How much does a roof lantern cost?

What moves it

Whether the opening exists. Fitting into a sound existing kerb is the cheapest version of this job by a wide margin. Forming a new opening in a finished roof is a different order of work.

Structure. A wide opening usually needs a steel, which has to be designed by an engineer, supplied and installed. On a large lantern the steel can cost more than the lantern.

Access. A single storey flat roof reached from a tower adds nothing. Anything higher, or over a conservatory or a fragile roof, usually needs scaffold, and scaffold is a real line rather than a rounding error.

Glass. Solar control, triple glazing and acoustic laminates all move the unit price. Solar control is the one worth paying for on a south facing kitchen.

Size, but less than you think. Doubling the glass area does not double the price, because a large part of the cost is labour, access and structure rather than glass.

Two smaller against one large

Worth pricing both. Two smaller lanterns often light a long room better and need less structure, which sometimes makes them cheaper as well as better. Ask for both on the quotation.

How we price is on the costs page.

Where the money goes

On a new extension a lantern is a moderate item. On a retrofit into an existing flat roof it is a substantial project, and the difference is almost entirely structural rather than glazing.

An indicative range comes by email from a photograph; a fixed price follows a survey, and we will not price a lantern without one.

The lantern itself

Manufactured to your dimensions, priced by size, frame material and glazing specification. Cost rises steeply rather than proportionally with size, because larger panes need thicker glass and a heavier frame.

Solar control glazing, acoustic laminate and an opening vent are each a step up. So is a bespoke shape or a hipped rather than a ridged configuration.

The structure

Frequently the largest single line on a retrofit and nothing to do with the glass.

On a new extension the opening is designed in and the joists either side are doubled up: little or no extra cost. On an existing roof, cutting joists means a designed trimmer, and beyond a certain span that means steel, plus an engineer’s fee.

Which way the joists run relative to the opening changes this more than the size of the lantern does, which is why it is established at survey.

The kerb

Forming it to at least 150mm, square and level, insulated on its outer face, with the covering dressed up it correctly. Where the existing roof has no fall or falls the wrong way, correcting it locally around the kerb is part of this line.

Access

A large lantern pane can weigh several hundred kilograms and cannot be manhandled. Crane or glass robot hire and a route to the roof are a real cost, and on a property with no side access they may constrain what size is realistic at all.

Making good

Forming the internal reveal, plastering and decorating. On a lantern this is a larger area than on a rooflight and it carries drying time before decoration.

What to compare

Ask any quotation to itemise: the lantern with dimensions and glazing specification; the structural work and whether an engineer’s fee is included; the kerb including insulation; access and how the unit reaches the roof; making good; disposal; and Building Control.

A single number against lantern supplied and fitted is not comparable with anything, and on a retrofit a quotation that does not mention structure has not been thought through.

The cheaper alternative worth considering

Two or three flat roof rooflights in a row. On a long narrow room they light it more evenly than one lantern, cost considerably less, need far less structural work, and fail independently.

A lantern earns its cost where the room is broad, the ceiling is flat and the volume it adds is the point.

Why every lantern is priced individually

There is no catalogue size. A lantern is made to the dimensions of your kerb, in your configuration, with your glazing specification, so a price comes from a specification rather than a list.

What moves the number

Size, and not linearly. Beyond a certain span the glass and the frame sections step up, and the cost steps with them.

The number of panes. A finely divided traditional lantern has far more frame than a modern one of the same footprint. More frame costs more and performs worse thermally, which is a real trade against the appearance.

Glazing specification. Solar control glass, a better spacer, self cleaning coating. Each is a modest addition and together they are worth having on a south facing lantern.

Opening vents. A powered vent adds the actuator, the control system, the electrical work and the certification.

Finish. Standard colours cost less than a specified colour, and a dual colour finish, dark outside and light inside, costs a little more and is frequently worth it.

The lines that are not the lantern

Structural design and steel around the opening. The kerb, built and insulated. The survey to the finished kerb. Access, and mechanical lifting where the panes are too heavy to carry through the house. Making good internally.

On a rear extension behind a terrace with no side access, the logistics of getting large panes to the roof can be a significant cost of its own, and it is worth establishing early rather than at delivery.