What Size Rooflight Do I Need? A Complete UK Sizing Guide

Choosing the right size flat roof skylight is one of the most important decisions in any extension or loft conversion project. A rooflight that is proportioned correctly for the room brings in consistent natural light without glare, overheating or a visual imbalance overhead. This guide covers a practical method for calculating the right rooflight size for any room, how different room types affect the calculation and when a standard size will work and when a custom size is needed.

The most widely used guideline in the UK is that the glazed area of a rooflight should represent 15 to 20 percent of the floor area of the room below. This proportion delivers a well-lit space without causing glare or significant heat gain on south-facing roofs.

How to Calculate Your Rooflight Size

Measure the floor area of the room (length x width in metres)
Multiply by 0.15 for the minimum glazed area and by 0.20 for the maximum
This gives the target glazed area in square metres
Choose a standard size rooflight with an area within this range, or combine multiple smaller units to reach the target area

Example: A 4m x 5m kitchen extension has a floor area of 20m². The target glazed area is 3.0m² to 4.0m². A single 1000x3000mm rooflight (3.0m²) sits at the lower end of this range. Two 1000x1500mm rooflights combined (3.0m²) achieve the same total area with a more balanced distribution of light across the room.

Note: The 15 to 20 percent rule is a general guideline, not a building regulation requirement. The actual minimum and maximum glazed area allowed will depend on local planning conditions, building control requirements and the specific roof structure. Always confirm with your building control officer for new builds and extensions.

Kitchen Extensions

Kitchen extensions benefit from generous overhead light, particularly over the worktop and island areas where task lighting is needed throughout the day. Aim for the upper end of the 15 to 20 percent range. A single large rooflight positioned above the kitchen island or a row of smaller units along the length of a narrow extension both work well. For south-facing kitchens, solar control glass is the appropriate specification to manage heat gain rather than increasing the rooflight size.

Living Rooms and Open-Plan Spaces

Open-plan living spaces often perform well at 15 percent of floor area because the space is used for relaxation rather than focused tasks. A single rooflight positioned centrally or towards the far end of the room from the main window draws light deep into the space. For large open-plan rooms, a roof lantern is a strong alternative to a flat rooflight — the raised ridge profile draws light from multiple angles and creates a sense of height that a flat rooflight cannot replicate at the same footprint.

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Bathrooms

Bathrooms do not require the same glazed area as kitchens or living rooms because they prioritise privacy over generous daylight. A 500x500mm or 600x900mm rooflight is typically sufficient for most domestic bathroom sizes. Satin privacy glass allows light through while obscuring the view from above.

Hallways and Corridors

Hallways are often the most underlit spaces in an extension because wall windows are limited. A rooflight positioned above a hallway or landing can transform a dark transitional space into a naturally lit route through the property. A 600x900mm or 1000x1000mm unit suits most standard hallway widths.

Bedrooms

Bedrooms benefit from natural light in the morning but may require blackout blinds in summer if the rooflight faces east or south. Size the rooflight at the lower end of the 15 to 20 percent range for bedrooms and consider solar control glass for south or west-facing roofs to manage early morning or late afternoon heat gain.

Standard flat rooflights from Rooflights and Skylights UK are available in internal dimensions from 500x500mm up to 1000x3000mm. The full standard range includes:

500x500mm — 0.25m²
600x900mm — 0.54m²
600x1200mm — 0.72m²
1000x1000mm — 1.0m²
1000x1500mm — 1.5m²
1000x2000mm — 2.0m²
1000x2500mm — 2.5m²
1000x3000mm — 3.0m²

Both framed and frameless rooflights are available across the full standard size range. Standard sizes are dispatched within 3 to 4 working days with free delivery to all UK mainland addresses.

When to Choose a Custom Size

A custom size rooflight is needed when the internal opening dimension falls outside the standard range, or when the precise size of the glazed area matters for structural or aesthetic reasons. Common scenarios include:

The internal opening is wider or longer than 1000x3000mm
The opening falls between standard sizes and a precise fit is required
The architect or project specification calls for an exact dimension
The existing structural opening cannot be modified to match a standard size

Custom size rooflights from Rooflights and Skylights UK are available in any dimension and are delivered within 7 to 10 working days. Contact our team on 0331 6303 671 with your internal opening dimensions for a quote and confirmed lead time.

How Glazing Type Affects Perceived Light Levels

The glazing specification affects how much light the rooflight delivers at a given size.

Clear Low-E glass: Maximises light transmission. A clear glazed rooflight at 15 percent of floor area will feel notably brighter than the same size unit with solar control glass.
Solar control glass: Reduces light transmission slightly in exchange for managing summer heat gain. Appropriate for south-facing roofs where overheating is a consideration.
Satin privacy glass: Diffuses light across the room rather than creating a direct beam of sunlight. The perceived light level is comparable to clear glass but the quality of light is softer and more even.

If specifying solar control or privacy glass, consider adding 10 to 15 percent to the target glazed area to compensate for the reduced light transmission and maintain comfortable light levels in the room.

Multiple Rooflights vs One Large Unit

For rooms where the target glazed area exceeds 3.0m², two or three smaller rooflights often perform better than a single very large unit. Multiple smaller units distribute light more evenly across the room, reduce the structural opening required in any one location and allow light to be directed to specific zones within the space.

For rooms where a single architectural overhead feature is the design intent, a roof lantern starting from 1000x1000mm delivers light from multiple angles and creates a genuine sense of height and volume that a flat rooflight of the same footprint cannot match.

Standard sizes from 500x500mm to 1000x3000mm, custom sizes available in any dimension. Framed and frameless designs, standard sizes dispatched in 3 to 4 working days with free UK mainland delivery and a 10-year guarantee. Browse the full range at rooflightsandskylightsuk.co.uk/collections/rooflights-skylights

The most widely used guideline is that the glazed area of a rooflight should represent 15 to 20 percent of the floor area of the room below. This delivers a well-lit space without causing glare or heat gain. For a 4x5m room with a 20m² floor area, the target glazed area would be 3.0m² to 4.0m².

For a kitchen extension, aim for the upper end of the 15 to 20 percent range. For a typical 4x4m kitchen extension (16m² floor area), the target glazed area is 2.4m² to 3.2m². A single 1000x3000mm rooflight (3.0m²) or two 1000x1500mm units combined (3.0m²) both suit this room size.

Standard flat rooflights from Rooflights and Skylights UK are available in internal dimensions from 500x500mm up to 1000x3000mm, including sizes at 600x900mm, 600x1200mm, 1000x1000mm, 1000x1500mm, 1000x2000mm, 1000x2500mm and 1000x3000mm. Custom sizes are available for openings outside this range, delivered within 7 to 10 working days.

Yes. Custom size rooflights from Rooflights and Skylights UK are available in any dimension. Custom orders are delivered within 7 to 10 working days to all UK mainland addresses. Contact our team on 0331 6303 671 with your internal opening dimensions for a quote.

Always measure the internal opening, not the external upstand. The internal opening is the clear aperture inside the upstand, measured from the inside face on all four sides in millimetres. This is the dimension to use when ordering. Never use the external footprint of an existing rooflight or the outside dimension of the upstand.

Yes. Solar control glass and satin privacy glass reduce light transmission slightly compared to clear Low-E glass. If specifying either of these glazing types, consider adding 10 to 15 percent to the target glazed area to compensate for the reduced light transmission and maintain comfortable light levels in the room.

A rooflight proportioned correctly for the room consistently outperforms one that is too large. Oversizing can cause glare, overheating on south-facing roofs and a ceiling that feels visually dominated by glass. The 15 to 20 percent rule provides a well-tested range that balances light quality, thermal performance and visual proportion. For south-facing roofs where heat gain is a concern, solar control glass is a more effective specification than simply reducing the rooflight size.

A 600x900mm or 1000x1000mm rooflight suits most hallways and landings, transforming what's often the darkest part of an extension into a naturally lit route through the property. Bathrooms need less glazed area since privacy matters more than generous daylight — a 500x500mm or 600x900mm unit is typically enough, paired with satin privacy glass to let light in while obscuring the view from above.