Changing roof pitch and roof tiles to build a dormer

Hi,

I would like to ask the idea of changing the roof pitch of my house in order to accommodate a dormer to be added as an extra room is a cost effective way compare to getting to build a second storey ?

I do not require more than one room, all I need is just an additional room and the current floor plan require me to give up my alfresco if I need to add an additional room.

I also have a relative flat and old uninteresting roof which I hope by this exercise I get more out of this as this will allow me to :
1, change the roof tile colour to matching the render house colour
2. have all my gutters and fascia replaced since they are old
3. replace most of the ceilings since they are old, more than 20 years old
4. remove the asbestos in the ceiling

Can someone please let me know if this is a huge building job that required a substantial of money that it is not worth it just for adding a dormer ?

Thanks
Abbie