A roof lantern is a glazed pyramid or ridge that sits proud of a flat roof on an upstand called a kerb. Most of what people compare is the frame and the glass, and most of what decides the final bill is the kerb underneath, who builds it, and how the roof covering is dressed up to meet it. Those are three different trades on many jobs, which is why two quotes for the same lantern can look nothing alike.
Roof lantern cost: the kerb is the job
A lantern does not sit on a flat roof, it sits on a timber upstand built off the deck and weathered into the covering. The lantern supplier quotes the frame and the glass. Somebody still has to build the kerb square and level, insulate it, dress the membrane up its face, and finish the inside. If a price looks low, check whether the kerb is in it. A lantern fitted to an out of square kerb will not close cleanly, and the fault shows up months later as a draught or a leak at one corner.
Glass is where the specification hides
Units are usually double or triple glazed with a coating on one face. The three things worth asking about are the thermal performance of the whole lantern rather than the glass alone, whether the outer pane is toughened and the inner laminated, and whether there is a solar control or self cleaning coating. Overhead glazing is treated differently from a vertical window because anything that fails above your head has to hold together. Approved Document K covers the protection side of that and a supplier should be able to say which pane does which job.
Large roof lanterns: ridge and pyramid designs behave differently
A square opening takes a pyramid. A long rectangular opening takes a ridge lantern, which is two sloping planes meeting at a bar. Ridge designs get heavy quickly and the bar has to carry itself across the span, so the aluminium gets deeper as the lantern gets longer. That is the real limit on a large lantern, not the price of glass. Where the span is genuinely long, a structural opinion on the roof below is cheaper before the order than after it.
What roof lantern prices should name
Ask for the kerb, the lantern, the glazing specification, the making good inside, and the access. Roof work is the largest single source of fatal falls in construction and the HSE treats access as part of the job rather than an extra, so a quote with no scaffold or tower named has not costed the whole of it. Ask also who returns if a seal fails in year two, because a lantern is one product with two warranties, the manufacturer's on the unit and the installer's on the fitting.
Period buildings and the Victorian roof lantern
The classic Victorian lantern is a steep pyramid with slim glazing bars and a decorative finial, and modern aluminium copies of it exist in most ranges. On a listed building or in a conservation area the shape is not the only question, because the consent regime is separate from building control. Check the planning position before ordering anything with a long lead time.
Questions people ask about roof lanterns
Do I need building regulations approval for a roof lantern?
Usually yes, because you are forming or altering an opening in a roof and the new glazing has to meet the thermal and safety requirements. Your installer may be able to self certify parts of it under a competent person scheme, otherwise it goes through building control. Confirm which route is being used before work starts.
Does a roof lantern need planning permission?
Often not on a standard house, because roof alterations within limits can be permitted development, but the limits are real and they do not apply to flats, listed buildings or some conservation areas. The Planning Portal sets out the roof rules and your local authority gives the binding answer.
How much glass is too much for one lantern?
The constraint is the span of the ridge bar and the weight the roof structure below can carry, not the glazing itself. Long lanterns need deeper frames and sometimes a structural check of the deck and the walls. Ask the supplier for the maximum clear span in the range you are pricing.
Will a lantern make the room cold?
A modern insulated lantern performs far better than an old rooflight, but glass still loses more heat than an insulated roof, so a large lantern changes the room's balance. The bigger comfort issue in practice is summer gain on a south facing roof, which is what a solar control coating is for.