What is the 150mm permitted development rule?

How it is measured

From the existing roof plane, meaning the line of the roof surface, to the highest point of the new work.

Most standard roof windows sit within it comfortably. A conservation rooflight, designed to sit nearly flush, clears it easily, which is one of the reasons it is the product officers expect to see in sensitive locations.

Where it bites

Roof lanterns. A pitched glazed structure on a kerb is by definition taller than 150mm. On a rear extension this is normally assessed against the extension’s own roof and lanterns are routinely fitted, but on a main roof slope it is a real constraint.

Large rooflights on a kerb. A tall upstand plus the unit can exceed it.

Very large lanterns, where holding a sensible pitch produces real height and the pitch sometimes has to be reduced to comply.

The other permitted development conditions

The 150mm rule is one of several. Others include not extending beyond the highest part of the roof, and, on a principal elevation fronting a highway, side-facing windows being obscure glazed and non-opening below 1.7m.

Permitted development also does not apply to flats and maisonettes, or where rights have been removed by an Article 4 direction or a planning condition.

Conservation areas and listed buildings

The rule still applies, and additional restrictions sit on top of it. Complying with the 150mm rule does not mean permission is not needed in a conservation area, and it has no bearing at all on the need for listed building consent.

How far a rooflight may stand proud

On an unlisted house outside any Article 4 direction, a rooflight is permitted development provided it does not project more than 150mm beyond the plane of the existing roof slope.

That single dimension is the most important number in domestic rooflight planning, and it is why low profile units exist.

Taking the measurement

From the existing roof surface to the highest point of the finished installation, including any flashing upstand where it stands proud. Not from the top of the rafters and not from the underside of the covering.

Manufacturers publish a section drawing showing exactly this dimension for each model, and it is the document to keep with your records and to include with any application.

The two conditions that go with it

Not above the ridge. The unit must not be higher than the highest part of the existing roof.

Side elevations facing a boundary. Any opening must be obscure glazed, and any opening part must be either non opening below 1.7 metres from the floor of the room or restricted so it cannot open below that height.

Meet all three and no planning application is needed.

What fails it

Bulky domes standing well proud of the covering, which is why old polycarbonate rooflights on flat roofs are frequently outside permitted development even though nobody has ever queried them.

A roof lantern, which stands considerably higher than 150mm and is dealt with as part of the extension’s own permission rather than under the rooflight rule.

What the rule does not cover

Listed buildings, where consent is required regardless. Flats and maisonettes, which have no permitted development rights at all. Properties with an Article 4 direction. And buildings where permitted development rights were removed by a planning condition, which is common on barn conversions and some new estates.

And it says nothing about Building Regulations

Permitted development is a planning concept only. A new opening still engages Part A for structure and Part L for energy, and in a habitable loft room Part B for means of escape. All notifiable regardless of the planning position.

If there is genuine doubt

A lawful development certificate from the council confirms the position formally. It costs a fee and it is worth having on an unusual case, particularly if you expect to sell within a few years.

What is actually being measured

The projection of the rooflight above the existing roof plane, measured perpendicular to the slope, at its highest point.

That is not the same as the thickness of the unit, and it is not measured from the battens. It is how far the finished installation stands proud of the finished roof surface around it, which is what somebody standing in the street would see.

The measurement includes the flashing upstand and any kerb, not just the glazed part.

The other conditions that travel with it

The 150mm limit is one condition among several, and satisfying it alone does not make an installation permitted development.

The rooflight must not be higher than the highest part of the roof. A unit close to the ridge can fail this even when it satisfies the projection limit.

And where the rooflight faces a side elevation and the room is a habitable one, obscure glazing and restricted opening conditions can apply.

Where the rule does not help you

In a conservation area, on a listed building, or where an Article 4 direction is in force. In those cases permitted development rights are restricted or removed entirely, and a unit that satisfies every dimensional condition still needs consent.

Flats and maisonettes do not have these rights at all. Neither do many properties where earlier extensions used up the allowance or where a planning condition removed them.

The council planning department will confirm the position for a specific address, and that confirmation is worth having in writing before ordering anything.