The roof decides before taste does
A lantern and a flat rooflight are not two styles of the same thing. A lantern is a raised, pitched structure that sits proud of the roof line and throws light down and sideways as the sun moves. A flat rooflight sits close to the plane of the roof and delivers a single, even wash of light straight down. Which one is available to you is usually settled by the roof you already have, not by the picture the client brought in.
So the first three answers we need are the same for both: the structural opening, the kerb or upstand detail, and the roof build-up. Those set the finished height, how the unit weathers and what the ceiling looks like from underneath. Send them with the extension drawings and the specification stops being guesswork.
Both are quoted from drawings rather than picked from a size list, which is why an early sketch is worth more than a long conversation about the finish.
What each one reaches
On thermal performance the flat unit has the edge on paper. Thermally broken aluminium flat rooflights reach whole-unit U-values from 0.9 W/m²K where specified; lanterns reach from 1.0 W/m²K where specified. Both take double or triple glazing, and we confirm the figure for your unit at quote rather than quoting a catalogue number.
That gap is smaller than it looks in practice, because a roof aperture sits in the coldest plane of the building and faces straight up at the sky. The glass make-up moves the result more than it would in a wall, and solar gain matters as much as heat loss on a south-facing flat roof. Our glazing guide covers what the make-up is actually doing.
The flat rooflights we supply are fixed units. If the room needs ventilation as well as daylight, that has to come from the opening schedule rather than the roof, and it is better settled before the roof is closed up than after.
Weight, size and what the structure carries
Lanterns are sized from the structural opening, up to 75 kg where specified. Weight is the constraint people forget: the glass is the heavy part, and triple glazing over a wide span adds load to a structure that may already have been designed. Flat rooflight sizing is assessed per project for the same reason.
Finish is the easy part. Lanterns come in the full RAL range, and the internal face can differ from the external where the room and the roof want different colours. Flat rooflights take project colour choices alongside the glazing specification.
Ordering either one
Both are supplied as a package add-on to a wider trade glazing order rather than as a single-item purchase. In practice that suits the way the work arrives anyway: the extension that wants a lantern is usually the extension that wants bifolds on the garden elevation.
Put the roof unit on the same schedule as the doors and windows and it is quoted, fabricated and delivered with them. Send the schedule and the drawings through quote, or read the detail on the roof lantern and flat rooflight pages first.