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

There is no statutory minimum in England — the Building Regulations set glazing U-values (Part L) and overheating maxima (Part O), but no minimum daylight area. The widely-used good-practice rule of thumb is glazing of about 1/6 of the floor area for a habitable room (1/8 for a bathroom). For a 12 m² loft bedroom that suggests roughly 2 m² of glazing. For a robust daylight assessment use the current standard, BS EN 17037:2018+A1:2021, which uses daylight-factor or illuminance modelling rather than a flat ratio. Note that Scotland does have a statutory rule of 1/15 of floor area. Separately, a loft bedroom (or other habitable room above the ground floor) usually needs a Part B escape window — a clear opening of at least 0.33 m² with no dimension less than 450mm.

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.

This calculator sizes the window only — it does not select a flashing kit — but as a guide, VELUX flashings are coded by the roof covering. EDL suits slate and flat coverings up to 8mm thick (pitch 15°–90°). EDW suits profiled or interlocking tiles with a profile up to 120mm (pitch 15°–90°). EDP suits flat interlocking (plain) concrete or clay tiles. EDN (slate) and EDJ (tile) are recessed kits that let the window sit lower in the roof (pitch 20°–90°). Always confirm the kit against your exact roof covering, profile depth and pitch using VELUX's current fitting guidance and the window's minimum pitch (most centre-pivot windows are 15° minimum).

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.

If the loft room is a habitable room (bedroom, study, etc.) and there is no alternative protected route to a final exit, Approved Document B Vol 1 §2.10 requires an emergency escape window. The window must have an unobstructed openable area of at least 0.33 m², with no dimension less than 450mm, and the bottom of the openable area must be no more than 1100mm above the floor. Approved Document B sets no lower limit — the 600mm/800mm figures sometimes quoted are guarding (fall-protection) thresholds under Part K, not part of the escape rule. Most VELUX centre-pivot windows in size MK04 and larger meet the openable-area minimum when opened to the escape position.