A Mansard is usually used when a home has a butterfly or London roof and a Mansard loft conversion involves the replacement of the London roof structure using a flat crown and steeply pitched slopes. There will be radical changes made to the degree of pitch of the roof, thus it will require planning permission.
A Mansard loft conversion is aesthetically pleasing to home owners for the possibility of incorporated windows. The pitch of the roof will allow for the windows to appear as small dormers. A Mansard conversion usually is made at the rear of a home and usually they are built with gable ends which extend up to a party parapet and along the steep slope of the roof. Vertical windows are often installed to maximize light in the added loft conversion, although flush fitting roof lights are used very successfully as well.
Home owners within the North London area who have these types of roofs usually choose this type of loft conversion and Londons Lofts pride themselves to provide this loft solution to their customers. An advantage of a Mansard roof from the French Architect from the 17th century, Francois Mansard is the great advantage it provides for maximizing available space in a loft.
However, unless the original roof is a Mansard roof it will cost a home owner quite a bit more in constructing a loft conversion as the entire roof will need replacing. It is not often encountered in the suburbs, but quite common in older properties and especially cities like London.