Flat Roof to Pitched Roof

With the improvements in flat roofing technology, there is now much less need for this particular conversion.
Modern, properly constructed GRP flat roofs can be as reliable as pitched roofs.
However, there may be other reasons for a conversion.

Using the normal, (steeper), roof pitch angle, on a ground floor, single story extension might mean that the roof would cover any bedroom windows above the extension.
Low pitch angle tiles can be used here.

i.e: Centurion tiles allow a pitch angle of 12.5 degrees.