What is the minimum window size for a loft room?

Under Approved Document L (Conservation of fuel and power), habitable rooms must have a glazed area of at least 1/6th of the floor area. For a 12 m² loft bedroom, you would need at least 2 m² of glazing. If the room is used as a bedroom or is above the ground floor, it must also meet Part B escape window requirements — a minimum clear opening of 0.33 m² with no dimension less than 450mm.

What Velux sizes are available?

Velux windows come in a range of standard sizes identified by a two-letter code. Common sizes include: CK02 (550x780mm), MK04 (780x980mm), MK06 (780x1180mm), SK06 (1140x1180mm), and UK08 (1340x1400mm). The first letter indicates width (C=550mm, M=780mm, S=1140mm, U=1340mm) and the numbers indicate height. Multiple windows can be combined side-by-side or stacked vertically for larger openings.

What flashing kit do I need for my roof?

Velux flashing kits come in several profiles to match your roof covering. Standard profile (EDZ) suits flat tiles and slates. High-profile (EDW) suits interlocking concrete tiles with a deep profile. Low-profile (EDL) suits slate and flat roof coverings below 15° pitch. Recessed (EDN) kits allow the window to sit flush with the roof covering. Always match the flashing kit to your roof material and pitch for a watertight seal.

Can I install a roof window without planning permission?

In most cases, roof windows can be installed under Permitted Development rights without planning permission, provided: the window does not project more than 150mm beyond the existing roof plane; it is not on the front elevation in a conservation area; and the glass is obscure-glazed and non-opening if it overlooks a neighbour (side elevation). However, if you are creating a new habitable room in the loft, you will likely need Building Regulations approval regardless of planning.

Do loft windows need to be escape windows?

If the loft room is used as a habitable room (bedroom, study, etc.) and is at first-floor level or above, Part B of the Building Regulations requires an escape window. The window must have a clear opening of at least 0.33 m², with no dimension less than 450mm. The bottom of the opening should be between 600mm and 1100mm above the floor. Most Velux windows in sizes MK04 and larger meet these requirements when fully open.

Roof Window Sizing Calculator — What Size Velux Do I Need?

Calculate the size and number of Velux or roof windows needed to comply with Part L daylight requirements. Includes flashing kit recommendations for your roof pitch.

Room Floor Area

Measured floor area of the room

Room Type

Affects minimum glazing fraction and escape requirements

Roof Pitch

Angle of roof slope — affects flashing kit type

Rafter Spacing

Centre-to-centre spacing of roof rafters

How We Calculate This

This calculator determines the size and number of roof windows needed based on the room floor area, room type, rafter spacing, and roof pitch. It follows the Part L daylight requirement and recommends standard Velux window sizes.

The formula

Minimum glazing area = Floor area × Glazing fraction (1/6 for habitable rooms)

Number of windows = Minimum glazing area ÷ Single window glazing area (rounded up)

Glazing fractions

Habitable rooms (bedroom, living, kitchen, study): 1/6th of floor area
Bathrooms: 1/8th of floor area (no statutory minimum, but good practice)

Standard Velux sizes

CK02: 550 × 780mm — 0.27 m² glazing
MK04: 780 × 980mm — 0.54 m² glazing
MK06: 780 × 1180mm — 0.66 m² glazing
SK06: 1140 × 1180mm — 1.03 m² glazing
UK08: 1340 × 1400mm — 1.47 m² glazing

Standards

Glazing requirements comply with Approved Document L (Conservation of fuel and power). Escape window requirements comply with Approved Document B (Fire safety). Window installation should comply with BS 8213-4 (Code of practice for survey and installation of windows and external doorsets).

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Roof Window Sizing Calculator — What Size Velux Do I Need?

How We Calculate This

The formula

Glazing fractions

Standard Velux sizes

Standards

Frequently Asked Questions

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