Having a flat roof extension added to a property used to be a cost effective way to extend your ground floor living space, however they are becoming outdated.  You could update the felt roof having it re felted, however this would do much for improving its appearance or light.  A fantastic way to upgrade a flat you would be to add the addition of a contemporary glass roof light or a glass pyramid/lantern.  These will add both an upgraded interior and exterior aspect as well as adding a natural source of light to your room.  Roof lights and glass pyramids can be fixed pains or have an opening panel operated manually or electrically.   For most roof lights you will not need planning permission as long as they follow planning guidelines.  

The added addition of a Roof light or glass pyramid/lantern may add value to your property and make it more appealing with the modern upgraded feel.