Materials should have a similar appearance
Class B requires materials used in exterior work to have a similar appearance to those of the existing house. The government guidance explains that this does not require identical materials. The visual effect matters: the faces and sides of a dormer should relate to the existing exterior, while a flat roof with little visual impact may use a different suitable covering. [1]
Specify the proposed facing, roof covering and window finish on the drawings. 'To match existing' is not a useful description where the intended product will visibly contrast with the house. An expressly approved planning design can involve a different assessment; do not confuse that with satisfying Class B. [1] [2]
Understand the 200mm measurement
For the ordinary dormer case, the original eaves must be maintained or reinstated. The edge nearest those eaves must, so far as practicable, be at least 0.2 metres back, measured along the roof slope from the outside edge of the eaves. The guidance measures from the outer edge of the tiles or slates, not from projecting guttering. [1]
There are exceptions to this part of the condition for a hip-to-gable enlargement and for an enlargement joining the original roof to the roof of a rear or side extension. The 'so far as practicable' qualification is not an unrestricted choice to maximise the room: any reliance on it should be supported by the actual practical or structural circumstances. [1]
A separate condition generally prevents the enlargement extending beyond the outside face of an original external wall, with an exception for the specified joined-roof arrangement. The exceptions are not identical, so check each limb of the condition rather than applying one shortcut to the whole design. [1]
Side-facing windows
A window inserted in a wall or roof slope forming a side elevation must be obscure-glazed. It must also be non-opening unless the parts that open are more than 1.7 metres above the floor of the room. For a stair or landing, the government guidance explains how the measurement is taken from the point immediately below the centre of the window. [1]
Privacy glass does not turn an otherwise prohibited opening into permitted development. Nor should a restricted, obscure side window be assumed to provide the escape opening required by a particular fire strategy. Check the planning and fire requirements together. [1] [3]
The other conditions still apply
Class B also controls the highest roof level, principal-elevation slopes fronting a highway, total added volume and features such as balconies. It does not permit roof enlargements in conservation areas. A compliant set-back cannot override any of those restrictions. [1]
Before submission or construction, check the final roof section, window schedule and external materials against the same drawings used for the lawfulness assessment. Site changes to a certified design need to be assessed again rather than assumed to be covered by the earlier certificate. [2]