Class C: Velux windows, rooflights and roof alterations

Class C can grant planning permission for rooflights, including Velux windows, and other alterations to the roof of a qualifying house in England. Velux is a brand: the same planning tests apply to other makes.

The two main measurements refer to the original roof, not simply the roof you see today. Side-facing roof windows also have privacy conditions.

The two roof measurements

• The alteration must not project more than 0.15 m (150 mm) beyond the plane of the original roof slope, measured perpendicular to its external surface
• The highest part of the alteration must not be above the highest part of the original roof

Perpendicular means at a right angle to the roof surface. A vertical measurement is different on a sloping roof. Check the complete proposed installation, rather than relying on a manufacturer’s nominal frame dimension.

Original generally means as the house stood on 1st July 1948, or as first built if later. Previous alterations do not automatically establish a new reference roof for Class C.

Roof windows on a side elevation

A window on a roof slope forming a side elevation must meet both conditions:

• It must be obscure-glazed
• It must be non-opening unless the parts which can open are more than 1.7 m above the floor of the room

Exactly 1.7 m is not enough for an opening part. An opening above that height still needs obscure glazing. The height is measured from the internal room floor, not the garden or the bottom of the window frame.

Can a rooflight face the street?

Class C has no general prohibition on a rooflight on a highway-facing principal roof slope. That differs from the restriction on roof enlargements in Class B. A suitable low-profile rooflight may therefore follow a different planning route from a front dormer.

You still need to check the dimensional limits, side-window condition where relevant, planning history and any Article 4 direction. Read the actual direction: it may affect particular works or elevations.

Conservation areas and listed buildings

Class C does not contain a blanket exclusion for conservation areas or other article 2(3) land. A local Article 4 direction or a planning condition can nevertheless remove the right.

Listed building consent is separate from planning permission. Alterations affecting a listed building’s special character require the appropriate consent, even if the planning work would meet Class C. Do not assume that a conservation-style rooflight will automatically be acceptable.

Which houses cannot use Class C?

• Flats, maisonettes and buildings containing flats are outside Part 1’s dwellinghouse definition
• Class C is unavailable where permission to use the house as a house arose only through Part 3 Classes G, M, MA, N, P, PA or Q
• A dwellinghouse built under Part 20 is excluded
• Relevant planning conditions and Article 4 directions may remove the right

The exclusion list does not name the former office-to-house Class O, but the property must still be a qualifying house and meet the other rules. The existing building and use must be lawful.

Dormers, solar panels and roof balconies

Use Class B for an enlargement such as a dormer or hip-to-gable roof. Class C excludes the installation, alteration or replacement of solar photovoltaic and solar thermal equipment, which must be considered under the relevant Part 14 right. Chimneys, flues and soil and vent pipes have their own route in Class G.

A roof-balcony product needs an assessment of its actual design and operation. The brand name, or the description “window”, does not establish that it meets Class C. Do not treat a projecting terrace or balcony as an ordinary flush rooflight without checking the complete proposal.

How to obtain certainty

There is no Class C prior approval procedure. Where all the requirements are met, the GPDO grants planning permission. A certificate of lawfulness for proposed development can confirm the position before installation.

Provide drawings showing the original roof, the perpendicular projection, the highest roof point and any side-window glazing and opening details. Structural work, thermal performance, ventilation and other Building Regulations matters are separate from this planning assessment.

Sources and Class C legislation

Checked against the current GPDO Part 1 and article 2 definitions. The government householder technical guidance provides explanations and examples, but must be read with subsequent amendments to the legislation.

Page Updated: 25th September 2026