Roof lantern or flat skylight: which suits an extension?

Both count as rooflights in the Building Regulations, meaning glazing carried on a raised edge built up from the roof, not laid flush with its surface. The real difference is shape. A flat skylight is one sealed glass unit, level or very gently domed, fixed to a low kerb. A roof lantern is a small glazed roof of its own, with sloping sides, usually a ridge along the top, and a frame that stands well clear of the flat roof around it.

Fixed and opening versions come in both families, and a flat unit can also be a walk-on rooflight, made to be stood on. Our guide to flat roof skylights explains the flat types, and the one on roof lanterns covers frames and glazing. This page deals only with choosing between them for an extension you are building or re-roofing, and the wider skylights and roof windows page covers the rest.

A flat skylight drops a pool of daylight straight down onto whatever sits beneath it. That suits a kitchen island, a dining table or a dark corridor, where you want brightness in one defined place. Because the glass is horizontal, the light shifts a good deal with the hour and the weather, and a clear sky overhead is its main source.

A lantern brings light in from several directions. Its sloping panes catch the sky at different angles, so the glow falls across the walls as well as the floor, and a big room feels softer and more even. It also gives you a taller ceiling in the middle of the room, because the plasterboard lining can follow the frame upward. Whether that counts depends on the room. If the extension is already tall, the lantern is mostly a feature. If it feels low or cramped, the extra height is the thing you will notice every day, and it may be the strongest reason to choose one.

Here the two part company most. A flat skylight is light enough to go onto many existing flat roofs once the deck is trimmed and a kerb built around the opening. A lantern is heavier and taller, so the framing around the opening has to be sized for it, often with extra joists doubled up. A builder or structural engineer should confirm that before anything is ordered, because a lantern delivered to a roof that cannot take it is an expensive mistake.

The roof surface matters as much. The covering must turn up the kerb and finish beneath the unit's own flashing, and the whole roof must drain away from the base. BS 6229:2025 keeps a minimum finished fall of 1:80, and roofers usually design steeper so water never stands against the glazing. A lantern on a roof that holds puddles is a leak waiting to happen, whichever make it is. Our answer on rooflight upstands and kerbs shows how the upstand is built.

Under Approved Document L 2021 (Table 4.2) the U-value limit for a new rooflight is 2.2 in an existing home, with 0.35 as the ceiling for a made-on-site edge. On 24 March 2027 the 2026 edition's Table 3.2 replaces it, repeating the same numbers. A lantern carries more glass and more frame than a flat unit, so its specification deserves a closer look, and the supplier's data sheet should give the figure for the whole unit rather than for the glass alone. Your building control body can confirm whether the work must be notified.

On planning, a lantern stands proud of the roof, so ask your council's planning team whether a particular design needs permission. The answer differs for a flat, a listed building or a conservation area. Check before you order, not after the glass has arrived, and keep the reply with your paperwork for whoever buys the house next.

Start with the room, not the product. A galley kitchen, a utility room or a bathroom with no outside wall usually wants light more than drama, and one or two flat units supply it for the least structural work. A wide living area with bifold doors, or a room whose ceiling will be seen from the garden, is the case for a lantern, because the glass becomes part of the architecture.

Cost follows the same split. A lantern is a larger piece of manufactured joinery and glass, and fitting it takes longer and more careful sealing, so it costs more than a flat unit of similar footprint. Our answer on what a roof lantern costs gives typical UK figures. When we visit, we photograph the deck and the edges, and tell you plainly which option the structure suits. The quote is written and the price is one total, given after the roof has been seen and the choice made.