Glossary
Velux rooflights in a London loft conversion or extension
A roof window installed in line with the existing roof pitch. Cheapest loft option.
A roof window installed in line with the existing roof pitch. Cheapest loft option.
A rooflight is a window set in the roof plane. Under Class C of the GPDO a rooflight is permitted development provided it projects no more than 150 mm beyond the plane of the existing roof slope, does not exceed the highest part of the roof, and — on a side elevation — is obscure-glazed and non-opening below 1.7 m.
A rooflight is far cheaper, needs no structural alteration beyond trimming the rafters, and is almost always permitted development. It does not add head height or usable floor area, so a loft converted with rooflights alone only works where the existing ridge height already gives 2.2 m or more over enough of the floor.
A dormer adds volume and usable space but consumes the 40 or 50 m³ allowance and is not permitted in a conservation area. The usual London compromise on a conservation-area loft is rooflights to the front slope and a modest dormer or none at the rear.
Part L requires a rooflight U-value of 2.2 W/m²K in an extension. Part K requires guarding where the cill is below 800 mm and the drop outside exceeds 600 mm. Part B allows a rooflight to serve as an escape window in a loft provided the clear opening is at least 0.33 m² with a minimum 450 mm in each direction, and the cill is between 800 and 1,100 mm above floor level.
That cill height requirement is what dictates rooflight position in a loft conversion — it must be reachable, which usually means low in the slope.
Class C survives in conservation areas for rear-facing rooflights, but a rooflight on a slope fronting a highway is not permitted development on Article 2(3) land. Conservation officers usually prefer conservation-style rooflights — flush-fitting, with a central glazing bar and a slim black frame.
Rooflight (Velux) in a Southwark context: London Borough of Southwark (Southwark Council) runs 43 conservation areas including the Camberwell Grove Conservation Area and the Bermondsey Street Conservation Area, and an Article 4 direction is in force there, so permitted development cannot be assumed. That decides how rooflight (velux) is handled: £548 and 8 weeks for a Southwark householder application, or £274 for the Lawful Development Certificate that records rooflight (velux) as lawful. The law behind rooflight (velux) sits in the Town and Country Planning Act 1990, the GPDO 2015 and the Building Regulations 2010.
| Maximum projection | 150 mm beyond the roof plane |
|---|---|
| Must not exceed | The highest part of the existing roof |
| Side elevation | Obscure-glazed, non-opening below 1.7 m |
| U-value in an extension | 2.2 W/m²K |
| Escape opening | 0.33 m², min 450 mm each way, cill 800–1,100 mm |
| Conservation areas | Rear slopes generally still permitted |
Usually not, provided it projects no more than 150 mm and does not exceed the ridge.
Yes, if the clear opening and cill height meet Approved Document B.
Not by statute, but officers commonly require a flush conservation-style unit on visible slopes.
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