If you are planning to extend your roof space with dormer windows, The preferred form is likely to be influenced by any traditional dormers in your area

The following guidelines should be taken into account:

• The size and scale of the dormer should not dominate the original roof
• Dormers should not project above the original roof ridge
• Generally, dormers should not be build off the wallhead unless they are of a traditional design in a traditional building.
• Dormers should not extend to the gable end.

In Conservation Areas and on Listed Buildings, where the principle of dormer extensions has been accepted, dormers should be small, designed in a traditional manner and have a pitched roof. Wide 'box' dormers are likely to be out of scale and may spoil the appearance of your house. As an alternative two smaller dormers may greatly improve the overall appearance.