Flat rooflight or roof lantern: which suits your extension

A flat rooflight is a low glazed unit that sits on a kerb or frame on a flat roof. The glass is a sealed double or triple unit, often with a domed or flat outer pane, and the whole thing stands only a short way above the roof. Some are fixed, some open, and some are frameless, with the glass bonded over a hidden frame so the roof looks almost uninterrupted.

A roof lantern is a glazed roof structure with sloping sides, built up from bars and panes into a pyramid, a hipped ridge or a long gable. It stands on a kerb too, but it rises a good deal higher above the roof deck, and from inside it gives you a lifted, shaped ceiling feature. Lanterns are usually aluminium or timber, with factory-made glass panes.

Seen from the garden, a flat rooflight is almost a line in the roof, while a lantern is a visible part of the building. Seen from the room, a flat rooflight is a bright rectangle in the ceiling, and a lantern is a small glass room above it.

For the same footprint, a lantern lets in light from more directions. Its sloping sides catch low morning and evening sun as well as light from overhead, and the glazing bars break it up as it falls. A flat rooflight takes light from straight above, which is strongest around midday and in summer.

That affects how the room feels. A flat rooflight over a kitchen island gives bright, even working light and a crisp patch of sun on a clear day. A lantern over a dining table gives softer, more varied light that changes through the day. Neither is brighter in the abstract: a large flat rooflight can bring in more light than a small lantern, because the glazed area is what counts.

Orientation matters more than shape. A south-facing roof can overheat under either, which is a question of glass specification and blinds, not the form. Our answer on whether roof lanterns get hot in summer goes through the options, and the same glass choices apply to flat units.

This is often where the decision is made. A flat rooflight adds very little to the height of the roof, so it suits a single-storey extension that sits below a neighbour's window, a low roof seen from upstairs, or any site where a visible lantern might be unwelcome. A lantern adds a noticeable form, and on a small extension against a tall house wall it can look like it is sitting on top of the roof rather than belonging to it.

It also changes how the extension reads from upstairs rooms. People in the bedrooms above look down on a flat roof, so a flat rooflight is a rectangle of glass in a neutral surface, and a lantern is a more prominent object. Some homeowners like that. Others find a quiet, low roof more restful.

Internal ceiling height works the other way. A lantern needs headroom to be seen from below. In a room with a low flat ceiling, a lantern can only show its underside as a rectangle, so you pay for the shaped roof and see little of it. In a taller room, perhaps with a raised ceiling, it comes into its own.

A lantern is heavier than a flat rooflight of the same footprint, because of its frame, its several glass panes and its height. The kerb it stands on has to carry that weight and resist wind lifting a tall glazed shape, and the joists around the opening have to be sized for it. In a new extension, that is designed in from the start. On an existing flat roof, it needs a check on the joists first; we cover that in can a roof lantern go on an existing flat roof.

A flat rooflight puts less load on the roof, and its kerb is lower, so it is the easier unit to add to an existing deck. The upstand and falls still matter, because water has to run away from the unit, and our guide to flat rooflight upstand heights and falls sets out what a good kerb looks like.

Both depend on the roof covering. Whether the deck is GRP, EPDM or felt changes how the kerb is formed and how the covering is dressed up to it, and the sister guide on flat rooflights on GRP, EPDM and felt roofs covers those details. The same coverings apply to a lantern kerb.

This is where the two feel most different once you are standing in the room. A flat rooflight sits in a ceiling opening with a short lining, and the finish is a crisp plaster edge: a plain or slightly splayed return, a clean arris and a paint line against the frame. In a modern ceiling it can look like a precisely cut slot of light.

A lantern asks for more. There is the inner kerb face to board and plaster, the junction between plaster and the lantern's own frame, and the question of how the ridge and bars look against the ceiling. Corners and movement matter more because the unit is bigger and heavier. We go through those options in ceiling finishes around a roof lantern.

It is easy to underestimate this in the choice. A lantern with an untidy return looks worse than a flat rooflight with a neat edge, however grand the lantern. If the budget is tight, a well-finished flat rooflight beats a poorly finished lantern, and a finish schedule before the quote makes the difference clear.

The cost line in the table is deliberately general. Prices depend on size, frame, glass and access, and they only mean something after a survey, which is why we do not quote figures for a type of rooflight in advance.

Patterns are easier to see in real homes. A rear extension on a 1930s semi in Heavitree, with a modest flat roof and a first-floor window looking down on it, usually suits a flat rooflight or a pair of them: the roof stays low, the light reaches the back of the kitchen and nothing rises above the roof line. A wider, deeper kitchen and dining extension on a detached house outside Crediton or Newton Abbot, with a raised ceiling and a garden to the rear, has room for a lantern as a centrepiece.

Older houses add their own pull. A Georgian or Victorian terrace in Exeter or Topsham often has a narrow side return or a lean-to at the back, and the plot is tight enough that a low glazed strip along the return does more than a lantern could. A cottage extension with exposed beams or a vaulted ceiling can take a lantern or a long flat rooflight. In a barn conversion, a long lantern can echo the ridge of the original roof, though the structure behind it is usually a matter for an engineer.

New builds, such as those around Cranbrook, tend to have flat ceilings under pitched roofs with little spare depth. Flat rooflights fit those conditions without much to rearrange, and a sun tunnel can handle a hall or landing where a larger unit will not go.

None of that is a rule. It is only where the practical limits tend to push you. A site visit gives better evidence than any pattern, because the real joists and the real ceiling height are what count.

Approved Document L sets limiting U-values for both types, and both are available with the same range of glass: double or triple glazing, a low-emissivity coating, a warm-edge spacer, solar control coatings, self-cleaning outer panes and a laminated inner pane for safety overhead. Laminated inner panes are good practice for any overhead glazing, though they are not a Part K requirement.

The difference is in the frame and the number of joints. A lantern has many bars, each of which is a place where heat can leave, so the frame design and the thermal breaks matter. A flat rooflight, especially a frameless one with a single sealed unit, has fewer junctions and a simpler thermal line. A big pane of glass with a good spacer can be as efficient as a well-designed lantern, and sometimes more so, which surprises people who assume the bigger structure must perform better.

In winter, a large expanse of glass overhead feels cooler than a ceiling, whichever you choose, so insulating the kerb and the lining fully is as important as the glass. In summer, solar control glass and a way to shade the opening count for more than the shape.

Planning is where the two can differ. Permitted development for rooflights on a house comes from Class C, Part 1, Schedule 2 of the General Permitted Development Order, and it usually applies when the unit stands no more than 150mm off the roof plane and finishes below the roof's highest point. A low flat rooflight tends to fit those limits. A lantern is taller, and may well stand further out than that, so whether it falls inside permitted development or needs an application depends on the roof, the extension and any conditions on it. Our answer on whether roof lanterns need planning permission goes through that, and the local planning authority can confirm for your address.

If the extension is new, the lantern is often part of the design that was approved, so check the drawings. In a conservation area, or if the house is listed, the rules are stricter and permission or consent may be needed for either type.

Building Regulations apply to both. A new rooflight is building work, so Part L applies and the opening has to be structurally sound. Notification to Building Control, where one is required, is something we take on.

A flat rooflight is easy to make an opening unit, electric or manual, so it can ventilate a kitchen or let heat out of a stairwell. Opening lanterns exist, but they are less common and cost more. If purge ventilation matters, a flat unit is usually the simpler route; our guide to fixed or opening flat rooflights explains the choice.

Blinds are a factor too. A flat unit takes a simple roller or pleated blind in its frame. A lantern has sloping panes, so blinds are fitted to each, and the cost and fuss are higher; see can you fit blinds to a roof lantern.

Four questions settle most cases:

1. How much headroom does the room have, and can the ceiling rise?
2. Is there a neighbouring window or wall that would make a tall roof feature unwelcome?
3. Does the roof structure suit the weight, or is a lighter unit easier?
4. Do you want a feature in the ceiling, or the cleanest, lightest opening possible?

If the answers are low ceiling, close neighbours, light structure and a plain look, a flat rooflight is the likely choice. If they point to a tall room, open site, strong structure and a feature ceiling, a lantern is worth the extra cost. Plenty of extensions use both: a lantern over the dining area and a flat unit over the utility room or hall.

If you are still undecided, the most useful step is a survey, because the roof, the joists and the ceiling will narrow the choice quickly. We measure the roof and ceiling, and write up the finish for each option in a finish schedule that comes with the quote, so you can compare them as they will look from the room. The full flat roof service is on our flat roof rooflights page, and you can ask for a quote for either.