check the current edition before you order.