The skylight projection limit is one of the most important technical rules to understand when planning a rooflight installation in the UK. Under permitted development rights, skylights can often be installed without applying for planning permission, but only if they meet strict dimensional criteria. One of the most critical of these is the 150mm projection rule. When researching compliant rooflight options, many homeowners and professionals refer to technical guidance available from suppliers such as Skylights Roof Lanterns.
Misunderstanding this rule can result in non-compliant installations, enforcement action from local authorities, or the need for retrospective planning applications. This article explains how the 150mm rooflight rule works, how roof plane projection is measured, and when planning height restrictions apply.
Understanding Rooflight Planning Limits in the UK
Permitted development rights allow homeowners to make certain alterations to their properties without submitting a full planning application. Roof alterations, including skylights and rooflights, are often permitted, but only when specific conditions are met.
Among these conditions, the skylight projection limit is one of the most strictly enforced. Even where a rooflight appears visually discreet, exceeding the permitted projection height can remove permitted development status entirely.
What Is the 150mm Projection Rule?
Definition of the Skylight Projection Limit
The 150mm rooflight rule is set out in the General Permitted Development Order (GPDO). It states that a rooflight must not project more than 150 millimetres above the plane of the existing roof.
This measurement applies to all rooflights, regardless of size, glazing type, or whether the unit is fixed or opening. If any part of the skylight exceeds this height, it falls outside permitted development.
How Rooflight Projection Is Measured
What Is Considered on the Roof Plane?
The roof plane refers to the external surface of the existing roof before installation. This includes the finished surface such as tiles, slates, or metal roofing, but excludes internal finishes, insulation layers, or ceiling structures.
All measurements must be taken from this external roof plane.
Measuring the Projection Correctly
Roof plane projection is measured vertically from the roof surface to the highest external point of the installed rooflight. This includes:
- The glazing unit
- Any framing or capping
- Hinges or opening mechanisms
The total height must not exceed the 150mm planning height restriction.
Does the Rooflight Upstand Count?
Yes. Rooflight upstand height planning is a common source of confusion. If the upstand forms part of the rooflight system, it is included in the overall projection measurement.
Even low-profile glazing can become non-compliant if the combined height of the upstand and glazing exceeds the skylight projection limit.
Why the 150mm Limit Matters for Permitted Development
When a Rooflight Qualifies as Permitted Development
A rooflight may qualify as permitted development if:
- The property is a single dwelling house
- The rooflight does not project more than 150mm above the roof plane
- The roof shape is not materially altered
- No additional restrictions apply to the property
When these criteria are met, planning permission is generally not required.
What Happens If the Projection Exceeds 150mm
If a rooflight exceeds the 150mm limit, even marginally:
- Permitted development rights no longer apply
- Planning permission may be required
- Enforcement action may be taken by the local authority
This makes precise measurement and specification essential at the design stage.
Typical Rooflight Designs and Their Projection Heights
Low-Profile Flat Glass Rooflights
Flat glass rooflights are commonly used where compliance with the skylight projection limit is a priority. Many modern rooflight options are designed to sit close to the roof surface, making them suitable for permitted development installations.
Suppliers such as Skylights Roof Lanterns provide detailed technical drawings that show total external projection, helping homeowners and designers assess compliance before installation.
Domed and Lantern Rooflights
Domed rooflights and roof lanterns typically project significantly higher than flat units. In most cases, these designs exceed the 150mm roof plane projection limit and require planning permission.
They are rarely suitable for permitted development unless installed on properties where planning restrictions do not apply.
Exceptions and Situations Where the Rule May Not Apply
Listed Buildings and Conservation Areas
Permitted development rights may be restricted or removed entirely for properties that are:
- Listed buildings
- Located in conservation areas
- Within national parks or protected landscapes
In these cases, planning consent may be required regardless of the skylight projection limit.
Additional Consent Requirements
Even where planning permission is not required, other approvals may still apply, including:
- Building Regulations approval
- Listed Building Consent
- Leaseholder or freeholder consent
Compliance with the 150mm rooflight rule does not remove the need for these additional permissions.
Flats and Non-Eligible Properties
Permitted development rights for roof alterations apply only to dwelling houses. Flats, maisonettes, and commercial properties do not benefit from these rights.
For such properties, planning permission is usually required regardless of rooflight height.
Local Authority Interpretation Variations
Although the 150mm rule is set nationally, interpretation can vary between local authorities. Some councils may request detailed drawings or consider the cumulative impact of multiple roof alterations.
Because of this, homeowners often choose to apply for a Lawful Development Certificate to confirm compliance.
Specifying Compliant Rooflights at Design Stage
Working With Manufacturers’ Specifications
Accurate product data is essential when assessing planning height restrictions. Manufacturers that provide clear section drawings and verified dimensions allow designers to confirm whether a rooflight meets the skylight projection limit. Reviewing detailed skylight specifications from Skylights Roof Lanterns can help ensure that rooflight specifications align with permitted development requirements before work begins.
Final Pre-Installation Compliance Checklist
Confirm Property Eligibility
Before installation, confirm that:
The property qualifies as a dwelling house
Permitted development rights have not been removed
No additional planning constraints apply
Verify Rooflight Dimensions
Ensure that:
Total external projection does not exceed 150mm
Measurements include the upstand and glazing
Opening mechanisms do not increase overall height
Documenting these checks can provide valuable evidence of compliance.
Conclusion
The skylight projection limit plays a decisive role in determining whether a rooflight installation qualifies as permitted development in the UK. While the 150mm threshold may appear minor, exceeding it can have significant planning consequences. By understanding how roof plane projection is measured, accounting for rooflight upstand height planning, and reviewing manufacturer specifications carefully, homeowners and professionals can reduce planning risk and ensure regulatory compliance. Where clarification on product dimensions or technical details is required, it may be helpful to consult directly via the Skylights Roof Lanterns contact page.
