Front rooflights are not front dormers
Class B restricts enlargements beyond a roof slope forming the principal elevation and fronting a highway. The government guidance expressly explains that rooflights on that elevation may instead qualify under Class C. A projecting dormer cannot be re-labelled a rooflight to avoid the enlargement rules. [1]
For the Class C assessment, the government technical guidance limits projection to 0.15 metres, measured perpendicular to the external surface of the original roof slope. It also prevents the highest part of the alteration exceeding the highest part of the original roof. Use a product section showing the whole frame and installation, not just the glass. [1]
The word 'original' is significant in that guidance. It means the building as it stood on 1 July 1948, or as built if later, rather than automatically meaning every later roof alteration. Where the roof has already changed, establish the relevant baseline before measuring. [1]
Check the property, not just the window
Householder Class C rights are not available to flats. Planning history can also exclude particular houses from these rights, and an Article 4 direction or planning condition may remove a right that would otherwise be available. Planning Portal's roof guidance identifies these eligibility checks alongside the dimensional limits. [2]
Conservation-area designation does not impose Class B's roof-enlargement exclusion on every Class C alteration. Nevertheless, check the precise local direction and, for a listed building, whether the work affects historic significance and requires listed building consent. [1] [3]
Side-facing roof windows must be obscure-glazed. They must also be non-opening unless the opening parts are more than 1.7 metres above the room floor. This is a side-elevation condition, not a blanket rule that every front rooflight needs obscure glass. [1]
The rooflight must also work technically
The roof structure has to support the opening and any altered rafters. Insulation needs continuity around the frame, and the ventilation strategy must work with the selected window and any separate background or extract provision. A planning-compliant rooflight is not automatically a complete building-regulations solution. [4] [5] [6]
An escape rooflight needs a suitable clear opening and position, and must form part of an appropriate fire strategy. In a typical conversion adding a floor above 4.5 metres, a rooflight is not a substitute for the protected escape route. [7]
What to keep with the drawings
Keep the manufacturer's installation dimensions, proposed roof section and any applicable planning decision or lawful development certificate. When ordering, check that a different frame or flashing arrangement will not change the certified projection or external appearance. The certificate only establishes the lawfulness of the development it describes. [1] [8]
For work affected by the energy and ventilation changes from 24 March 2027, confirm the applicable Approved Document editions and transitional route before finalising the specification. [9]