What a roof lantern costs, and why

The first thing a lantern quote reflects is the opening it covers. A bigger lantern uses more glass, more frame and more of the roof, so the cost rises with it, though not in a straight line. Glazed units are made to order, and above a certain span the glass has to be thicker or the frame has to carry more glazing bars, which adds weight as well as material.

Shape matters almost as much as size. A square lantern on four equal hips is the simplest form. A rectangular lantern adds a ridge between the two hipped ends, and the longer that ridge, the more frame there is. Lanterns built around a corner, in an L-shape, or with one end butting a wall rather than hipped need extra pieces and extra flashing where they meet the house, and each adds its own lines to the quote.

Letting the room set the size

We size a lantern from the room below it. The aim is to put daylight where the windows stop reaching. In a long single-storey side extension with garden doors at one end, that is the stretch furthest from the doors, and a lantern placed there can be smaller than one centred over the whole room and still light more of it. A well-placed smaller unit means less glass and less frame on the quote.

The glazing is where specifications vary most between quotes. A new or replacement roof lantern has to meet Part L of the Building Regulations, which sets a limit of 2.2 W/m²K for a rooflight or lantern on an existing home. Double glazing with a low-emissivity coating, a good warm-edge spacer and an inert gas fill, in a suitable frame, can meet that for the whole unit. Triple glazing goes further, keeping the room warmer near the glass on a winter night and cutting down on condensation, but it is heavier and dearer, and the frame has to be designed for it.

On top of the basic unit come coatings and layers. Solar control glass reflects some of the sun's heat, which matters on a south or west-facing extension that can get warm on a summer afternoon. A tinted or blue-grey finish does a similar job and changes the colour of the light a little. A self-cleaning outer coating breaks down dirt so that rain washes it off more easily. A laminated inner pane holds together if it breaks, which is a sensible choice over a kitchen or a dining table.

None of these extras is compulsory, and some homes need none of them. A north-facing lantern over a side extension may want as much light as it can get, so solar control would work against it. We pick the glass at the survey, after looking at which way the roof faces, what shades it and what the room is used for.

A lantern does not sit straight on the roof. It sits on a kerb, or upstand, which lifts it clear of rainwater and gives the roof covering something to turn up against. Some lanterns come with a factory kerb; others go on a timber kerb built on site and insulated to match the roof. Either way the kerb is part of the price, and so is the way the roof covering is dressed up its sides.

The covering itself matters. A felt roof, a rubber membrane and a fibreglass roof each need a different method to join the new kerb into the old surface. If the existing covering is near the end of its life, patching a new kerb into it is a false economy, and the quote may include a fresh area of covering around the lantern, or a full recover if that is the only sensible route, with the area to be renewed marked on the quote.

Warm roof or cold roof

How the roof is built changes the work too. In a warm roof the insulation sits on top of the deck, under the covering, and the kerb has to carry that insulation up its sides. In a cold roof the insulation sits between the joists, with a ventilated gap above it, and cutting a hole means making sure that ventilation still works around the new opening. We look at the roof from above and, where we can, from inside, so the quote is based on what is really there.

Structural work around the opening

Unless it happens to fit between two joists, a lantern opening cuts through at least one. The joists either side then need trimming: new timbers fixed across the gap to carry the load of the ones that were cut. A modest opening running the same way as the joists can need very little. A wide opening across them, or one in a roof that was only just strong enough to begin with, may need deeper timbers, a steel beam or a calculation from a structural engineer. It can be the biggest single difference between lanterns of the same size on different houses.

A single-storey extension with a clear path down the side of the house is a straightforward place to work from ladders or a small tower. A mid-terrace house with no side passage is different: every tool, length of timber and glazed unit has to come through the house, and the larger sections of a lantern may need lifting over the roof instead. Where the extension roof can only be reached from above a neighbour's garden, or where a large lantern needs mechanical lifting, the quote will show it.

Inside, the opening has to be finished. The sides of the shaft between the ceiling and the lantern, called the reveals, are lined and plastered so the light spills cleanly into the room. If there is a pendant light or a run of downlights where the opening will be, they may need moving by an electrician. Decoration is usually left for you or your decorator, and the quote should state where our work stops.

One way to keep this part of the cost down is timing. If you are building an extension, fitting the lantern while the roof is going on avoids cutting into a finished roof and ceiling later. When the extension is already up and decorated, that saving has passed, but careful planning of the route through the house still keeps the mess and the hours down.

Forming a new opening in a roof and glazing it is building work. The structure, the energy standard of the glass and, in some rooms, fire safety all have to meet the Building Regulations, so the work is notifiable. The council's building control team or a registered building control approver checks it and signs it off, and they charge for doing so. We handle the Building Control notification where the work is notifiable, and your quote shows that fee as a line of its own.

Planning is a separate matter. A roof lantern on a flat roof usually stands more than 150 mm above the roof plane, which puts it outside the Class C permitted development allowance for rooflights. In practice a lantern then normally requires a planning application unless the council confirms otherwise, or the lantern is already shown on the approved plans of an extension that has planning permission. An application carries its own fee and, sometimes, drawings; our guide to roof lantern planning and Building Regulations sets out when each applies, and your house's position is checked before anything is ordered.

Replacing an old lantern like for like can be simpler on both counts, but a new unit still has to meet the current energy standard, so even a replacement is worth discussing before it is priced.

A good lantern quote reads like a description of the job, listing what is and is not included alongside the total.

The work comes with our 10-year workmanship guarantee in writing. If you are weighing a lantern against a flat rooflight or a roof window, our guide on roof lantern or VELUX window may help, and if your roof is already finished, see adding a lantern to an existing roof.

When you are ready, send a photo of the room and the roof on WhatsApp to 07880 284133, or book a free survey and we will come and measure.