The right roof lantern size is usually one whose glazed area covers around 10–20% of the ceiling, in proportion to the room and centred over its main feature. To measure, take the width and length of the roof opening in several places, check the diagonals, and allow for a kerb or upstand around 150mm high. The room's orientation, roof structure and what sits beneath the lantern all affect the final choice.
Various factors contribute to the size of your roof lantern, such as the roof’s structure, position, and where the room faces.
How Do You Choose the Right Roof Lantern Size?
Choose a roof lantern whose glazed area is roughly 10–20% of the ceiling area, in proportion to the room and centred over its focal point. Go larger in north-facing rooms that get less direct sun, and check the roof structure can take the weight before settling on a size.
Picking the correct size for your roof lantern depends on your taste, but it is also influenced by what you’re working with. You would usually see roof lanterns installed in the middle of the room, but a flat roof accommodates installation in any area.
Let’s look at the factors that determine the size of your roof light.
Which way is the room facing?
The direction your room faces greatly affects the size of roof lanterns. For example, north-facing roof lanterns will only get sun in the morning, whereas south-facing roof lanterns usually get sunlight during the day, so sizing should balance daylight with energy efficiency for better comfort and thermal control.
This means the north-facing room would benefit from a larger roof lantern. Have a look at our other blog to answer the question: does a roof lantern let in more light?
Will Privacy or Planning Affect the Size?
When thinking about the planning permission, also consider your privacy. Some plans may be rejected if your roof lantern exposes too much of your home. On the other hand, you don’t want the roof lantern to be too big that it can be peered into if your home is overlooked. You just may be letting in more than just natural light!
Can Your Roof Structure Support a Large Lantern?
If you have an old house, it may not be strong enough to hold a big roof lantern. You must evaluate your existing roof structure, check for rafters and other obstructions, and rule out structural issues before determining the size of roof lights you need. This helps greatly.
If you’re thinking ahead, work out how the roof lantern will increase your house’s value. A well-placed and right size roof lantern can elevate your house spec significantly.
What is below the roof lantern?
You should also consider what will go straight below the roof lantern, as it will be exposed to direct sunlight. This is often a kitchen counter, kitchen island or dining table. No matter what it is, the roof lantern will place a lot of attention to it, so deciding on the size is important.
For example, you may want your roof lantern to be big enough to run parallel with the bifold doors, and this will allow your roof lantern to blend in effortlessly with the style of the room and enhance the space.
When judging whether a large or small roof lantern will look balanced, consider the room layout. Large roof lanterns can be overwhelming, look out of proportion if it is a small room, and look flashy instead of classy. However, a small roof lantern can be easily forgotten and even pointless in a large space.
How Do You Measure for a Roof Lantern?
Measure the width and length of the roof opening in at least three places, check the diagonals match to confirm it is square, and allow for a kerb or upstand around 150mm high and 100mm wide. Roof lantern sizes are usually quoted as the external kerb size, so give your supplier the kerb dimensions.
If you have looked at our different types of roof lanterns and are now asking, "what size roof lantern do I need"? Don't worry: we have you covered. Roof lantern sizes are normally quoted as the external kerb size, so the roof opening itself must be measured correctly, not just the gap, and the internal dimensions should match the product specification for a perfect fit.
Taking the correct measurements of your roof is essential when deciding on the size of your glass roof lantern. For accurate measurements on site, use a tape measure when measuring the opening, and give us the width x length of the area, also allowing for the overall width where the lantern sits on a timber upstand.
Measure the width and length in multiple places and note any variation.
Verify that the opening is square by measuring from corner to corner, and double check the figures for a snug fit.
Our advice is to compensate for an upstand of 150mm tall and 100mm wide, with the width being in the measurements you give us. The edges of the roof lantern must sit properly on this upstand, with some overhang for adequate rainwater runoff and weathering.
The external kerb must stick out 150mm from the roof for this as well, and the external upstand size should include the frame footprint rather than just the hole. You can find more tips in our guide to installing a roof lantern on a flat roof.
Allow for the thickness of any roofing membrane or waterproof membrane, as well as insulation, to prevent water ingress and potential leaks.
What Glass Options Are Available for Roof Lanterns?
Most roof lanterns use double-glazed units with toughened outer glass and laminated inner glass, low-E coatings and argon fill, with triple glazing, solar-control and self-cleaning glass available as upgrades. Your glazing choice affects the U-value, weight and price of the lantern.
Not only will the type of glass play a role in the comfort of the room, but it also affects the size of the lantern.
You can use toughened glass on the outside and laminated glass on the inside, which can accommodate most sizes of roof lanterns.
The outside glass is very strong, and laminated glass can still hold together if broken - a vital safety feature. You can also expect a soft-coat low-emissivity coating, argon gas fill, and warm edge spacer bars.
You can also upgrade the glazing to improve insulation, with the u value varying between glazing choices and affecting thermal performance. High-specification, double-glazed or triple-glazed lanterns can be quite costly, so you have to be sure of your size before purchasing.
Most glass lanterns these days offer the options of solar control and self-cleaning glass. You can get these together or separately. Compare the options in our guide to the different types of roof lanterns.
Is a Roof Lantern Worth It?
Yes, for most flat-roofed kitchens and extensions a correctly sized roof lantern is worth it: it brings far more daylight into the centre of the room, makes the ceiling feel higher and adds an architectural feature buyers like. See the full list of benefits of roof lanterns.
With the correct size, a roof lantern can transform a room and add value to your home. They complement anything set below it, whether it is a dining room, aluminium door, or windows.
Lanterns are popular with homeowners for the following reasons:
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The perfect roof lantern can improve the room’s appearance and comfort while making it feel brighter and bigger.
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It alters the room dimensions and increases the roof height.
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It is aesthetic and a unique way to add natural light to your home, which increases its value.
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Lanterns are energy-efficient, retain heat, and save money on energy bills.
Roof Lantern Size FAQs
What size roof lantern do I need for a 6m x 4m room?
For a 6m x 4m room (24m²), a roof lantern of roughly 2000mm x 1000mm to 3000mm x 1500mm usually gives good, even daylight without overheating, which is around 10–20% of the ceiling area.
In extensions, building regulations generally limit the total area of windows, doors and rooflights to 25% of the floor area (plus the area of any openings the extension covers), so balance the lantern size against your doors.
What are the common roof lantern sizes?
They are available in standard sizes, from compact options such as 1000mm x 1000mm up to larger formats like 2000mm x 4000mm, alongside many choices in styles, shapes, and colours. Bespoke sizes are widely available for non-standard openings.
How much is a roof lantern?
A new roof lantern cost varies depending on the size, glazing, and glass type. Prices start from around £350 for a small uPVC lantern and around £600 for aluminium, rising past £2,000 for large or high-specification units before installation. See our roof lantern cost guide for a full breakdown.
Do I need planning permission for a roof lantern?
Not always. Permitted development only allows rooflights that project no more than 150mm above the existing roof plane, so a lantern added to an existing flat roof often needs planning permission, whereas one included in a new extension is usually covered by that extension's approval. Permitted development conditions include:
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It must not stick out more than 150mm from the roof.
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It is not higher than the highest point of your house.
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Any side-facing glazing is obscure-glazed and non-opening below 1.7m above floor level.
If there is any doubt about the roof pitch or whether necessary adjustments are needed, consult a professional before installation.
This applies to average UK houses; however, converted houses, flats, and maisonettes may differ. You should look into the building regulations in these cases.
How high are roof lanterns?
Height depends on width and pitch. A typical lantern with a 25-degree pitch rises roughly 230mm for every 1000mm of width, so a 2000mm-wide lantern stands around 450–500mm above its upstand. Add the kerb, usually around 150mm above the finished roof, for the total height.
What size roof lantern do I need for a 4m x 4m room?
A square room of around 16m² usually suits a square lantern of about 1500mm x 1500mm to 1800mm x 1800mm, centred in the ceiling. This keeps the proportions balanced and brings light evenly to all four walls.
Is it better to have one large roof lantern or two smaller ones?
One large lantern makes a stronger focal point, while two smaller lanterns spread light more evenly across long rooms and may avoid heavier structural steelwork. Multiple lanterns cost more in units and labour, as our roof lantern prices guide explains.
