A rear dormer extension is the process by which the back slope of the roof is removed and then built up into a box shape from the rear elevation. Rear dormers are usually built off the back wall and up off both the sides.
A mansard loft conversion, named after the 17th-century French Architect Francois Mansard, is situated to the rear of the property. This type of conversion has a flat roof, with the back wall sloping inwards at an angle of 72 degrees.
An L-shaped dormer conversion is where two dormer builds are constructed in a way that they join together. Usually, one dormer will be built on the rear outrigger roof and the other on the main roof, which is what gives it the instantly recognisable L-shape.