Conversion types
Rooflight loft conversions
A rooflight conversion puts windows into the existing roof slope without changing its shape. It falls under Class C of the GPDO, uses none of the 40m³/50m³ volume allowance, and is not withdrawn on designated land — so it is normally still Permitted Development in a Conservation Area. Rooflights must not protrude more than 150mm beyond the roof plane.
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What a rooflight conversion is
A rooflight conversion converts the loft without changing the shape of the roof. The structure is upgraded, a stair goes in, the floor is built up and insulated, and daylight comes from windows set into the existing slope.
Because nothing is added above the original roof plane, it adds no volume — which changes its legal position completely.
Why Class C is a different world
| Rooflight (Class C) | Rear dormer (Class B) | |
|---|---|---|
| Volume allowance used | None | Typically 15–35m³ |
| Permitted in a Conservation Area | Yes | No |
| Permitted in an AONB / National Park | Yes | No |
| Restricted on a highway-fronting slope | No, subject to the 150mm limit | Yes |
| Adds headroom | None | Substantial |
| Typical cost | Lower | Considerably higher |
The conditions
- C.1(a) — 150mm. No part may protrude more than 150mm beyond the plane of the original roof slope, measured perpendicular to the external surface. Standard rooflights meet this; deep units and raised installations may not.
- C.1(b) — ridge height. No part may be higher than the highest part of the original roof. Rules out ridge-mounted units that break the line.
- C.2 — side windows. Obscure-glazed, and non-opening unless the opening part is more than 1.7m above the floor of the room.
When it actually works
The honest constraint on a rooflight conversion is not planning, it is geometry. It adds no headroom whatsoever, so everything depends on what your roof already has.
- Works well: a Victorian or Edwardian roof with a steep pitch and a high ridge, where 2.2m or more is available across a usable strip either side of the ridge line.
- Works marginally: a 1930s semi, where you often get standing height in the middle and a room that is usable but tight, particularly once the floor and roof build-up is deducted.
- Usually does not work: a post-1965 trussed-rafter roof with a shallow pitch. There is rarely enough height, and the trusses have to be restructured before the space exists at all.
The deduction people forget is the build-up. The finished headroom is measured from the top of the new floor to the underside of the new insulated ceiling — and both of those are considerably thicker than the joists and plaster they replace.
It still needs Building Regulations
Adding no volume does not make it a small job in Building Control terms. A rooflight conversion still turns a two-storey house into a three-storey one, which means a protected stairway, fire doors and interlinked alarms — see Building Regulations for loft conversions. The rooflights themselves also usually have to serve as emergency egress windows, which sets minimum opening dimensions and a maximum sill height.
Common questions
- Do I need planning permission for Velux windows?
- Almost never. Rooflights are permitted under Class C of the GPDO provided they protrude no more than 150mm beyond the plane of the existing roof slope and no part sits above the highest part of the roof. Side-elevation rooflights must be obscure-glazed and non-opening below 1.7m. Class C is not withdrawn on designated land, though an Article 4 Direction can remove it.
- What is the 150mm rule?
- Class C.1(a) requires that the alteration does not protrude more than 150mm beyond the plane of the slope of the original roof, measured perpendicular to the external surface. Standard rooflights sit within this comfortably; deeper units and some out-of-plane installations do not, which is why flush-fitting conservation rooflights are specified on sensitive roofs.
- Is a rooflight conversion enough for a bedroom?
- It depends entirely on your existing roof. A rooflight conversion adds no headroom at all — it only lets in light — so it works where the ridge is high and the pitch steep enough to give 2.2m or more at the apex with usable floor area either side. On a shallow modern trussed roof it usually does not.
Sources
- GPDO 2015 — Schedule 2, Part 1, Class C — rooflights and other roof alterations that add no volume
- Permitted development rights for householders: technical guidance — the government's plain-English reading of Class B, with diagrams
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Related guides
Rear dormer loft conversion rules
The Permitted Development limits that shape a rear dormer: volume, ridge height, the 200mm eaves setback, highway frontage, matching materials and side windows.
Loft conversions in a Conservation Area
A dormer is not Permitted Development in a Conservation Area — but a rooflight conversion usually still is. Why Class B is withdrawn and Class C is not.
Loft conversion head height
There is no legal minimum ceiling height for a loft room. 2.2m is a practical guideline, not a regulation — and the number that matters is the finished height after build-up.
Permitted Development rules for loft conversions
Class B and Class C of the GPDO 2015, explained clause by clause: volume limits, ridge height, eaves setback, highway frontage, materials and side windows.
Written by the LoftNow team from the legislation and Approved Documents cited above, and reviewed against the rules our assessment engine implements. It is general guidance for properties in England, not advice on your particular property — Permitted Development is self-assessed, and the definitive answer for a specific house comes from your local planning authority, normally via a Lawful Development Certificate.