What is the minimum window size for a room in the UK?

Under Part L of the Building Regulations, habitable rooms (bedrooms, living rooms) and kitchens should have a glazing area of at least 1/6th (16.7%) of the floor area. For example, a 15 m² bedroom needs at least 2.5 m² of window area. Bathrooms do not have a mandatory minimum but natural light is recommended where possible.

What is the glazing-to-floor area ratio?

The glazing-to-floor area ratio is the total window area divided by the room floor area, expressed as a percentage. For example, 3 m² of windows in a 15 m² room gives a ratio of 20%. Part L requires a minimum of 16.7% (1/6th) for habitable rooms. Part O flags overheating risk above 25%.

What is Part O and how does it affect window sizes?

Part O (Overheating) was introduced in the 2022 Building Regulations and applies to new residential buildings in England. It limits solar gains to prevent overheating, particularly relevant for south and west-facing rooms with large areas of glazing. Rooms with glazing exceeding 25% of floor area on these orientations may fail the simplified Part O assessment and require mitigation such as solar shading.

What is a daylight factor and what is a good target?

The daylight factor is the ratio of indoor illuminance to outdoor illuminance on an overcast day, expressed as a percentage. BS EN 17037 recommends a minimum of 2% for habitable rooms and a target of 3% for bedrooms and 4% for living rooms. A room with a 2% daylight factor receives about 2% of the available outdoor daylight.

How does room depth affect natural light?

A common rule of thumb is that a room should not be deeper than 2.5 times the window head height. For a standard window with a head at 2.1 m, the room should not be deeper than about 5.25 m for adequate daylight at the back. Deeper rooms will need additional measures such as roof lights, light pipes, or higher windows to achieve good daylight levels.

Natural Light Calculator — Does My Room Meet Part L Daylight Requirements?

Natural Light Calculator — Check Part L Daylight Compliance

Calculate the glazing-to-floor area ratio for your room and check compliance with Part L (daylight), Part O (overheating), and BS EN 17037 daylight factor standards.

Room Floor Area

Total floor area of the room

Total Window Area (glazed area)

Total glazed area of all windows in the room

Room Type

Different room types have different requirements

Room Depth (optional)

Distance from window wall to back of room

How We Calculate This

This calculator checks three aspects of natural light in your room: the glazing-to-floor area ratio (Part L), estimated daylight factor (BS EN 17037), and overheating risk (Part O).

Part L — Daylight Requirement

Habitable rooms and kitchens require glazing area of at least 1/6th (16.7%) of floor area. Bathrooms have no mandatory minimum.

Daylight Factor Estimate

We provide a simplified daylight factor estimate based on window area, floor area, and an assumed transmittance of 0.6 for double glazing. This is an approximation — accurate daylight factor calculations require 3D modelling software considering room geometry, surface reflectances, and external obstructions.

Part O — Overheating

Glazing exceeding 25% of floor area, particularly on south and west-facing elevations, is flagged as a potential overheating risk under Part O (2022). Mitigation measures include external shading, openable windows, and reduced glazing area.

References

Approved Document L — Conservation of fuel and power
Approved Document O — Overheating (2022)
BS EN 17037:2018 — Daylight in buildings

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Natural Light Calculator — Check Part L Daylight Compliance

How We Calculate This

Part L — Daylight Requirement

Daylight Factor Estimate

Part O — Overheating

References

Frequently Asked Questions

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