The design is similar to a Rear dormer although the rear slope is at a 70 degree angle instead of a 90 degree. Mansards are quite often seen built up with brick sides but this can only be achieved under planning permission.

Due to the degree of the rear slope, Mansards look softer on appearance and less top heavy than that of a dormer construction, however this will also be a cost increase as more time and effort will be required to cut the timbers to the required angle.

Do check with your local authority if they are likely to approve planning permission for a Mansard, alot of authorites do not seem to like the design of a Mansard construction. You can call your local planning department, or search online in their planning database records, or a good way to tell is to have a look in your street or in the area and see what has been built, this should give you a good indication of what you will be allowed.