Do I Need Planning Permission for a Rooflight or Velux Window?

It is important to understand the difference between a rooflight and a dormer, because they are covered by different rules:

• A rooflight (including Velux-style windows) sits within or very close to the plane of the existing roof. It does not create any additional internal volume. Rooflights are covered by Class C.
• A dormer is a projecting structure that extends out from the roof slope, creating additional headroom and floor space. Dormers are covered by Class B and have much stricter rules, including volume limits.

If you are simply installing windows into the roof without changing the roof shape, you are dealing with Class C — and the rules are very straightforward.

The key Class C rule is that the rooflight must not protrude more than 150mm beyond the plane of the existing roof slope. This is measured from the outer surface of the roof tile or slate to the highest point of the rooflight frame.

Most standard Velux-style centre-pivot rooflights protrude around 50–100mm when closed, well within the 150mm limit. Top-hung rooflights have a similar profile.

Conservation rooflights are designed to sit completely flush with the roof surface. They are the safest option if you are unsure about protrusion, and they are required in some conservation areas (see below).

The 150mm rule applies to the rooflight itself, not to any flashing kit around it. Standard flashing kits do not protrude beyond 150mm.

Any rooflight on a side elevation must be:

• Obscure glazed to Pilkington Level 3 or equivalent, and
• Either non-opening or top-hung with the opening part at least 1.7 metres above the internal floor level.

This rule protects neighbour privacy. It applies even if the rooflight is on a roof slope rather than a vertical wall — it is the orientation that matters, not whether the window is vertical or angled.

Rooflights on front-facing and rear-facing roof slopes have no obscure glazing requirement under Class C. Note that this is different from dormers under Class B, which are not permitted on highway-facing slopes. Rooflights on front slopes are fine.

Flat roof rooflights and roof lanterns are increasingly popular, especially on single-storey extensions. They fall under Class C if they are installed into an existing roof.

The 150mm protrusion rule still applies, measured from the plane of the flat roof surface. Most flat roof rooflights sit low-profile and easily meet this limit. Larger roof lanterns (sometimes called "lantern lights") can protrude more, particularly if they have a pitched glass structure. If the lantern exceeds 150mm above the roof surface, it would fall outside Class C.

If the rooflight is being installed as part of a new extension, it does not trigger Class C separately — it is assessed as part of the extension under Class A. Class C applies to alterations to the existing roof of the house.

There is no limit on the number of rooflights you can install under Class C, provided each one meets the protrusion and glazing rules. You can install as many rooflights as you want on rear and front slopes without restriction.

However, if you are installing rooflights as part of a loft conversion that also involves a dormer, the rooflights do not count towards the Class B volume limit (they do not add external volume). They are still assessed under Class C independently.

If you are converting a loft into habitable space, building regulations may require an escape window. Where a standard window in a gable wall is not possible, a rooflight can serve as the fire escape route.

An escape rooflight must have a clear opening of at least 0.33m² with no dimension less than 450mm. It must be openable from inside without a key, and the bottom of the opening must be no more than 1.1 metres above the floor. These are building regulations requirements (Approved Document B), not planning rules, but they affect which rooflight model you choose.

Class C is not specifically restricted in conservation areas, national parks, or Areas of Outstanding Natural Beauty — the same protrusion and glazing rules apply everywhere.

However, many councils in conservation areas have planning guidance that recommends or requires conservation-style (flush-fitting) rooflights. This is not a legal requirement under Class C, but if you are applying for an LDC or if a planning officer queries the installation, using conservation rooflights demonstrates good practice.

Listed buildings do not benefit from Class C permitted development rights. Installing a rooflight on a listed building requires listed building consent and may also need planning permission.

Replacing a rooflight like-for-like (same size, same position) generally does not need building regulations approval, though the new unit should meet current thermal performance standards under the FENSA or CERTASS scheme if it is a glazed unit.

Installing a new rooflight where none existed before will usually trigger building regulations requirements covering:

• Structural support — cutting roof timbers to create the opening may require additional trimming or support.
• Thermal performance — the glazing unit must meet minimum U-value requirements.
• Weather resistance — correct flashing and waterproofing.
• Fire safety — if the rooflight is part of a loft conversion, escape window requirements apply.

For standard rooflights that clearly meet the 150mm rule, most homeowners do not apply for an LDC. However, for large roof lanterns or installations where the protrusion is close to the 150mm limit, a Lawful Development Certificate provides formal confirmation. It can also be helpful if you are installing multiple rooflights and want to pre-empt any queries from neighbours or the council.