A roof lantern quote is not one number. It is seven or eight separate costs added together, and the reason two quotes for what looks like the same lantern can land £1,500 apart is almost always that one of them is missing a line item, not that one installer is cheaper. Supply-and-fit for the lantern itself typically runs from around £2,400 for a small 1.0 x 1.0m unit to £8,400 for a 3.0 x 2.0m aluminium lantern, on a sound structure with reasonable access. On a retrofit into an existing flat roof, structural work, access and making good can add a comparable amount again. This page breaks those costs into their parts so you can see exactly where the money goes.
What actually makes up a roof lantern quote
Every fixed quote we write is built from the same set of line items. Not every job needs all of them. A new-build kitchen extension with the ring beam already in the structural drawings skips the structural opening cost almost entirely. A retrofit into a 1990s flat roof usually cannot.
- Unit cost. The lantern itself: frame, glazing, factory finish and the glazing specification you have chosen.
- Upstand or kerb works. Building or rebuilding the insulated kerb the lantern sits on.
- Structural opening and engineer’s fee. Trimmers or a ring beam, and the calculation that specifies them.
- Glazing upgrades. Solar control coatings, triple glazing, self-cleaning outer panes, tint or acoustic layers.
- Access. Tower scaffold for single-storey work, full scaffold for two-storey or restricted sites.
- Making good. Internal reveals, plasterboard, skim and trims once the frame is in.
- Disposal. Skip hire and removal of the old lantern, old roofing material and site waste.
- Building Control fee. The local authority charge for a building notice or full plans application on retrofit work.
The line items, in detail
Unit cost
This is the biggest single number and it scales with glazed area, frame material and glazing specification rather than a flat per-square-metre rate. A 2.0 x 1.0m lantern is not simply double the price of a 1.0 x 1.0m unit, because the smaller lantern carries proportionally more frame relative to glass and the larger one needs a stronger bar section to span the extra width. Aluminium with solar control double glazing is our default specification and the baseline every other figure on this page assumes.
Upstand and kerb works
On a new-build extension the kerb is built once, as part of the roof structure, and its cost sits inside the main fitting figure. On a replacement, the existing kerb has to be exposed and inspected before anyone can say whether it needs a repair or a full rebuild. A sound kerb needing only a new covering adds very little. A rotten timber kerb needing full reconstruction, insulation and a new membrane can add a meaningful sum, and it is genuinely not knowable from a phone call. Our page on how a roof lantern upstand is built covers the construction detail behind this line item.
Structural opening and engineer’s fee
On openings up to roughly 1.2m, doubled timber trimmers usually manage the load and this cost stays modest. Beyond 1.5m, a ring beam is typically needed, and an engineer’s calculation is required before Building Control will sign it off. The calculation itself is not expensive, generally a few hundred pounds. The ring beam construction, in steel or engineered timber running the full perimeter of the opening, is the bigger figure and it is genuinely variable depending on span, roof loading and how the load transfers into the existing structure. This is consistently the single biggest source of quote-to-quote variation on retrofit lanterns.
Glazing upgrades
Triple glazing typically adds roughly 15 to 25 per cent to the unit cost and around 40 per cent to the glass weight, which can in turn affect the structural calculation. A self-cleaning outer coating adds a modest amount and is worth it on anything hard to reach. An electric opening vent, factory-fitted, typically adds £600 to £1,100 depending on the system and how close the nearest power supply already is. Dual-colour finish, grey outside and white inside for example, typically adds £250 to £500.
Access and scaffold
A single-storey extension with a clear approach usually only needs a tower, priced modestly for the duration of the job. Two-storey access, a restricted side passage, or a roof that needs full scaffold rather than a tower changes this figure considerably, and it is quoted once the survey has seen the actual approach to the roof rather than guessed from a description.
Making good and plastering
Once the frame is in, the internal reveals need forming, plasterboarding, skimming and trimming to a finish that matches the rest of the ceiling. On a straightforward rectangular opening this is a modest, fairly predictable cost. On a large lantern with deep reveals, or where the existing ceiling needs matching to an unusual profile, it costs more. Plaster drying time also sits outside our programme and is worth building into your own schedule if decorators are booked separately.
Disposal
Old lanterns, old roofing felt or membrane, cut-off timber and packaging all need removing from site. On a replacement this is a fixed, fairly small cost. On a job where the old lantern is aluminium with old sealed units, there is sometimes a modest credit for scrap value, though it rarely moves the total noticeably.
Building Control fee
We are not part of a competent-person scheme, so any retrofit lantern goes through your local authority as a building notice or a full plans application. The fee itself is a local authority charge, not ours, and it is a genuinely modest line relative to the rest of the job, typically a few hundred pounds depending on which route and which council. New-build extension work is usually covered under the extension’s own Building Control application, so this line does not apply twice.
What pushes a quote up
| Variable | Typical effect |
|---|---|
| Timber frame instead of aluminium | Adds 40 to 70 per cent to the unit cost |
| Triple glazing | Adds roughly 15 to 25 per cent to the unit cost |
| Marine specification coating and 316L fixings | A modest addition, standard within a mile of open sea |
| Electric opening vent | £600 to £1,100 fitted |
| Dual-colour finish | £250 to £500 |
| Ring beam or structural upgrade | Quoted separately once the engineer’s design is confirmed. The largest variable on retrofit work |
| Two-storey or restricted access | Quoted separately once the approach has been surveyed |
| Conservation-style timber unit for a listed building | Quoted individually, always |
Location plays a smaller role than most homeowners expect. A lantern at Felixstowe or Aldeburgh costs a little more than the same lantern at Stowmarket, because of marine-grade fixings and coating, but that difference is modest against the structural and access variables above. What actually separates a £5,000 job from a £9,000 job on paper-identical lantern sizes is almost always structure, access or a glazing upgrade, not the postcode.
A worked example: 2.5 x 1.5m lantern into an existing flat roof
Take a genuinely common enquiry: a 2.5 x 1.5m aluminium lantern going into a 1990s single-storey kitchen extension with a felt flat roof, on a site with straightforward access for a tower. The kerb inspection finds it undersized and not insulated, so it needs rebuilding rather than repairing, and the opening at 1.5m across needs a modest ring beam rather than trimmers alone.
| Line item | Typical range |
|---|---|
| Unit supply and fit, solar control double glazing | £4,600 to £6,000 |
| Structural opening: engineer’s calculation and ring beam | £800 to £1,800 |
| Access: single-storey tower | £300 to £600 |
| Making good and internal plastering | £300 to £600 |
| Disposal | £150 to £300 |
| Building Control fee | £200 to £400 |
| Approximate total | £6,350 to £9,700 |
Compare that with a new-build extension of the same lantern size, where the ring beam is already part of the roof’s structural design and the kerb is built once, correctly, from new. There, the structural opening line and most of the kerb-rebuild cost drop out, and the total sits much closer to the unit’s supply-and-fit figure alone. That is the practical reason a new extension and a retrofit into an old roof can carry very different totals for what is, on paper, the same size lantern.
The survey removes the guesswork. Everything above the “unit cost” line is a range because it genuinely cannot be fixed without seeing your roof. Once we have surveyed it, every one of these line items becomes a single number on a written quote, and that number does not move once you have accepted it.
What this does not cover
This breakdown is for a standard aluminium lantern. Timber units for period and listed properties are quoted individually because the specification, from bar profile to consent requirements, is set case by case. Our page on timber roof lanterns for period and listed properties covers what changes for that route. And if you are still deciding between a lantern and a flat rooflight before any of these numbers become relevant, our comparison of roof lantern versus flat rooflight sets out where a flat unit is the cheaper and better answer. For how the actual fitting programme runs once you have a fixed price, see the roof lantern fitting process, stage by stage.
Why is the ring beam not included in the published lantern price?
Because it depends entirely on your specific roof, in a way a lantern’s glazing and frame do not. Span, existing joist condition, and how the load transfers into the walls below all vary from one house to the next, and none of that is knowable before a survey. Quoting it separately, after an engineer’s calculation, is more honest than folding a guessed figure into a headline price.
Does a small lantern still need a structural opening cost?
Usually not much of one. Openings up to around 1.2m are typically managed with doubled timber trimmers, which is a modest cost rather than a ring beam. The structural line item grows significantly once you are past about 1.5m, which is where a ring beam and an engineer’s sign-off generally become necessary.
Is the Building Control fee the same for every job?
Roughly, though it depends on your local authority and whether the job goes through as a building notice or a full plans application. New-build extension work is usually covered under the extension’s own application, so a retrofit lantern is the case where this line item most often applies on its own.
Why do two quotes for the same size lantern differ so much?
Almost always because one quote is missing a line item, not because one installer is simply cheaper. A price that only covers unit supply and fit, with the kerb, structural work, access and making good quoted as extras later, will always look lower on the day than a quote that has priced the whole job up front.
For the full published price table by lantern size, see the main roof lantern installation page, or our costs page for prices across every service we offer. To get every line item above turned into one fixed number for your own roof, request a fixed quote and we will survey it properly before we price anything.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.